Best Accounting APIs for SaaS Platforms and Marketplaces (2026)
Seven options a platform team actually considers, sorted by what each one is for, with the limits and pricing models that decide the choice.
"Best accounting API" is an unanswerable question until you say what the API is for. The products that appear in this search are not competitors to each other. They fall into four categories that solve different problems, and picking from the wrong category is the most expensive mistake available here.
This guide sorts the realistic options by category, states what each is good at, and names the limits that decide the choice. Facts about other vendors come from their own documentation and pricing pages, linked throughout; anything we could not verify is marked as unconfirmed rather than guessed.
The four categories
| Category | What it gives you | Who it suits |
|---|---|---|
| Embedded accounting backend | Statutory double-entry books your product owns, with tax and documents | Platforms that must be the system of record |
| App-first accounting with an API | A mature accounting application, plus programmatic access to it | Products serving businesses that already keep books in that app |
| Ledger infrastructure | Fast, immutable balance tracking without statutory accounting | Wallets, in-app balances, payment products |
| Aggregator / unified API | One interface into whichever accounting app each customer uses | Tools that read or write their customers' books |
| Tax engine | Rates, calculation, sometimes filing | Anyone needing tax decisions, alongside books kept elsewhere |
Quick comparison
| Product | Category | Books you own | EU VAT depth | Pricing model |
|---|---|---|---|---|
| Nordlet | Embedded accounting backend | Yes | Per-country VAT, VIES, OSS/IOSS returns | Metered by request |
| Xero API | App-first accounting | Yes, in Xero | VAT returns per supported country | Per organisation, per month |
| QuickBooks Online API | App-first accounting | Yes, in QuickBooks | VAT by region; no standard IOSS procedure | Per company subscription |
| Odoo | ERP with RPC API | Yes, self-hosted or hosted | Localisation modules per country | Per user, API on Custom plan |
| Modern Treasury Ledgers | Ledger infrastructure | Balances, not statutory books | Not a tax product | Contract |
| Codat / Rutter / Merge | Aggregator | No, your customer's tool owns them | Inherited from the underlying app | Contract, usually per connection |
| Stripe Tax | Tax engine | No | Calculation and optional filing | Per calculation and transaction |
1. Nordlet — embedded double-entry accounting with EU VAT
Nordlet is an accounting backend a platform calls from its own code. It exposes an immutable double-entry ledger, invoicing, purchase records, bank reconciliation, inventory, payroll, and EU tax through a single REST interface, currently more than 400 endpoints across 29 modules generated from one OpenAPI specification.
What matters for platform work specifically: balanced postings are enforced by a database trigger at commit time rather than by application code, locked periods reject any posting dated inside them, document numbers are gapless and allocated concurrency-safely, and every mutation is recorded with actor, action, entity and the change itself. Retries are protected by an Idempotency-Key header that replays the stored response rather than creating a second document. Webhook events are written to a transactional outbox in the same database transaction as the change, so a rolled-back document never emits an event.
Pricing is metered: plans from €10 per month with 3,000 requests, €50 with 30,000, and €300 with 200,000, with additional requests billed per request. Every plan includes every module and the full API; companies, users, and sandbox companies are unlimited.
The limits to know: Nordlet is in early access and onboarding design partners, so production timelines are a real question to ask. EU VAT rate reference covers the EU-27 and OSS/IOSS returns are computed from your invoices, but domestic return packs ship for three countries today — Lithuania (FR0600), Germany (UStVA) and Poland (JPK_V7M) — payroll is Lithuanian only, and no return is submitted to a tax authority on your behalf.
Best for: marketplaces, SaaS platforms, and e-commerce products that need to own their books and their EU VAT evidence inside their own product.
2. Xero API — a mature app-first accounting API
Xero is an accounting application with a well-documented REST API, an app store, and a large accountant community. If your customers already keep their books in Xero, integrating with it meets them where they are.
For embedded use the constraints are structural rather than technical. Xero documents API limits of 60 calls per minute and 5,000 calls per day per connected organisation, 5 concurrent calls, and 10,000 calls per minute across an entire app. Uncertified apps may connect to at most 25 organisations; going beyond that requires App Partner certification, a review process with multiple checkpoints and commercial terms. Pricing is per organisation per month, published on Xero's pricing pages by country.
Best for: products that integrate with the books their customers already keep. Harder to use as the accounting backend for hundreds or thousands of tenants, because each tenant is a separate subscription and a separate connection against those limits.
3. QuickBooks Online API — the largest install base
QuickBooks Online has the widest small-business footprint in several markets, an OAuth 2.0 REST API, webhooks, and a large developer ecosystem. Its throttling is commonly documented at 500 requests per minute per company, with lower limits on batch and report endpoints; Intuit's own developer documentation is the authority to confirm current figures against.
