Free online calculators
Fast, accurate tools for finance, math, health, dates, and everyday life — each with the formula, assumptions, and a worked example in plain view.
Popular calculators
View all 25 →- Mortgage CalculatorEstimate the monthly payment, total interest, and total cost of a fixed-rate mortgage.
- Loan CalculatorCalculate the monthly payment and total interest for any fixed-rate installment loan.
- Compound Interest CalculatorSee how an investment grows with compounding, with optional regular contributions.
- Percentage CalculatorFind what a percentage of a number is, what percent one number is of another, and the percentage change between two values.
- Average CalculatorCalculate the arithmetic mean of a set of numbers, along with the count, sum, minimum, and maximum.
- BMI CalculatorCalculate body mass index from height and weight, in metric or imperial units, with the WHO classification.
- Age CalculatorCalculate an exact age in years, months, and days between a date of birth and any date.
- Date Difference CalculatorCount the days, weeks, months, and years between two calendar dates.
- Tip CalculatorWork out the tip, total bill, and per-person split for any check.
Browse by category
- FinanceMortgage, loan, interest, and savings calculations.5 calculators →
- MathPercentages, fractions, ratios, averages, and algebra.9 calculators →
- HealthBMI, BMR, and calorie estimates based on standard formulas.3 calculators →
- Time & DateAge, date differences, and business day calculations.4 calculators →
- EverydayTips, discounts, grades, fuel costs, and daily math.4 calculators →
- Unit conversionsLength, weight, temperature, area, volume, speed, data, and energy.8 converters →
How our calculators work
- 01
Enter your numbers
Plain-labeled inputs with sensible defaults. Invalid entries get a clear message, never a silent failure.
- 02
Get a clear result
The answer up front, plus the breakdown behind it — monthly payment, total interest, category, or next milestone.
- 03
See the method
Every page states the exact formula, defines each variable, and walks through a worked example.
- 04
Know the limits
Assumptions and limitations are listed beside every result, so you know exactly what an estimate does and doesn't cover.
Why use Free Online Calculator?
Because a number you can't check is a number you can't trust.
- Every calculator shows its formula, assumptions, and sources — nothing is a black box.
- Results are computed instantly in your browser; your numbers are never sent to a server.
- Content is written to explain, not just to answer: worked examples, FAQs, and plain-language guides.
- No accounts, no paywalls, no download gates — every tool is free and immediate.
Guides
All guides →- Math guideHow to Calculate a PercentageThe three basic percentage problems and the one idea that solves all of them.
- Math guideHow to Calculate Percentage Increase (and Decrease)The change formula, why the base matters, and the trap of reversing percentages.
- Math guidePercentage Difference vs. Percentage ChangeTwo similar-sounding measures that answer different questions — and how to choose correctly.
- Finance guideHow Compound Interest WorksWhy interest-on-interest changes everything about saving and borrowing over time.