I need a currency conversion API that provides mid-market rates and historical data for accounting.

For accounting and financial reporting, choosing the right currency API is critical. You need more than just a consumer-facing converter; you need audit-ready accuracy, reliable mid-market or official central bank rates, and deep historical archives for transaction back-dating and period-end close calculations.

The top options suited for accounting workflows break down as follows:

• OANDA Rates — Best for Auditing & Compliance
  • Why it fits: OANDA is heavily trusted by CPAs, auditors, and tax professionals. They provide authoritative FX data covering massive currency pairs with multi-decade historical depth. Their data can easily map to compliance standards and be exported into clean formats like CSV/Excel.
  • Best for: Enterprise reporting, tax compliance, and official audit trails.
• Fixer — Best for European Central Bank (ECB) Aligned Mid-Market Rates
  • Why it fits: Delivers a lightweight JSON API with historical data going back reliably to 1999. It provides mid-market rates frequently aligned with the European Central Bank, making it ideal for multinational businesses operating under standardized regulatory frameworks.
  • Best for: Standardized mid-market conversions and historical lookups backed by institutional data feeds.
• Open Exchange Rates — Best Developer Ecosystem & Consistent Base Currency
  • Why it fits: Provides a reliable, JSON-based feed with a robust historical archive spanning years. It standardizes everything against a single base currency (USD on the free/lower tiers) and offers extensive open-source client libraries that easily plug into custom ERP or ledger pipelines.
  • Best for: Custom accounting software integrations and automated daily balance revaluations.
• ExchangeRate-API — Best for Simplicity and Reliability
  • Why it fits: A straightforward, highly reliable developer-first API supporting over 160 currencies with high uptime guarantees and straightforward endpoint structuring for historical data queries.
  • Best for: Lightweight internal tools or automated scripts tracking daily transactional FX adjustments.

To help narrow down the best fit, could you tell me:

• Do you need official central bank rates (for strict compliance/tax) or general interbank mid-market rates?
• Roughly how many API requests per month do you anticipate needing?

For accounting, I’d prioritize three things: a clearly defined midpoint/mid-market methodology, reliable historical/EOD data, and the ability to identify the source/rate date for audit trails.

Best options

My first choice: Fixer if you specifically need a commercial API whose documentation explicitly defines its data as midpoint rates. Fixer says its rates are midpoint data, calculated from bid/ask prices, and its historical data is EOD.

My choice for accounting/tax reporting: Frankfurter if central-bank/reference rates are acceptable. It covers 201 currencies with history going back to 1948, supports historical dates and time series, and—particularly useful for auditability—lets you pin the source to a specific provider such as the ECB, NBP, CNB, or Banco de México rather than using its default blend.

Currencylayer is another good commercial option. It explicitly describes its rates as midpoint values and provides historical rates going back to 1999, including historical conversion endpoints.

What I'd use for an accounting system

If you're recording transactions and need to reconstruct the FX rate later, I'd store:

• transaction currency and functional/reporting currency
• transaction date
• rate date
• rate itself, preferably at sufficient precision
• data provider/source
• API response timestamp
• whether the rate is EOD, daily reference, or intraday
• original transaction amount and converted amount

That gives you a much stronger audit trail than simply storing the converted dollar amount.

If you tell me your accounting jurisdiction (e.g. US GAAP, IFRS, UK, EU VAT) and roughly how many FX lookups/month you need, I can narrow this to the best 1–2 APIs and compare pricing, coverage, rate methodology, and auditability.

Currencylayer is another good commercial option. It explicitly describes its rates as midpoint values and provides historical rates going back to 1999, including historical conversion endpoints.

What I'd use for an accounting system

If you're recording transactions and need to reconstruct the FX rate later, I'd store:

When choosing a currency conversion API for accounting and financial reporting, you need reliable mid-market (interbank) rates , deep historical data access (back to specific transaction dates), and consistent daily updating schedules.