The API is scoped to a company file, so a platform serving many businesses manages many connections rather than one multi-company account. On EU cross-border tax, Intuit's own support material notes there is no standard procedure for recording IOSS VAT and recommends asking a bookkeeper or accountant, which is worth knowing before you plan distance-selling flows around it.
Best for: reaching businesses that already use QuickBooks. Not designed to be the ledger inside someone else's platform.
4. Odoo — open-source ERP with a model-level API
Odoo covers accounting alongside inventory, manufacturing, CRM, and much else, can be self-hosted, and is genuinely extensible. Its external API is model-level rather than domain-level: you call ORM methods on models such as account.move and res.partner over XML-RPC or JSON-RPC. That is powerful and means you inherit responsibility for using the models correctly.
Two facts change the calculation for platform builders. Odoo's documentation states that external API access is available on the Custom pricing tier and not on the One App Free or Standard plans. And Odoo has announced that the /xmlrpc, /xmlrpc/2 and /jsonrpc endpoints are scheduled for removal in Odoo 22 and Odoo Online 21.1, with a newer JSON-2 API as the replacement — so an integration built today has a migration ahead of it. Pricing is per user per month, published on Odoo's pricing page.
Best for: companies that want one system across operations and finance, with in-house capacity to run and upgrade it. A fuller treatment is in Nordlet vs Odoo for platform builders.
5. Modern Treasury Ledgers — ledger infrastructure
Modern Treasury Ledgers provides high-throughput, immutable, double-entry balance tracking built for developers, aimed at wallets, in-app balances, and payment products. It is engineered for consistency and concurrency at volume.
It is not statutory accounting. There is no chart of accounts for your annual report, no VAT engine, no invoice numbering, no tax registers. Teams frequently need both: infrastructure for real-time balances, and an accounting system for the books that get filed.
Best for: real-time balances at high transaction rates, with statutory accounting handled separately.
6. Codat, Rutter and Merge — unified APIs into your customer's books
These products give you one interface that reads and writes across many accounting systems, handling authentication, field mapping, and syncing. Codat leans toward lending and banking use cases, Rutter toward commerce and payments data, and Merge offers a broad horizontal surface across many software categories.
The thing to be clear about: with an aggregator, the books are still your customer's, in your customer's tool. That is exactly right if you are building lending, spend management, or analytics. It is exactly wrong if your platform needs to be the system of record, because you inherit whatever each underlying app does and cannot enforce a consistent model across them.
Best for: reading or writing your customers' existing books at scale.
7. Stripe Tax — tax calculation, not accounting
Stripe Tax calculates and collects tax inside the Stripe stack and offers managed filing on higher tiers, with clear public pricing per calculation call and per transaction. Its transaction records are tax records, not general accounting journals.
Best for: businesses already on Stripe that need correct tax quickly, with books kept elsewhere.
Which to pick for which job
| Your job | Reach for |
|---|---|
| Be the system of record for your platform's own books | Embedded accounting backend |
| Sell to businesses that keep books in Xero or QuickBooks | That vendor's API, or an aggregator if you need several |
| Track wallet balances at thousands of transactions per second | Ledger infrastructure |
| Read many customers' books for lending or analytics | Aggregator |
| Get the right tax rate on a checkout | Tax engine |
| Run finance and operations in one system, in-house | ERP |
Most platforms end up with two of these, not one. The pairing worth planning for is an accounting backend that owns the books plus a tax engine or aggregator at the edges — and the decision to make first is which product owns the record, because everything else is arranged around it.
FAQ
What is the best accounting API for a marketplace?
For a marketplace that must own its books, an embedded accounting backend is the right category, because seller payables, commission revenue, refunds, and per-country VAT all have to be recorded in one place with an audit trail. App-first APIs such as Xero and QuickBooks are better suited to integrating with books your customers already keep than to acting as your ledger for hundreds of tenants.
Can I just use Stripe for marketplace accounting?
Stripe moves and splits money and can calculate tax. It does not keep double-entry books for your platform: commission earned, VAT owed on fees, seller liabilities between payment and payout, refunds, and reserves still need recording somewhere, and that record has to be reconciled against Stripe rather than assumed from it.
Is a unified accounting API a replacement for having my own ledger?
No. A unified API is a way into other people's ledgers. If your platform is the system of record, you need your own.
How much do accounting APIs cost?
The models differ more than the prices. Per-organisation and per-user pricing charges for the shape of the business; metered pricing charges for usage. Nordlet's published plans start at €10 per month with 3,000 requests included. Xero, QuickBooks and Odoo publish subscription prices per organisation, company, or user on their own sites, and those are the figures to budget against.
What should I test before committing to any of them?
Post a split transaction and check the journal. Send the same request twice and count the results. Try to edit a posted entry and to post into a closed period. Export a month of data and see whether an accountant can read it. Those four tests separate the categories faster than any feature list.