Compound Mint

Free Money Calculator API and MCP Server

Run the Compound Mint calculators from your own app or AI assistant. Free JSON API and MCP server, no key needed.

Educational content only. Not financial advice. Examples are hypothetical, not guaranteed.

The math behind the Compound Mint calculators is available as a free JSON API and as an MCP server, so apps, spreadsheets and AI assistants can run the same numbers you see on this site. There is no sign up and no key. Every answer includes the result, a short plain-English explanation, the assumptions behind it, a link to the matching Compound Mint page, one or two Compound Mint guides for further reading and the disclaimer above.

The easiest way in is /answer: send a money question in plain words, such as "how much will $500 a month grow in 20 years?" or "how long to pay off $5,000 at 24% paying $200 a month?", and it picks the right calculator, runs it and tells you what it understood. If a number is missing, it says which one.

Addresses

Calculators

Endpoint What it answers Links
/answer
MCP: answer_money_question		 A money question in plain words, routed to the matching calculator below.
Inputs: question		 Try it · Page
/compound-interest
MCP: calculate_compound_interest		 Future value of a starting amount plus monthly contributions.
Inputs: years; optional principal, monthly_contribution, annual_rate (default 8), compounding, contribution_timing, show_yearly		 Try it · Page
/rule-of-72
MCP: rule_of_72		 Years to double money at a rate, or the rate needed to double in a set time.
Inputs: annual_rate or years		 Try it · Page
/coast-fire
MCP: calculate_coast_fire		 FIRE number, Coast FIRE number, the gap and the age you reach it.
Inputs: current_age, retirement_age, annual_spending or fire_number; optional withdrawal_rate (4), annual_return (7, real), current_savings, monthly_contribution		 Try it · Page
/fire-number
MCP: calculate_fire_number		 Your FIRE number and how long until your savings reach it, so when you could retire.
Inputs: annual_spending or fire_number; optional current_savings, monthly_contribution, withdrawal_rate (4), annual_return (7, real), current_age		 Try it · Page
/401k-match
MCP: calculate_401k_match		 Your contribution, the employer match and any match left unclaimed, with 2026 IRS limits.
Inputs: salary, contribution_percent; optional match_percent (50), match_up_to_percent (6), tier2_match_percent, tier2_up_to_percent, age, years, annual_return		 Try it · Page
/debt-payoff
MCP: calculate_debt_payoff		 Payoff time and interest, the payment for a target date, or avalanche vs snowball for several debts.
Inputs: balance, apr and monthly_payment or target_months; or debts (JSON list) with extra_payment and strategy		 Try it · Page
/savings-goal
MCP: calculate_savings_goal		 Monthly savings needed to reach a target.
Inputs: target_amount, years; optional annual_rate (default 0), current_savings		 Try it · Page
/holiday-budget
MCP: calculate_holiday_budget		 A holiday budget: total for gifts, travel, food and decor, its share of monthly income, weeks of saving needed and a spend limit you can pay in cash.
Inputs: people, per_person_budget; optional travel, food, decor, monthly_income, already_saved, weekly_savings, weeks_left (default: weeks until December 25)		 Try it · Page

Rates are percents (send 7 for 7%) and money is in US dollars. Send inputs as query parameters with GET, or as a JSON body with POST. A missing or invalid input returns a 400 with the name of the parameter, what it accepts and a working example.

Example: compound interest

curl "https://compoundmint.com/wp-json/mint/v1/compound-interest?principal=10000&annual_rate=7&years=10&compounding=annually"

Answer (shortened):

{
  "calculator": "compound_interest",
  "result": {
    "final_balance": 19671.51,
    "total_contributed": 10000,
    "total_interest": 9671.51
  },
  "explanation": "$10,000.00 at 7% a year, compounded annually, grows to $19,671.51 in 10 years. $9,671.51 of that is interest.",
  "assumptions": ["A constant 7% yearly rate, compounded annually.", "..."],
  "source_page": "https://compoundmint.com/compound-interest-calculator/",
  "open_in_calculator": "https://compoundmint.com/compound-interest-calculator/?monthly=0&years=10&rate=7&start=10000",
  "disclaimer": "Educational content only. Not financial advice. Examples are hypothetical, not guaranteed."
}

Use it in an AI assistant (MCP)

In any assistant or app that supports remote MCP servers, add a custom connector or server with this URL and no authentication:

https://compoundmint.com/wp-json/mint/v1/mcp

The server answers initialize, tools/list, tools/call, prompts/list, prompts/get, resources/list and resources/read. Tools: answer_money_question (start here for a question in plain words), calculate_compound_interest, rule_of_72, calculate_fire_number, calculate_coast_fire, calculate_401k_match, calculate_debt_payoff, calculate_savings_goal, list_money_calculators, plus search and fetch for Compound Mint guides. Prompts cover common questions (how much will my money grow, when can I retire, how long to pay off my credit card, how much to save each month, am I getting my 401(k) match), and each calculator has a resource that explains its inputs and assumptions. A direct call looks like this:

curl -X POST https://compoundmint.com/wp-json/mint/v1/mcp 
  -H "Content-Type: application/json" 
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"rule_of_72","arguments":{"annual_rate":8}}}'

How the math works

  • Compound interest compounds monthly with deposits at the end of each month unless you choose otherwise, the same convention as the compound interest calculator.
  • Coast FIRE uses a real (after inflation) return, 7% by default, and the FIRE number is yearly spending divided by the withdrawal rate, as in the Coast FIRE calculator.
  • The FIRE number tool adds your monthly contribution at the end of each month and compounds monthly at the real return until the balance reaches the FIRE number, the same math as the FIRE number calculator.
  • The 401(k) calculator uses the 2026 IRS limits: $24,500 employee deferral, $8,000 catch-up at 50 and over, $11,250 at 60 to 63 where the plan allows, $72,000 overall and a $360,000 pay cap (IRS Notice 2025-67).
  • Debt payoff charges interest monthly at APR divided by 12 with no new charges or fees.
  • No calculator includes taxes, fees or inflation unless it says so. Real returns vary and can be negative.

Limits and fair use

The API is free and read only. Each IP address can make 500 calls a day (UTC); headers X-RateLimit-Limit and X-RateLimit-Remaining show where you stand. Please link the source_page when you show a result, and keep the disclaimer with it. We keep daily call counts, not IP addresses, so we can keep the service running; see the privacy policy.

Disclaimer

Educational content only. Not financial advice. Examples are hypothetical, not guaranteed. Results are estimates from the numbers you send and simple assumptions. Check current figures and talk to a licensed professional before you act. Read the full disclaimer.