Best Free Finance API for Developers in 2026

You need a finance API. You do not need to build the math yourself.

Mortgage formulas, amortization schedules, compound interest, loan payoff projections: these are solved problems. Implementing them from scratch means debugging floating point edge cases, handling negative amortization, and maintaining tax bracket tables. A good API handles this and returns numbers you can trust.

Here is what to look for, and how ToolKnife's finance API compares.

What a finance API should give you

At minimum:

• Mortgage calculator with amortization schedule
• Loan calculator with monthly payments and total interest
• Compound interest with configurable compounding periods
• ROI and CAGR for investment analysis
• Currency conversion with live exchange rates
• Tax calculations for multiple jurisdictions

Most free finance APIs give you one of these. ToolKnife gives you all of them through a single endpoint.

Quick start

Every finance tool on ToolKnife is available through the same REST API pattern:

curl -X POST https://toolknife.com/api/v1/tools/mortgage-calculator_calculate-mortgage \
  -H "Content-Type: application/json" \
  -d '{"principal": 400000, "annualRate": 6.5, "years": 30, "downPayment": 80000}'

{
  "monthlyPayment": 2023.65,
  "totalPayment": 728515.57,
  "totalInterest": 408515.57,
  "loanAmount": 320000
}

The API is free at 200 calls/day. No API key needed for basic usage.

Python example

import requests

def calculate_mortgage(principal, rate, years, down_payment=0):
    resp = requests.post(
        "https://toolknife.com/api/v1/tools/mortgage-calculator_calculate-mortgage",
        json={
            "principal": principal,
            "annualRate": rate,
            "years": years,
            "downPayment": down_payment,
        },
    )
    return resp.json()

result = calculate_mortgage(500000, 6.5, 30, 100000)
print(f"Monthly payment: ${result['monthlyPayment']:,.2f}")

JavaScript example

const response = await fetch(
  "https://toolknife.com/api/v1/tools/compound-interest-calculator_calculate-compound-interest",
  {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({
      principal: 10000,
      rate: 7,
      years: 20,
      compoundingFrequency: 12,
      monthlyContribution: 500,
    }),
  }
);

const data = await response.json();
console.log(`Future value: $${data.futureValue.toLocaleString()}`);

Available finance endpoints

Full list at toolknife.com/docs.

For AI agents: MCP

If you are building with Claude, GPT, or other LLM agents, ToolKnife also works as an MCP server. The agent calls finance functions directly:

{
  "mcpServers": {
    "toolknife": {
      "type": "streamable-http",
      "url": "https://toolknife.com/api/mcp"
    }
  }
}

No wrapper code needed. The agent discovers available tools and calls them with structured parameters.

Pricing

The free tier is enough for prototyping, side projects, and low-traffic apps. Pro covers most production apps.

Browse the full API documentation or try the finance tools in the browser first.