diff --git a/localenv/mock-account-servicing-entity/app/routes/rates.ts b/localenv/mock-account-servicing-entity/app/routes/rates.ts
index 2e59b3e512..c841d4fdc2 100644
--- a/localenv/mock-account-servicing-entity/app/routes/rates.ts
+++ b/localenv/mock-account-servicing-entity/app/routes/rates.ts
@@ -1,15 +1,33 @@
 import type { LoaderArgs } from '@remix-run/node'
 import { json } from '@remix-run/node'
 
+interface Rates {
+  [currency: string]: { [currency: string]: number }
+}
+
+const exchangeRates: Rates = {
+  USD: {
+    EUR: 1.1602,
+    ZAR: 17.3792
+  },
+  EUR: {
+    USD: 0.8619,
+    ZAR: 20.44
+  },
+  ZAR: {
+    USD: 0.0575,
+    EUR: 0.0489
+  }
+}
+
 // eslint-disable-next-line @typescript-eslint/no-unused-vars
 export function loader({ request }: LoaderArgs) {
+  const base = new URL(request.url).searchParams.get('base') || 'USD'
+
   return json(
     {
-      base: 'USD',
-      rates: {
-        EUR: 1.1602,
-        ZAR: 17.3792
-      }
+      base,
+      rates: exchangeRates[base] ?? {}
     },
     { status: 200 }
   )
diff --git a/packages/backend/src/open_payments/payment/outgoing/service.test.ts b/packages/backend/src/open_payments/payment/outgoing/service.test.ts
index 612dd739e8..01c0b896b6 100644
--- a/packages/backend/src/open_payments/payment/outgoing/service.test.ts
+++ b/packages/backend/src/open_payments/payment/outgoing/service.test.ts
@@ -229,6 +229,7 @@ describe('OutgoingPaymentService', (): void => {
     Config.exchangeRatesUrl = 'https://test.rates'
     nock(Config.exchangeRatesUrl)
       .get('/')
+      .query(true)
       .reply(200, () => ({
         base: 'USD',
         rates: {
diff --git a/packages/backend/src/open_payments/quote/service.test.ts b/packages/backend/src/open_payments/quote/service.test.ts
index 0305d88162..d683e4e4bc 100644
--- a/packages/backend/src/open_payments/quote/service.test.ts
+++ b/packages/backend/src/open_payments/quote/service.test.ts
@@ -72,6 +72,7 @@ describe('QuoteService', (): void => {
     Config.signatureSecret = SIGNATURE_SECRET
     nock(Config.exchangeRatesUrl)
       .get('/')
+      .query(true)
       .reply(200, () => ({
         base: 'USD',
         rates: {
diff --git a/packages/backend/src/open_payments/quote/service.ts b/packages/backend/src/open_payments/quote/service.ts
index 772959841d..f1c5bf5e2a 100644
--- a/packages/backend/src/open_payments/quote/service.ts
+++ b/packages/backend/src/open_payments/quote/service.ts
@@ -221,9 +221,11 @@ export async function startQuote(
   deps: ServiceDependencies,
   options: StartQuoteOptions
 ): Promise<Pay.Quote> {
-  const rates = await deps.ratesService.rates().catch((_err: Error) => {
-    throw new Error('missing rates')
-  })
+  const rates = await deps.ratesService
+    .rates(options.receiver.assetCode)
+    .catch((_err: Error) => {
+      throw new Error('missing rates')
+    })
 
   const plugin = deps.makeIlpPlugin({
     sourceAccount: options.paymentPointer,
diff --git a/packages/backend/src/openapi/exchange-rates.yaml b/packages/backend/src/openapi/exchange-rates.yaml
index bae5cb5a70..a5bca19191 100644
--- a/packages/backend/src/openapi/exchange-rates.yaml
+++ b/packages/backend/src/openapi/exchange-rates.yaml
@@ -16,7 +16,14 @@ tags:
     description: Exchange rates
 paths:
   /:
-    parameters: []
+    parameters:
+      - schema:
+          type: string
+          minLength: 1
+        name: base
+        in: query
+        required: true
+        description: Base exchange rate Base exchange rate
     get:
       summary: Fetch exchange rates
       operationId: get-rates
diff --git a/packages/backend/src/rates/service.ts b/packages/backend/src/rates/service.ts
index ce98f266ac..04192ce7a6 100644
--- a/packages/backend/src/rates/service.ts
+++ b/packages/backend/src/rates/service.ts
@@ -5,7 +5,7 @@ import { convert, ConvertOptions } from './util'
 const REQUEST_TIMEOUT = 5_000 // millseconds
 
