diff --git a/BankingClientModel/build.gradle.kts b/BankingClientModel/build.gradle.kts
index 25aae74b..24fa0d53 100644
--- a/BankingClientModel/build.gradle.kts
+++ b/BankingClientModel/build.gradle.kts
@@ -79,6 +79,8 @@ kotlin {
 
     val kotlinxDateTimeVersion: String by project
     val jsJodaTimeZoneVersion: String by project
+    val ionspinBigNumVersion: String by project
+
 
     sourceSets {
         commonMain {
@@ -102,12 +104,26 @@ kotlin {
         jsMain {
             dependencies {
                 api(npm("@js-joda/timezone", jsJodaTimeZoneVersion))
+
+                implementation(npm("big.js", "6.0.3"))
             }
         }
         jsTest {  }
 
         nativeMain { }
         nativeTest { }
+
+        linuxMain {
+            dependencies {
+                implementation("com.ionspin.kotlin:bignum:$ionspinBigNumVersion")
+            }
+        }
+
+        mingwMain {
+            dependencies {
+                implementation("com.ionspin.kotlin:bignum:$ionspinBigNumVersion")
+            }
+        }
     }
 }
 
diff --git a/BankingClientModel/src/appleMain/kotlin/net/codinux/banking/client/model/Amount.apple.kt b/BankingClientModel/src/appleMain/kotlin/net/codinux/banking/client/model/Amount.apple.kt
new file mode 100644
index 00000000..28010e65
--- /dev/null
+++ b/BankingClientModel/src/appleMain/kotlin/net/codinux/banking/client/model/Amount.apple.kt
@@ -0,0 +1,29 @@
+package net.codinux.banking.client.model
+
+import platform.Foundation.NSDecimalNumber
+import net.codinux.banking.client.model.config.NoArgConstructor
+
+@NoArgConstructor
+actual class Amount actual constructor(amount: String) : NSDecimalNumber(string = amount) {
+
+    actual companion object {
+        actual val Zero = Amount("0.00")
+    }
+
+
+    actual operator fun plus(other: Amount): Amount =
+        Amount(this.decimalNumberByAdding(other).stringValue)
+
+    actual operator fun minus(other: Amount): Amount =
+        Amount(this.decimalNumberBySubtracting(other).stringValue)
+
+    actual operator fun times(other: Amount): Amount =
+        Amount(this.decimalNumberByMultiplyingBy(other).stringValue)
+
+    actual operator fun div(other: Amount): Amount =
+        Amount(this.decimalNumberByDividingBy(other).stringValue)
+
+
+    actual override fun toString(): String = this.stringValue
+
+}
\ No newline at end of file
diff --git a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/Amount.kt b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/Amount.kt
index bd217748..5e240bde 100644
--- a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/Amount.kt
+++ b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/Amount.kt
@@ -1,21 +1,31 @@
 package net.codinux.banking.client.model
 
 import net.codinux.banking.client.model.config.NoArgConstructor
-import kotlin.jvm.JvmInline
 
-@JvmInline
+
+internal const val DecimalPrecision = 20 // 20 to match Big.js's behavior
+
+
+fun Amount.toFloat() =
+    this.toString().toFloat()
+
+fun Amount.toDouble() =
+    this.toString().toDouble()
+
+
 @NoArgConstructor
-value class Amount(val amount: String = "0") {
+expect class Amount(amount: String = "0") {
 
     companion object {
-        val Zero = Amount("0")
-
-        fun fromString(amount: String): Amount = Amount(amount)
+        val Zero: Amount
     }
 
 
-    constructor(amount: Double) : this(amount.toString())
+    operator fun plus(other: Amount): Amount
+    operator fun minus(other: Amount): Amount
+    operator fun times(other: Amount): Amount
+    operator fun div(other: Amount): Amount
 