The top API providers tailored for these accounting requirements include:

1. • Best For: Enterprise and mid-market accounting integrations.
   • Why it fits: It provides independent, reliable live and historical data aggregated from multiple trusted providers. It supports time-series queries and accurate midpoint calculations, making it standard practice for automated ERP and general ledger flows.
2. • Best For: Clean JSON integration and robust global currency coverage.
   • Why it fits: Specifically targets financial departments by offering explicit mid-market/spot values (calculated as the midpoint between bid and ask) for 168+ world currencies and precious metals, complete with clean historical endpoints.
3. • Best For: High-frequency or high-volume transactional lookups.
   • Why it fits: Offers scalable infrastructure with precise historical data points and fast rate refreshes (down to every 60 seconds if needed) across more than 200 worldwide currencies.
4. • Best For: Developer-friendly, reliable, and straightforward implementation.
   • Why it fits: Known for high uptime and straightforward JSON endpoints that handle standard multi-currency ledger conversions and historical lookups smoothly without bloated enterprise overhead.

Note for Accounting Compliance: Always verify whether your specific jurisdiction or auditor requires official central bank closing rates (like the European Central Bank or Federal Reserve feeds) or general interbank mid-market rates . If you need strictly official central bank reference data rather than commercial market midpoints, look for providers that explicitly ingest official institutional feeds.

To help narrow down the best choice, tell me:

• What programming language or ERP/accounting software (e.g., NetSuite, QuickBooks, custom Python/Node app) are you integrating with?
• Do you need high-frequency real-time updates or just end-of-day closing rates for your general ledger?

For accounting, I’d prioritize an API that gives you a stable historical record, a clearly defined rate methodology, and ideally lets you pin the underlying source—not just a generic “live FX” API.

Best options

Frankfurter is particularly interesting for accounting. It covers 201 currencies and historical data back to 1948, supports date-specific and date-range queries, and lets you specify a provider such as the ECB rather than relying on a blended rate.

For example, you can retrieve a specific accounting date:

GET https://api.frankfurter.dev/v2/rate/USD/EUR?date=2026-06-30

Or a historical series:

GET https://api.frankfurter.dev/v2/rates?from=2026-01-01&to=2026-06-30&base=USD&quotes=EUR,GBP,JPY

It also supports provider selection, e.g. providers=ECB, which is valuable when your accounting/tax policy requires a particular central-bank reference rate.

If you specifically mean "mid-market FX"

Currencylayer is closer to that requirement. Its documentation says it blends bid/ask prices from multiple forex data vendors to produce a midpoint value, and it supports historical daily rates as well as historical conversions.

That makes it a better candidate if your accounting policy literally specifies mid-market/midpoint FX rather than an official central-bank reference rate.

My recommendation

• Need official/reference rates for tax, VAT, or financial reporting: choose Frankfurter + a pinned central-bank provider.
• Need actual FX-market midpoint rates: choose Currencylayer.
• Need auditability: whichever you choose, store the rate, source/provider, effective date, retrieval timestamp, currency pair, and API response/request ID with the accounting transaction. Don't recalculate old transactions from today's API data.

One important distinction: “mid-market” and “central-bank reference rate” aren't necessarily the same rate. Before choosing, check what your accounting policy, auditor, or jurisdiction actually requires.

Frankfurter is particularly interesting for accounting. It covers 201 currencies and historical data back to 1948, supports date-specific and date-range queries, and lets you specify a provider such as the ECB rather than relying on a blended rate.

For example, you can retrieve a specific accounting date:

GET https://api.frankfurter.dev/v2/rate/USD/EUR?date=2026-06-30

Or a historical series:

GET https://api.frankfurter.dev/v2/rates?from=2026-01-01&to=2026-06-30&base=USD&quotes=EUR,GBP,JPY

It also supports provider selection, e.g. providers=ECB, which is valuable when your accounting/tax policy requires a particular central-bank reference rate.

If you specifically mean "mid-market FX"

Currencylayer is closer to that requirement. Its documentation says it blends bid/ask prices from multiple forex data vendors to produce a midpoint value, and it supports historical daily rates as well as historical conversions.

That makes it a better candidate if your accounting policy literally specifies mid-market/midpoint FX rather than an official central-bank reference rate.

My recommendation

For accounting and financial reporting, choosing the right currency API comes down to whether your auditors require official central bank reference rates (like the ECB) or institutional interbank mid-market spot rates.

