MegaTools · Independent · Free · No account
Calculators that show their work.
75 calculators across 8 subjects. Each one gives you the answer, the formula behind it, and an honest account of when that answer stops being reliable.
- Calculators
- 75
- Categories
- 8
- Sources per tool
- 2+
- Cost
- £0
- Last updated
- 12 Aug 2026
Every formula on this site traces to the body that defines it. Where a method is contested, the page says so. Editorial policy →
The index
Start with the subject
Every section opens with what its tools assume before you use them, and where those assumptions stop holding.
- 01Finance10 toolsFree finance calculators for mortgages, loans, compound interest, salary, and tax. Each one shows the formula, a worked example, and what it leaves out.
- 02Health10 toolsFree health calculators for BMI, BMR, TDEE, body fat, and calorie needs. Formulas cited to WHO, NIH and peer-reviewed sources, with limitations stated.
- 03Math9 toolsFree math calculators for percentages, fractions, averages, ratios, and standard deviation. Step-by-step working shown for every result.
- 04Conversion10 toolsFree unit converters for length, weight, temperature, area, speed, and data. Exact conversion factors, not rounded approximations.
- 05Date & Time9 toolsFree date and time calculators for age, date differences, and time zone conversion. Handles leap years and daylight saving correctly.
- 06Developer10 toolsFree developer utilities for password generation, JSON formatting, case conversion, and encoding. Everything runs in your browser, nothing is uploaded.
- 07Writing & SEO9 toolsFree tools for writers and SEOs: word counter, character counter, readability scoring, slug generation, and domain age lookup. No sign-up, no limits.
- 08Lifestyle8 toolsFree everyday calculators for tipping, splitting bills, and fuel costs. Quick answers with the arithmetic shown.
What we promise
Four things, on every page
None of these is a claim about this site you have to take on trust. Open any calculator and check.
- 01
The formula is on the page
Every calculator shows the equation it runs and works through a real example with real numbers, so you can check the result by hand if it matters.
- 02
Sources you can follow
Reference values are cited to the bodies that publish them — WHO, NIH, NICE, HMRC, the IETF — with links you can open and read.
- 03
Nothing leaves your browser
Calculations run on your device. Weights, salaries and everything else you type stay on your machine. No account, no upload, no analytics on your inputs.
- 04
We say where it breaks
Every tool has a section on when its answer stops being reliable. A number without its caveats is worse than no number at all.
How it works
Three steps to an answer you can check
- 01
Find it
Search from any page with the / key, or read down the index below. Nothing is more than two clicks from here.
- 02
Type your numbers
Results update as you type. There is no submit button and nothing is sent anywhere. Units switch between metric and US where it matters.
- 03
Check the working
Under every result sits the formula, a worked example, the limits of the method, and links to the sources the constants came from.
Get started
Find the calculator you need.
75 tools across 8 categories, each with its formula, a worked example, and the sources behind it. More are being written.