Financial math behind one endpoint.
The engine behind FinCalc’s hosted calculators, as a typed REST API. Send inputs, get verified results, and build them into your own apps, digital banking, or advisor tools.
Free plan, no sales call required.
curl -X POST \ https://api.fincalc.com/v2/calculators/mortgage-payment \ -H "Authorization: Bearer $FINCALC_KEY" \ -H "Content-Type: application/json" \ -d '{ "loanAmount": 300000, "annualRate": 6.25, "termYears": 30 }'
const res = await fetch( "https://api.fincalc.com/v2/calculators/mortgage-payment", { method: "POST", headers: { Authorization: `Bearer ${process.env.FINCALC_KEY}`, "Content-Type": "application/json", }, body: JSON.stringify({ loanAmount: 300000, annualRate: 6.25, termYears: 30 }), } ); const result = await res.json();
// 200 OK { "monthlyPayment": 1847.15, "totalInterest": 364974.58, "schedule": [ { "month": 1, "principal": 284.65, "interest": 1562.50 }, // ...359 more rows ] }
Nobody hand-writes the math.
Every calculation starts as a workbook an analyst builds and checks. A compiler turns its formulas into code, and a test harness compares the result with decades of production output before anything ships.
- Stage 1
Analyst workbook
The calculation is authored and reviewed as spreadsheet formulas, like
=PMT(B3/12,B4*12,-B2)in cellB14. - Stage 2
Formula parsing
Each formula is parsed into a syntax tree, so the logic is read exactly as written.
- Stage 3
Code generation
The tree is compiled into Java for the API and JavaScript from the same source.
- Stage 4
Differential testing
Generated code runs case by case against the production service that has quoted real customers for over 30 years, with a pinned golden suite.
- Stage 5
Compatibility gate
A publish that would break an existing contract is blocked before it reaches the API.
The result: when a number changes, it changed in a reviewed workbook, not in a line of code someone typed.
Built to integrate cleanly.
A consistent contract across every calculator, documented in a developer portal.
OpenAPI 3.1 specification
Every calculator is a typed POST endpoint. Generate clients in your language and validate requests before you send them.
Inputs with display metadata
Each input carries its type, default, and a human-readable label, so you can build forms directly from the spec.
Schedules and tables
Calculators that produce amortization schedules, projections, or comparisons return them as structured rows.
Standard error responses
Errors follow RFC 9457 problem details, with the same shape across every endpoint.
Versioned paths
Endpoints live under /v2, and breaking changes are blocked from publishing.
Self-service keys
Sign up in the developer portal, create keys, and try endpoints without talking to sales.
API, hosted, or both.
The API and FinCalc’s hosted calculators share one engine, so results match wherever members see them.
| Compared | API | Hosted calculators |
|---|---|---|
| Best for | Your own apps, digital banking, and advisor platforms | Adding calculators to a website without development |
| You build | The interface and the experience | Nothing. Configure in the console and embed |
| Lead tools | Build your own | Capture, routing, appointment requests, attribution |
| Math | Same verified engine | |
Working with a bank or credit union’s marketing team? Point them to FinCalc for institutions.
From key to first result.
Four steps, most of them in the developer portal.
- Step 1
Create an account
Sign up in the developer portal and create an API key on the free plan.
- Step 2
Pick a calculator
Browse the reference or the calculator library and try requests in the portal.
- Step 3
Integrate
Generate a client from the OpenAPI spec, or call the endpoint directly.
- Step 4
Go to production
Move to Pro or Enterprise for higher limits and production use.
Plans and rate limits.
Every plan can call every published calculator. Plans differ in request rate, support, and terms.
| Plan | Rate limit |
|---|---|
| Free | 5 / min |
| Pro | 20 / min |
| Enterprise | 1,000+ / min |
What’s next.
We share direction so you can plan, and label anything that isn’t available yet.
- MCP serverRoadmap
Give AI agents access to FinCalc calculators as tools, with the same verified results as the API.
- AI assistance for institutionsRoadmap
Conversational help grounded in calculator results, designed for regulated financial institutions.
Developer questions.
How do you handle breaking changes?
Endpoints are versioned under /v2, and a compatibility gate blocks publishing any change that would break an existing contract.
Can I generate a client library?
Yes. The OpenAPI 3.1 specification works with standard generators for Java, TypeScript, Python, C#, and other languages.
What happens when I hit a rate limit?
The API returns HTTP 429 with a problem details body. Wait and retry, or move to a plan with a higher limit.
We use the CalcXML API today. What changes?
CalcXML v1.3 is frozen and keeps running through December 31, 2027. We’ll give you a map from each v1.3 calculator to its v2 endpoint, and our engineers will help with the switch. Details for API clients.
Where does the API run?
On AWS in U.S. regions, behind an API gateway. See Security and trust for data handling details.
Start with the free plan.
Create a key and make your first request in minutes.