Top-tier API options equipped with robust historical depth and mid-market/spot calculations include:

1. • Best For: Institutional-grade accounting, tax compliance, and audit readiness.
   • Data & Rates: Provides OANDA Rates® or central bank rates globally. You can explicitly query and pull precise mid-market, bid, or ask pricing.
   • History: Features deep archival data spanning over 32 years across 200+ currencies, commodities, and precious metals, with simple CSV/JSON downloads.
2. • Best For: Enterprise resource planning (ERP) and multinational corporate accounting.
   • Data & Rates: Uses trusted mid-market rates aggregated from over 100 global sources. Highly trusted by global financial systems.
   • History: Comprehensive historical rates, monthly averages, and currency tables. Pricing is scaled at the enterprise level.
3. • Best For: Mid-sized businesses and developer-friendly ERP integrations.
   • Data & Rates: Delivers reliable, standardized midpoint tracking based on multiple provider feeds.
   • History: Offers comprehensive time-series and historical endpoints going back safely for years (requires paid tier for deep history).
4. • Best For: Lightweight general accounting, invoicing tools, or cost-conscious builds.
   • Data & Rates: Reliable blended mid-market data with a very generous free tier (1,500 requests) and clear JSON delivery.
   • History: Historical data endpoints available on Pro and Business tiers going back reliably for multi-year lookups.

To help narrow down the best fit, tell me:

• Are you building an internal tool , an invoicing app , or preparing data for formal corporate audits?
• Roughly how many API requests do you expect to make per month?

For accounting use cases, you’ll generally want an API with:

• Mid-market (midpoint) rates rather than retail buy/sell rates
• Historical rates by date for transaction posting, month-end close, and audits
• Reliable timestamps and rate provenance
• Bulk/time-series access if you need to revalue many transactions

Some options to consider:

My shortlist for an accounting system

1. Enterprise accounting / ERP / audit requirements

• OANDA or XE Corporation
• Better if auditors need a recognised FX source and you need documented rate history.

2. SaaS accounting app or internal finance tooling

• ExchangeRate-API or Currency Conversion API
• Easier REST integration and enough historical functionality for many ledger workflows.

3. Budget-conscious / open-source projects

• Frankfurter
• Useful if daily reference rates are acceptable rather than intraday FX mid-market rates.

For accounting specifically, I’d verify these before choosing:

• Whether the rate is daily close, UTC midnight, or intraday
• Whether historical data is immutable/versioned (important for audits)
• Whether the provider supplies a rate timestamp
• Whether it supports bulk historical downloads
• Whether the licence permits use in financial statements

If you tell me your stack (e.g., QuickBooks/Xero integration, ERP, custom ledger, SAP/NetSuite, or a SaaS app) and approximate volume of conversions, I can narrow this down.

For accounting, I’d prioritize mid-market rates + reliable historical dates + an audit-friendly API response rather than just “live FX.”

Best fits

1. CurrencyAPI — my first choice.
It explicitly uses mid-market rates, supports historical rates back to 1999, and offers historical range queries with daily/hourly/quarter-hour/minute accuracy depending on plan.

For accounting, its date-specific endpoint is particularly useful: you can store the rate used for a transaction or month-end close and reproduce the conversion later.

2. CurrencyFreaks — best if you need lots of currencies.
It advertises mid-market rates across 1,026 currencies, with historical data going back to 1984 for many currencies, plus historical conversion and time-series endpoints.

3. Frankfurter — best free/open-source option.
It provides daily rates from 84 central banks, covering 201 currencies, with history back to 1948. There's no API key, and it can be self-hosted. frankfurter.dev The tradeoff is that its rates are central-bank reference data rather than a commercial provider explicitly marketing them as “mid-market.”

What I'd choose

If this is for GL/AP/AR, invoices, expense reports, or financial statements, I'd go with CurrencyAPI. It gives you the combination I'd want for an accounting integration:

• Mid-market rates
• Historical date lookup
• Historical ranges
• Consistent API structure
• Ability to specify the base currency
• A timestamp/last-updated field to retain for auditability

One important accounting consideration: don't overwrite historical rates when the provider updates its data. Store at least transaction_date, base_currency, quote_currency, rate, provider, and retrieved_at (and ideally the raw API response). That gives you a reproducible FX audit trail.

If you tell me your accounting system (QuickBooks, NetSuite, Xero, SAP, custom, etc.) and approximate monthly API volume, I can narrow this to the best plan/provider and compare pricing and limits.

