42 calculators, and every one has an open API
Free online calculators, built for people and AI agents.
Fast, clean calculators for loans, percentages, BMI, dates and more. Each one is also a free JSON endpoint, so an AI agent can compute exactly what you see on the page. No sign-up, no key.
Browse by category
Financial
Loan, mortgage, compound interest, tip and sales tax calculators for everyday money decisions.
Math
Percentage, average, ratio, GCD and rounding calculators for quick number work.
Health
BMI, BMR, calorie and ideal weight calculators built on standard medical formulas.
Geometry
Area, perimeter, volume and surface area calculators for circles, rectangles and solids.
Time & Date
Age, days between dates and add days calculators for anything on a calendar.
Conversions
Length, weight and temperature converters covering metric and imperial units.
Popular calculators
Mortgage Calculator
Mortgage calculator with taxes and insurance. Estimate your monthly house payment, principal and interest, and total interest over the loan term.
Loan Calculator
Loan calculator for any amount, rate and term. See your monthly payment, total interest and total cost instantly, with the amortization math shown.
Percentage Calculator
Percentage calculator that finds what X percent of a number is, like 15% of 200, with the working shown. Quick answers for tips, tests and discounts.
Compound Interest Calculator
Compound interest calculator with monthly contributions. See the future value of your savings, total contributed and interest earned over any period.
BMI Calculator
BMI calculator for adults: enter height and weight in metric or imperial units to get your body mass index and weight category, with the formula shown.
Tip Calculator
Tip calculator that adds the gratuity and splits the bill. Enter the bill, tip percent and party size to see the tip, total and each person's share.
Age Calculator
Age calculator that shows exactly how old you are in years, months and days from your date of birth, plus your total age in months and days.
Percentage Change Calculator
Percentage change calculator: compare two numbers to get the percent increase or decrease and the raw difference, with the formula and steps shown.
An API built for agents
Every calculator is a JSON endpoint. An agent can discover the full list, read each input schema and run calculations without a key. There is an OpenAPI spec and an llms.txt as well.
curl "https://moltcalc.com/api/calc/loan?amount=20000&rate=7&years=5"
{
"slug": "loan",
"outputs": [
{ "key": "monthly_payment",
"value": 396.02,
"display": "$396.02" }
]
}