Free online calculators that show their working
551 calculators for money, maths, health, home projects and HVAC. Each one gives you the answer, the formula behind it and a worked example you can check.
No sign-up · Runs in your browser · Every formula sourced and tested
Popular calculators
- Percentage CalculatorPercent of a number, percent change and 'X is what % of Y'.
- Mortgage Payment CalculatorPrincipal, interest, tax and insurance in one monthly figure.
- BMI CalculatorBody mass index with the healthy weight range for your height.
- Compound Interest CalculatorGrow a lump sum plus regular contributions at any compounding frequency.
- Loan Payment CalculatorMonthly payment, total interest and payoff total for any fixed-rate loan.
- GPA CalculatorWeighted GPA from grades and credit hours.
- Age CalculatorExact age in years, months, days and total time lived.
- TDEE CalculatorTotal daily energy expenditure and macro targets.
- Concrete CalculatorCubic yards and bags for slabs, footings and posts.
- Square Footage CalculatorRoom area for flooring, paint and materials.
- CFM CalculatorAirflow in cubic feet per minute from room size and air changes.
- Standard Deviation CalculatorSample and population spread, variance and standard error.
Browse by topic
Finance
Loans, mortgages, interest, savings and retirement — with amortization and the maths shown.
- Loan Payment Calculator
- Paycheck Calculator
- Savings Goal Calculator
- 50/30/20 Budget Calculator
- Discount Calculator
Math
Percentages, fractions, algebra, geometry and calculus, solved step by step.
- Logarithm Calculator
- Volume Calculator
- Riemann Sum Calculator
- Arithmetic Sequence Calculator
- Combination and Permutation Calculator
Health
BMI, calories, body measurements and pregnancy dates, with the formulas clinicians use.
- Calories Burned Calculator
- Macro Calculator
- BMR Calculator
- Calorie Deficit Calculator
- Daily Water Intake Calculator
Home & DIY
Materials and measurements for building, landscaping, flooring, paint and repairs.
- Electricity Cost Calculator
- Gravel Calculator
- Deck Calculator
- Flooring Calculator
- Gutter Size Calculator
HVAC & Airflow
CFM, duct sizing, BTU and ventilation rates for homes, workshops and commercial spaces.
- CFM Calculator
- ACH to CFM Calculator
- BTU to CFM Calculator
- Fresh Air CFM per Person Calculator
- Register Size Calculator
Business
Margins, pricing, payroll, break-even and the everyday numbers of running a business.
- Freelance Hourly Rate Calculator
- Food Cost Percentage Calculator
- Margin & Markup Calculator
- Markup Calculator
- Commission Calculator
Statistics
Averages, spread, probability and hypothesis tests with every step written out.
- Percentile Rank Calculator
- T Test Calculator
- Average Calculator
- Chi Square Calculator
- Correlation Coefficient Calculator
Fitness
Training paces, one-rep max, heart-rate zones and energy expenditure.
- Race Time Predictor
- Cycling FTP Calculator
- Plate Loading Calculator
- Steps to Distance Calculator
- Creatine Dosage Calculator
Education
Grades, GPA, test scores and study planning for students and teachers.
- GPA Calculator
- Attendance Percentage Calculator
- Test Grade Calculator
- Grade Curve Calculator
- Final Grade Calculator
Everyday
Dates and time, tips and bills, travel, cooking, pets and other everyday questions.
- Road Trip Cost Calculator
- Split Bill Calculator
- Aquarium Volume Calculator
- Carbon Footprint Calculator
- Coffee to Water Ratio Calculator
Science
Chemistry, weather and physical science calculations with units handled for you.
- Half-Life Calculator
- Combined Gas Law Calculator
- Ideal Gas Law Calculator
- Molar Mass Calculator
- Momentum Calculator
Engineering
Electrical, mechanical, structural and computing formulas.
- Duct Size Calculator
- Gear Ratio Calculator
- Subnet Calculator
- Battery Runtime Calculator
- LED Resistor Calculator
Physics
Motion, energy, forces and waves, worked through with units.
- Kinetic Energy Calculator
- Doppler Effect Calculator
- Sound Intensity Decibel Calculator
- Thin Lens Equation Calculator
- Torque Calculator
Real Estate
Rental yield, affordability, closing costs and property maths.
- Closing Cost Calculator
- Down Payment Calculator
- HELOC Calculator
- Real Estate Cash Flow Calculator
- 1031 Exchange Calculator
Investing
Returns, dividends, portfolio growth and investment comparison.
- Dollar Cost Averaging Calculator
- Portfolio Rebalancing Calculator
- Stock Average Calculator
- Capital Gains Tax Calculator
- Crypto Profit Calculator
Auto
Fuel economy, running costs, tyres, speed and vehicle maths.
- Fuel Cost Calculator
- Car Depreciation Calculator
- EV Charging Cost Calculator
- EV vs Gas Cost Calculator
- Speedometer Error Calculator
Conversions
Unit conversions with the exact factors shown.
- Download Time Calculator
- Video File Size Calculator
- Area Converter
- Aspect Ratio Calculator
- Base Converter
Why you can trust these numbers
Formulas from named sources
Each calculator uses a published method — a standard, textbook or official guidance — and lists it on the page.
Worked examples you can check
Examples on every page are produced by the calculator itself, so the numbers in the text always match the tool.
Reviewed and corrected
Pages are reviewed before publishing and re-checked when someone reports an error. Every page has a report link.