+    override fun toString(): String
 
-    override fun toString() = amount
 }
\ No newline at end of file
diff --git a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/extensions/AmountExtensions.kt b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/extensions/AmountExtensions.kt
deleted file mode 100644
index 025826f7..00000000
--- a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/extensions/AmountExtensions.kt
+++ /dev/null
@@ -1,6 +0,0 @@
-package net.codinux.banking.client.model.extensions
-
-import net.codinux.banking.client.model.Amount
-
-// TODO: really map to BigDecimal
-fun Amount.toBigDecimal(): Double = this.amount.toDouble()
\ No newline at end of file
diff --git a/BankingClientModel/src/commonTest/kotlin/net/codinux/banking/client/model/AmountTest.kt b/BankingClientModel/src/commonTest/kotlin/net/codinux/banking/client/model/AmountTest.kt
new file mode 100644
index 00000000..6a7e28aa
--- /dev/null
+++ b/BankingClientModel/src/commonTest/kotlin/net/codinux/banking/client/model/AmountTest.kt
@@ -0,0 +1,36 @@
+package net.codinux.banking.client.model
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class AmountTest {
+
+    @Test
+    fun add() {
+        val result = Amount("0.1") + Amount("0.2")
+
+        assertEquals(Amount("0.3"), result)
+    }
+
+    @Test
+    fun minus() {
+        val result = Amount("0.1") - Amount("0.2")
+
+        assertEquals(Amount("-0.1"), result)
+    }
+
+    @Test
+    fun multiply() {
+        val result = Amount("0.1") * Amount("0.2")
+
+        assertEquals(Amount("0.02"), result)
+    }
+
+    @Test
+    fun divide() {
+        val result = Amount("1") / Amount("3")
+
+        assertEquals(Amount("0.33333333333333333333"), result)
+    }
+
+}
\ No newline at end of file
diff --git a/BankingClientModel/src/jsMain/kotlin/net/codinux/banking/client/model/Amount.js.kt b/BankingClientModel/src/jsMain/kotlin/net/codinux/banking/client/model/Amount.js.kt
new file mode 100644
index 00000000..25981e7d
--- /dev/null
+++ b/BankingClientModel/src/jsMain/kotlin/net/codinux/banking/client/model/Amount.js.kt
@@ -0,0 +1,51 @@
+package net.codinux.banking.client.model
+
+import net.codinux.banking.client.model.config.NoArgConstructor
+
+@JsModule("big.js")
+@JsNonModule
+open external class Big(value: String) {
+    fun plus(other: Big): Big
+    fun minus(other: Big): Big
+    fun times(other: Big): Big
+    fun div(other: Big): Big
+    override fun toString(): String
+}
+
+
+@NoArgConstructor
+actual class Amount actual constructor(amount: String): Big(amount) {
+
+    actual companion object {
+        actual val Zero = Amount("0.00")
+    }
+
+
+    actual operator fun plus(other: Amount): Amount {
+        return Amount(super.plus(other).toString())
+    }
+
+    actual operator fun minus(other: Amount): Amount {
+        return Amount(super.minus(other).toString())
+    }
+
+    actual operator fun times(other: Amount): Amount {
+        return Amount(super.times(other).toString())
+    }
+
+    actual operator fun div(other: Amount): Amount {
+        return Amount(super.div(other).toString())
+    }
+
+
+    override fun equals(other: Any?): Boolean {
+        return other is Amount && this.toString() == other.toString()
+    }
+
+    override fun hashCode(): Int {
+        return super.hashCode()
+    }
+
+    actual override fun toString(): String = super.toString()
+
+}
\ No newline at end of file
diff --git a/BankingClientModel/src/jvmMain/kotlin/net/codinux/banking/client/model/Amount.jvm.kt b/BankingClientModel/src/jvmMain/kotlin/net/codinux/banking/client/model/Amount.jvm.kt
