Everyday numbers, sorted in seconds.
JustCalcs is a set of small, focused calculators for the questions that come up at home, at work and on the road. Every page shows the formula and a worked example, so you can check the answer instead of trusting it.
Everything runs in your browser. No account, no upload, no stored data.
Example results
What a calculation looks like
- 10 cm in inches
- 3.937 in
- 10 km in 55:00
- 5:30 /km
- 20% off 49.90
- 39.92
1 inch = 2.54 cm, exactly
running pace per kilometre
you save 9.98
Everything in one place
Calculator sections
Pick the section that matches the task. Each calculator has a page of its own with the method, the limits of the result and links to the neighbouring tools.
Unit Converters
unit converter
- Time Unit Converter
- Temperature Converter
- Length Converter
- Weight Converter
- Scale Calculator
- Data Storage Converter
- Area Converter
- Power Converter
- Speed Converter
- Pressure Converter
- Volume Converter
Dates & Time
date calculators
- Work Hours Calculator
- Date Calculator
- Time Duration Calculator
- Age Calculator
- Days Between Dates Calculator
- Days Until Calculator
Everyday Money
personal finance calculators
- Discount Calculator
- Split Bill Calculator
- Margin and Markup Calculator
- Tip Calculator
- Unit Price Calculator
How it works
Three steps, no learning curve
The calculators are built to be read as much as used: the numbers you enter stay visible, and so does the arithmetic that turns them into a result.
01 · Enter what you know
Fields start with a sensible example so you can see the shape of the calculation before replacing the values with your own.
02 · Read the whole answer
Along with the result you get the formula, the units used and the intermediate values, rounded only at the point of display.
03 · Share the exact result
The inputs live in the address bar, so copying the link sends someone the calculation itself, not a screenshot of it.
Why these calculators
Small tools that stay out of the way
One task per page
No dashboard to configure and no unrelated fields competing for attention.
Exact conversion factors
Unit ratios are kept exact in the calculation; rounding happens only in the output.
Works on a phone
Full-width inputs, native pickers and no horizontal scrolling on a narrow screen.
Free and open
No sign-up wall, no trial period and no lead form between you and the answer.
Ready when you are
Find the calculator you need.
The catalogue lists every calculator on the site, grouped by the kind of question it answers.