 export interface RatesService {
-  rates(): Promise<Rates>
+  rates(baseAssetCode: string): Promise<Rates>
   convert(
     opts: Omit<ConvertOptions, 'exchangeRate'>
   ): Promise<bigint | ConvertError>
@@ -59,7 +59,7 @@ class RatesServiceImpl implements RatesService {
     if (sameCode && sameScale) return opts.sourceAmount
     if (sameCode) return convert({ exchangeRate: 1.0, ...opts })
 
-    const rates = await this.sharedLoad()
+    const rates = await this.sharedLoad(opts.sourceAsset.code)
     const sourcePrice = rates[opts.sourceAsset.code]
     if (!sourcePrice) return ConvertError.MissingSourceAsset
     if (!isValidPrice(sourcePrice)) return ConvertError.InvalidSourcePrice
@@ -73,11 +73,11 @@ class RatesServiceImpl implements RatesService {
     return convert({ exchangeRate, ...opts })
   }
 
-  async rates(): Promise<Rates> {
-    return this.sharedLoad()
+  async rates(baseAssetCode: string): Promise<Rates> {
+    return this.sharedLoad(baseAssetCode)
   }
 
-  private sharedLoad(): Promise<Rates> {
+  private sharedLoad(baseAssetCode: string): Promise<Rates> {
     if (this.ratesRequest && this.ratesExpiry) {
       if (this.ratesExpiry < new Date()) {
         // Already expired: invalidate cached rates.
@@ -89,7 +89,7 @@ class RatesServiceImpl implements RatesService {
       ) {
         // Expiring soon: start prefetch.
         if (!this.prefetchRequest) {
-          this.prefetchRequest = this.loadNow().finally(() => {
+          this.prefetchRequest = this.loadNow(baseAssetCode).finally(() => {
             this.prefetchRequest = undefined
           })
         }
@@ -99,7 +99,7 @@ class RatesServiceImpl implements RatesService {
     if (!this.ratesRequest) {
       this.ratesRequest =
         this.prefetchRequest ||
-        this.loadNow().catch((err) => {
+        this.loadNow(baseAssetCode).catch((err) => {
           this.ratesRequest = undefined
           this.ratesExpiry = undefined
           throw err
@@ -108,14 +108,16 @@ class RatesServiceImpl implements RatesService {
     return this.ratesRequest
   }
 
-  private async loadNow(): Promise<Rates> {
+  private async loadNow(baseAssetCode: string): Promise<Rates> {
     const url = this.deps.exchangeRatesUrl
     if (!url) return {}
 
-    const res = await this.axios.get<RatesResponse>(url).catch((err) => {
-      this.deps.logger.warn({ err: err.message }, 'price request error')
-      throw err
-    })
+    const res = await this.axios
+      .get<RatesResponse>(url, { params: { base: baseAssetCode } })
+      .catch((err) => {
+        this.deps.logger.warn({ err: err.message }, 'price request error')
+        throw err
+      })
 
     const { base, rates } = res.data
     this.checkBaseAsset(base)
diff --git a/packages/backend/src/rates/util.test.ts b/packages/backend/src/rates/util.test.ts
index 807bb971b7..eb2526f074 100644
--- a/packages/backend/src/rates/util.test.ts
+++ b/packages/backend/src/rates/util.test.ts
@@ -1,60 +1,64 @@
 import { convert, Asset } from './util'
 