new file mode 100644
index 00000000..6ac8ffbd
--- /dev/null
+++ b/BankingClientModel/src/jvmMain/kotlin/net/codinux/banking/client/model/Amount.jvm.kt
@@ -0,0 +1,40 @@
+package net.codinux.banking.client.model
+
+import net.codinux.banking.client.model.config.NoArgConstructor
+import java.math.BigDecimal
+import java.math.RoundingMode
+
+@NoArgConstructor
+actual class Amount actual constructor(amount: String) : BigDecimal(amount) {
+
+    actual companion object {
+        actual val Zero = Amount("0.00")
+    }
+
+
+    actual operator fun plus(other: Amount): Amount =
+        Amount(this.add(other).toString())
+
+    actual operator fun minus(other: Amount): Amount =
+        Amount(this.subtract(other).toString())
+
+    actual operator fun times(other: Amount): Amount =
+        Amount(this.multiply(other).toString())
+
+    actual operator fun div(other: Amount): Amount =
+        // without RoundingMode a java.lang.ArithmeticException: Non-terminating decimal expansion; no exact representable decimal result. will be thrown
+        Amount(this.divide(other, DecimalPrecision, RoundingMode.HALF_UP).toString()) // 20 to match Big.js's behaviour
+
+
+    /*      why are these methods required when deriving from BigDecimal?      */
+
+    override fun toByte(): Byte {
+        1 + 1
+        return 0 // will never be called; where is this method coming from?
+    }
+
+    override fun toShort(): Short {
+        return 0 // will never be called; where is this method coming from?
+    }
+
+}
\ No newline at end of file
diff --git a/BankingClientModel/src/linuxMain/kotlin/net/codinux/banking/client/model/Amount.linux.kt b/BankingClientModel/src/linuxMain/kotlin/net/codinux/banking/client/model/Amount.linux.kt
new file mode 100644
index 00000000..0794807e
--- /dev/null
+++ b/BankingClientModel/src/linuxMain/kotlin/net/codinux/banking/client/model/Amount.linux.kt
@@ -0,0 +1,47 @@
+package net.codinux.banking.client.model
+
+import com.ionspin.kotlin.bignum.decimal.BigDecimal
+import com.ionspin.kotlin.bignum.decimal.DecimalMode
+import com.ionspin.kotlin.bignum.decimal.RoundingMode
+import net.codinux.banking.client.model.config.NoArgConstructor
+
+@NoArgConstructor
+actual class Amount actual constructor(amount: String) {
+
+    actual companion object {
+        actual val Zero: Amount = Amount("0.00")
+
+        private val decimalMode = DecimalMode(DecimalPrecision.toLong(), RoundingMode.ROUND_HALF_CEILING)
+    }
+
+
+    internal val amount: BigDecimal = BigDecimal.parseString(amount)
+
+    actual operator fun plus(other: Amount): Amount {
+        return Amount(amount.add(other.amount).toString())
+    }
+
+    actual operator fun minus(other: Amount): Amount {
+        return Amount(amount.subtract(other.amount).toString())
+    }
+
+    actual operator fun times(other: Amount): Amount {
+        return Amount(amount.multiply(other.amount).toString())
+    }
+
+    actual operator fun div(other: Amount): Amount {
+        return Amount(amount.divide(other.amount, decimalMode).toString())
+    }
+
+
+    override fun equals(other: Any?): Boolean {
+        return other is Amount && this.amount == other.amount
+    }
+
+    override fun hashCode(): Int {
+        return amount.hashCode()
+    }
+
+    actual override fun toString(): String = amount.toPlainString()
+
+}
\ No newline at end of file
diff --git a/BankingClientModel/src/mingwMain/kotlin/net/codinux/banking/client/model/Amount.mingw.kt b/BankingClientModel/src/mingwMain/kotlin/net/codinux/banking/client/model/Amount.mingw.kt