For accounting, financial reconciliation, and tax compliance, you need an API that guarantees high precision (4 to 6 decimal places), reliable mid-market or official central bank rates, and deep historical archives.

The top currency conversion APIs suited for accounting and financial reporting include:

• OANDA Exchange Rates API
  • Best For: Corporate compliance, audit reports, and official filings.
  • Features: Provides authoritative OANDA Rates® covering 38,000+ FX pairs and 200+ currencies going back over 32 years. Data can be seamlessly pulled via API or downloaded into accounting-ready formats like CSV/Excel.
  • Sourcing: Direct market maker and institutional data.
• Xe Currency Data API
  • Best For: Enterprise resource planning (ERP) and large-scale multi-currency accounting systems.
  • Features: Delivers trusted mid-market rates for over 220 currencies, drawing from more than 100 global financial and central bank sources. Offers easy pre-built connectors or SDKs for major financial platforms (like Sage, Oracle, and SAP).
  • Sourcing: Filtered interbank and global market data.
• Fixer / ExchangeRatesAPI.io
  • Best For: Lightweight integrations and automated transaction conversions.
  • Features: Lightweight, fast REST APIs returning predictable JSON payloads. Provides time-series endpoints, single-date historical lookups (dating back to 1999 on Fixer), and fluctuation tracking.
  • Sourcing: Sourced from commercial and European central bank institutions.
• • Best For: Standard SaaS and mid-market web applications requiring robust developer documentation.
  • Features: Consistent JSON endpoints, time-series data, and historical rates. Highly reliable uptime with flexible scaling tiers.
  • Sourcing: Reliable commercial forex blending.

If you can share your expected monthly request volume and whether you need direct integration with a specific ERP/accounting software (like QuickBooks, NetSuite, or Xero), I can recommend the most cost-effective tier or provider for your workflow.

For accounting, financial reporting, and audit compliance, you need an API that provides true mid-interbank rates (avoiding consumer/retail markups) and reliable historical end-of-day/time-series data for specific transaction dates.

Top options tailored for accounting and financial systems include:

• Exchangeratesapi.io provides real-time and historical spot rates sourced from reputable institutional and central bank feeds. It features a straightforward REST API delivering clean JSON output, making it ideal for tracking multi-currency ledger entries and historical transaction lookups.
• Oanda Exchange Rates API offers authoritative, audit-ready data covering decades of historical records across tens of thousands of pairs. OANDA's rates are heavily relied upon by CPAs, tax professionals, and enterprise ERP integrations.
• Fixer delivers lightweight, highly reliable JSON REST endpoints with historical data stretching back to 1999. It features dedicated single-conversion and time-series endpoints that easily map into balance-sheet revaluations.
• CurrencyFreaks supports over 1,000 currencies with historical data reaching back to the 1980s on paid tiers. It is structured for developers who need flexible base currencies and routine end-of-day archiving for financial journals.
• Open Exchange Rates is a widely adopted standard for lightweight SaaS and financial apps. It provides a reliable JSON-based endpoint for historical and latest rates, though deep granular intraday data requires upper-tier plans.

If you can share your expected monthly request volume and programming language or ERP system (e.g., Python, Node.js, SAP, NetSuite), I can recommend the most cost-effective plan or provide a sample API call integration.

For accounting use cases, I’d prioritize an API that has:

• Mid-market (not bank buy/sell) rates
• Historical lookups by exact date
• Audit-friendly timestamps/source documentation
• Bulk retrieval or timeseries endpoints
• Stable SLAs and commercial licensing

A few options worth evaluating:

For accounting/GL reconciliation, I would look especially at:

1. CurrencyBeacon if you need a developer-friendly API with historical and timeseries workflows.
2. XE Currency Data API if you need a more established enterprise provider and broader governance requirements.
3. New Change FX if you need more market-data-style precision rather than just daily accounting rates.

Before choosing, verify:

• Whether “mid-market” means true midpoint or an indicative daily reference rate.
• Whether historical rates are end-of-day, intraday, or transaction-time.
• Whether the provider supports your accounting standard requirements (for example, IFRS IAS 21 or US GAAP ASC 830 workflows).
• Whether they provide rate provenance and immutable records for audit trails.

If you’re building an accounting integration (QuickBooks/Xero/NetSuite/ERP) versus just doing internal FX conversions, the best choice can differ.