-describe('Rates util', function () {
-  describe('convert', function () {
-    it('converts same scales', () => {
-      // Rate > 1
-      expect(
-        convert({
-          exchangeRate: 1.5,
-          sourceAmount: 100n,
-          sourceAsset: asset(9),
-          destinationAsset: asset(9)
-        })
-      ).toBe(150n)
-      // Rate < 1
-      expect(
-        convert({
-          exchangeRate: 0.5,
-          sourceAmount: 100n,
-          sourceAsset: asset(9),
-          destinationAsset: asset(9)
-        })
-      ).toBe(50n)
-      // Round down
-      expect(
-        convert({
-          exchangeRate: 0.5,
-          sourceAmount: 101n,
-          sourceAsset: asset(9),
-          destinationAsset: asset(9)
-        })
-      ).toBe(50n)
+describe('Rates util', () => {
+  describe('convert', () => {
+    describe('convert same scales', () => {
+      test.each`
+        exchangeRate | sourceAmount | assetScale | expectedResult | description
+        ${1.5}       | ${100n}      | ${9}       | ${150n}        | ${'exchange rate above 1'}
+        ${1.1602}    | ${12345n}    | ${2}       | ${14323n}      | ${'exchange rate above 1 with rounding up'}
+        ${1.1602}    | ${10001n}    | ${2}       | ${11603n}      | ${'exchange rate above 1 with rounding down'}
+        ${0.5}       | ${100n}      | ${9}       | ${50n}         | ${'exchange rate below 1'}
+        ${0.5}       | ${101n}      | ${9}       | ${51n}         | ${'exchange rate below 1 with rounding up'}
+        ${0.8611}    | ${1000n}     | ${2}       | ${861n}        | ${'exchange rate below 1 with rounding down'}
+      `(
+        '$description',
+        async ({
+          exchangeRate,
+          sourceAmount,
+          assetScale,
+          expectedResult
+        }): Promise<void> => {
+          expect(
+            convert({
+              exchangeRate,
+              sourceAmount,
+              sourceAsset: createAsset(assetScale),
+              destinationAsset: createAsset(assetScale)
+            })
+          ).toBe(expectedResult)
+        }
+      )
     })
 
-    it('converts different scales', () => {
-      // Scale low → high
-      expect(
-        convert({
-          exchangeRate: 1.5,
-          sourceAmount: 100n,
-          sourceAsset: asset(9),
-          destinationAsset: asset(12)
-        })
-      ).toBe(150_000n)
-      // Scale high → low
-      expect(
-        convert({
-          exchangeRate: 1.5,
-          sourceAmount: 100_000n,
-          sourceAsset: asset(12),
-          destinationAsset: asset(9)
-        })
-      ).toBe(150n)
+    describe('convert different scales', () => {
+      test.each`
+        exchangeRate | sourceAmount | sourceAssetScale | destinationAssetScale | expectedResult | description
+        ${1.5}       | ${100n}      | ${9}             | ${12}                 | ${150_000n}    | ${'convert scale from low to high'}
+        ${1.5}       | ${100_000n}  | ${12}            | ${9}                  | ${150n}        | ${'convert scale from high to low'}
+      `(
+        '$description',
+        async ({
+          exchangeRate,
+          sourceAmount,
+          sourceAssetScale,
+          destinationAssetScale,
+          expectedResult
+        }): Promise<void> => {
+          expect(
+            convert({
+              exchangeRate,
+              sourceAmount,
+              sourceAsset: createAsset(sourceAssetScale),
+              destinationAsset: createAsset(destinationAssetScale)
+            })
+          ).toBe(expectedResult)
+        }
+      )
     })
   })
 })
 
-function asset(scale: number): Asset {
-  return { code: '[ignored]', scale }
+function createAsset(scale: number): Asset {
+  return { code: 'XYZ', scale }
 }
diff --git a/packages/backend/src/rates/util.ts b/packages/backend/src/rates/util.ts
index 9d49b70f1d..5cb911ce63 100644
--- a/packages/backend/src/rates/util.ts
+++ b/packages/backend/src/rates/util.ts
@@ -12,11 +12,8 @@ export interface Asset {
 }
 
 export function convert(opts: ConvertOptions): bigint {
-  const maxScale = Math.max(opts.sourceAsset.scale, opts.destinationAsset.scale)
-  const shiftUp = 10 ** maxScale
   const scaleDiff = opts.destinationAsset.scale - opts.sourceAsset.scale
   const scaledExchangeRate = opts.exchangeRate * 10 ** scaleDiff
-  return (
-    (opts.sourceAmount * BigInt(scaledExchangeRate * shiftUp)) / BigInt(shiftUp)
-  )
+
+  return BigInt(Math.round(Number(opts.sourceAmount) * scaledExchangeRate))
 }