new file mode 100644
index 00000000..0794807e
--- /dev/null
+++ b/BankingClientModel/src/mingwMain/kotlin/net/codinux/banking/client/model/Amount.mingw.kt
@@ -0,0 +1,47 @@
+package net.codinux.banking.client.model
+
+import com.ionspin.kotlin.bignum.decimal.BigDecimal
+import com.ionspin.kotlin.bignum.decimal.DecimalMode
+import com.ionspin.kotlin.bignum.decimal.RoundingMode
+import net.codinux.banking.client.model.config.NoArgConstructor
+
+@NoArgConstructor
+actual class Amount actual constructor(amount: String) {
+
+    actual companion object {
+        actual val Zero: Amount = Amount("0.00")
+
+        private val decimalMode = DecimalMode(DecimalPrecision.toLong(), RoundingMode.ROUND_HALF_CEILING)
+    }
+
+
+    internal val amount: BigDecimal = BigDecimal.parseString(amount)
+
+    actual operator fun plus(other: Amount): Amount {
+        return Amount(amount.add(other.amount).toString())
+    }
+
+    actual operator fun minus(other: Amount): Amount {
+        return Amount(amount.subtract(other.amount).toString())
+    }
+
+    actual operator fun times(other: Amount): Amount {
+        return Amount(amount.multiply(other.amount).toString())
+    }
+
+    actual operator fun div(other: Amount): Amount {
+        return Amount(amount.divide(other.amount, decimalMode).toString())
+    }
+
+
+    override fun equals(other: Any?): Boolean {
+        return other is Amount && this.amount == other.amount
+    }
+
+    override fun hashCode(): Int {
+        return amount.hashCode()
+    }
+
+    actual override fun toString(): String = amount.toPlainString()
+
+}
\ No newline at end of file
diff --git a/BankingClientModel/src/wasmJsMain/kotlin/net/codinux/banking/client/model/Amount.wasmJs.kt b/BankingClientModel/src/wasmJsMain/kotlin/net/codinux/banking/client/model/Amount.wasmJs.kt
new file mode 100644
index 00000000..e26ba15f
--- /dev/null
+++ b/BankingClientModel/src/wasmJsMain/kotlin/net/codinux/banking/client/model/Amount.wasmJs.kt
@@ -0,0 +1,51 @@
+package net.codinux.banking.client.model
+
+import net.codinux.banking.client.model.config.NoArgConstructor
+
+@JsModule("big.js")
+@kotlin.js.JsNonModule
+open external class Big(value: String) {
+    fun plus(other: Big): Big
+    fun minus(other: Big): Big
+    fun times(other: Big): Big
+    fun div(other: Big): Big
+    override fun toString(): String
+}
+
+
+@NoArgConstructor
+actual class Amount actual constructor(amount: String): Big(amount) {
+
+    actual companion object {
+        actual val Zero = Amount("0.00")
+    }
+
+
+    actual operator fun plus(other: Amount): Amount {
+        return Amount(super.plus(other).toString())
+    }
+
+    actual operator fun minus(other: Amount): Amount {
+        return Amount(super.minus(other).toString())
+    }
+
+    actual operator fun times(other: Amount): Amount {
+        return Amount(super.times(other).toString())
+    }
+
+    actual operator fun div(other: Amount): Amount {
+        return Amount(super.div(other).toString())
+    }
+
+
+    override fun equals(other: Any?): Boolean {
+        return other is Amount && this.toString() == other.toString()
+    }
+
+    override fun hashCode(): Int {
+        return super.hashCode()
+    }
+
+    actual override fun toString(): String = super.toString()
+
+}
\ No newline at end of file
diff --git a/FinTs4jBankingClient/build.gradle.kts b/FinTs4jBankingClient/build.gradle.kts
index bb2597b8..cb571522 100644
--- a/FinTs4jBankingClient/build.gradle.kts
+++ b/FinTs4jBankingClient/build.gradle.kts
@@ -10,7 +10,7 @@ plugins {
 
 
 kotlin {
-    jvmToolchain(8)
+    jvmToolchain(11)
 
     jvm {
         withJava()
diff --git a/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kMapper.kt b/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kMapper.kt
index 753c3e81..cb169281 100644
--- a/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kMapper.kt
+++ b/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kMapper.kt
@@ -8,7 +8,6 @@ import net.codinux.banking.client.model.*
 import net.codinux.banking.client.model.AccountTransaction
 import net.codinux.banking.client.model.Amount
 import net.codinux.banking.client.model.extensions.EuropeBerlin
-import net.codinux.banking.client.model.extensions.toBigDecimal
 import net.codinux.banking.client.model.tan.*
 import net.codinux.banking.client.model.options.GetAccountDataOptions
 import net.codinux.banking.client.model.request.GetAccountDataRequest
@@ -238,38 +237,41 @@ open class FinTs4kMapper {
         holding.pricingTime ?: statementDate, holding.buyingDate
     )
 
-    private fun getTotalBalance(holding: Holding): Amount? {
+    // visible for testing
+    internal fun getTotalBalance(holding: Holding): Amount? {
         return if (holding.totalBalance != null) {
             mapNullableAmount(holding.totalBalance)
         } else if (holding.quantity != null && holding.marketValue != null) {
-            Amount(holding.quantity!! * mapAmount(holding.marketValue!!).toBigDecimal())
+            Amount((holding.quantity!! * holding.marketValue.toString().toDouble()).toString())
         } else {
             null
         }
     }
 
-    private fun getTotalCostPrice(holding: Holding): Amount? {
+    // visible for testing
+    internal fun getTotalCostPrice(holding: Holding): Amount? {
         return if (holding.totalCostPrice != null) {
             mapNullableAmount(holding.totalCostPrice)
         } else if (holding.quantity != null && holding.averageCostPrice != null) {
-            Amount(holding.quantity!! * mapAmount(holding.averageCostPrice!!).toBigDecimal())
+            Amount((holding.quantity!! * holding.averageCostPrice.toString().toDouble()).toString())
         } else {
             null
         }
     }
 
-    private fun calculatePerformance(holding: Holding): Float? {
+    // visible for testing
+    internal fun calculatePerformance(holding: Holding): Float? {
         val totalBalance = getTotalBalance(holding)
         val totalCostPrice = getTotalCostPrice(holding)
 
         if (totalBalance != null && totalCostPrice != null) {
-            return ((totalBalance.toBigDecimal() - totalCostPrice.toBigDecimal()) / totalCostPrice.toBigDecimal() * 100).toFloat()
+            return ((totalBalance - totalCostPrice) / totalCostPrice).toFloat() * 100
         }
 
         val marketValue = mapNullableAmount(holding.marketValue)
         val costPrice = mapNullableAmount(holding.averageCostPrice)
         if (marketValue != null && costPrice != null) {
-            return ((marketValue.toBigDecimal() - costPrice.toBigDecimal()) / costPrice.toBigDecimal() * 100).toFloat()
+            return ((marketValue - costPrice) / costPrice).toFloat() * 100
         }
 
         return null
@@ -277,11 +279,11 @@ open class FinTs4kMapper {
 
     protected open fun mapNullableMoney(amount: Money?) = amount?.let { mapMoney(it) }
 
-    protected open fun mapMoney(amount: Money) = Amount.fromString(amount.amount.string.replace(',', '.'))
+    protected open fun mapMoney(amount: Money) = Amount(amount.amount.string.replace(',', '.'))
 
     protected open fun mapNullableAmount(amount: net.codinux.banking.fints.model.Amount?) = amount?.let { mapAmount(it) }
 
-    protected open fun mapAmount(amount: net.codinux.banking.fints.model.Amount) = Amount.fromString(amount.string.replace(',', '.'))
+    protected open fun mapAmount(amount: net.codinux.banking.fints.model.Amount) = Amount(amount.string.replace(',', '.'))
 
 
     open fun mapTanChallenge(challenge: net.codinux.banking.fints.model.TanChallenge): TanChallenge {
@@ -388,7 +390,7 @@ open class FinTs4kMapper {
             mapError(response)
         }
 
-    open fun mapToMoney(amount: Amount, currency: String): Money = Money(amount.amount, currency)
+    open fun mapToMoney(amount: Amount, currency: String): Money = Money(amount.toString(), currency)
 
 
     protected open fun <T> mapError(response: net.dankito.banking.client.model.response.FinTsClientResponse): Response<T> {
diff --git a/FinTs4jBankingClient/src/commonTest/kotlin/net/codinux/banking/client/fints4k/FinTs4kMapperTest.kt b/FinTs4jBankingClient/src/commonTest/kotlin/net/codinux/banking/client/fints4k/FinTs4kMapperTest.kt
new file mode 100644
index 00000000..fc8f2594
--- /dev/null
+++ b/FinTs4jBankingClient/src/commonTest/kotlin/net/codinux/banking/client/fints4k/FinTs4kMapperTest.kt
@@ -0,0 +1,53 @@
+package net.codinux.banking.client.fints4k
+
+import net.codinux.banking.client.model.Amount
+import kotlin.test.Test
+import net.codinux.banking.fints.transactions.swift.model.Holding
+import kotlin.test.assertEquals
+
+class FinTs4kMapperTest {
+
+    private val underTest = FinTs4kMapper()
+
+
+    @Test
+    fun getTotalBalance_TotalBalanceIsNull_CalculateByQuantityAndMarketValue() {
+        val holding = Holding("", null, null, null, 4, null, null, null, fints4kAmount("13.33"))
+
+        val result = underTest.getTotalBalance(holding)
+
+        assertEquals(Amount("53.32"), result)
+    }
+
+    @Test
+    fun getTotalCostPrice_TotalCostPriceIsNull_CalculateByQuantityAndAverageCostPrice() {
+        val holding = Holding("", null, null, null, 47, fints4kAmount("16.828"), null)
+
+        val result = underTest.getTotalCostPrice(holding)
+
+        assertEquals(Amount("790.9159999999999"), result)
+    }
+
+    @Test
+    fun calculatePerformance_ByTotalBalanceAndTotalCostPrice() {
+        val holding = Holding("", null, null, null, null, null, fints4kAmount("20217.12"), null, null, null, fints4kAmount("19027.04"))
+
+        val result = underTest.calculatePerformance(holding)
+
+        assertEquals(6.2546773f, result!!, 0.000001f) // for JS the result has too many decimal places, so add a tolerance
+    }
+
+    @Test
+    fun calculatePerformance_ByMarketValueAndAverageCostPrice() {
+        val holding = Holding("", null, null, null, null, fints4kAmount("16.828"), null, null, fints4kAmount("16.75"), null, null)
+
+        val result = underTest.calculatePerformance(holding)
+
+        assertEquals(-0.4635132f, result!!, 0.0000001f) // for JS the result has too many decimal places, so add a tolerance
+    }
+
+
+    private fun fints4kAmount(amount: String) =
+        net.codinux.banking.fints.model.Amount(amount)
+
+}
\ No newline at end of file
diff --git a/gradle.properties b/gradle.properties
index 56958e4d..4e033d05 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -6,3 +6,6 @@ kotlinVersion=2.0.10
 kotlinxDateTimeVersion=0.5.0
 jsJodaTimeZoneVersion=2.3.0
 coroutinesVersion=1.8.1
+
+# 0.3.10 uses Kotlin 2.0.0
+ionspinBigNumVersion=0.3.9
diff --git a/kotlin-js-store/yarn.lock b/kotlin-js-store/yarn.lock
index 9e7a7758..d9072388 100644
--- a/kotlin-js-store/yarn.lock
+++ b/kotlin-js-store/yarn.lock
@@ -556,6 +556,11 @@ batch@0.6.1:
   resolved "https://registry.yarnpkg.com/batch/-/batch-0.6.1.tgz#dc34314f4e679318093fc760272525f94bf25c16"
   integrity sha512-x+VAiMRL6UPkx+kudNvxTl6hB2XNNCG2r+7wixVfIYwu/2HKRXimwQyaumLjMveWvT2Hkd/cAJw+QBMfJ/EKVw==
 
+big.js@6.0.3:
+  version "6.0.3"
+  resolved "https://registry.yarnpkg.com/big.js/-/big.js-6.0.3.tgz#8b4d99ac7023668e0e465d3f78c23b8ac29ad381"
+  integrity sha512-n6yn1FyVL1EW2DBAr4jlU/kObhRzmr+NNRESl65VIOT8WBJj/Kezpx2zFdhJUqYI6qrtTW7moCStYL5VxeVdPA==
+
 binary-extensions@^2.0.0:
   version "2.3.0"
   resolved "https://registry.yarnpkg.com/binary-extensions/-/binary-extensions-2.3.0.tgz#f6e14a97858d327252200242d4ccfe522c445522"