🧮 Calculators
Math, finance, health, and everyday calculators — 27 free tools available. No signup required.
This section collects the calculations people look up rather than memorize — percentages and fractions, the statistics used in coursework, the trigonometry that turns up in a DIY measurement, and the kitchen and photography maths that has awkward units baked into it.
They run entirely in your browser. Nothing you type is sent anywhere, there is no account, and each one shows the formula it used rather than just an answer, so you can check the working or reproduce it by hand.
If you are not sure which one you need, the difference is usually in what you already know. A percentage problem with two knowns and one unknown belongs in the percentage calculator; a problem where the unknown is inside an equation belongs in the quadratic solver or the logarithm calculator.
Where to start
- Percentage Calculator — Handles all three directions — what percent of, percent change, and reverse percentage — which is where most manual attempts go wrong.
- Unit Converter — Length, mass, volume, temperature, area and speed in one place, with exact conversion factors rather than rounded ones.
- Standard Deviation & Variance Calculator — Distinguishes sample from population, which is the single most common error in coursework statistics.
- Fraction Arithmetic & Simplifier Calculator — Works in exact fractions rather than converting to decimals, so results stay precise.
- Recipe Portion & Serving Size Scaler — Scales ingredients properly, including the ones that do not scale linearly.
- Golden Hour & Blue Hour Photography Lighting Calculator — Computes actual solar position for a date and location instead of quoting a rule of thumb.
All calculators (27)
BMI Calculator
Work out your BMI, and the measurement current NICE guidance asks for alongside it: waist-to-height ...
Age Calculator
Your exact age in years, months and days plus every other unit, the weekday you were born, your next...
Percentage Calculator
A percentage calculator with the working shown: percent of a number, percent change, percent differe...
Tip Calculator
A tip calculator that works on the pre-tax or post-tax amount and shows what the choice costs, split...
Unit Converter
A unit converter across 12 categories and 90+ units that shows the factor used — and whether it is e...
Standard Deviation & Variance Calculator
A standard deviation calculator for sample and population, with the full working, relative standard ...
Standard Normal Z-Score & Percentile Calculator
A z score calculator both ways — score to percentile and back — with the step most skip: for a sampl...
Scientific & Engineering Notation Converter
A scientific notation converter for decimal, E-notation and engineering form without losing digits —...
Quadratic Equation Formula Solver
A quadratic equation solver for ax² + bx + c = 0: the discriminant step by step, real or complex roo...
Right Triangle Trigonometry & Pythagorean Calculator
A right triangle trig calculator solving from any two values: sides, angles, area, altitude, inradiu...
Circle Area, Circumference & Diameter Calculator
Solve a circle from radius, diameter, circumference or area — plus arc length, sector, chord and seg...
Fraction Arithmetic & Simplifier Calculator
Add, subtract, multiply and divide fractions — including mixed numbers and a third fraction, which t...
Significant Figures Counter & Rounding Calculator
Count significant figures and round correctly — including the ambiguous cases like 100 and 1000, and...
Matrix Determinant Calculator (2×2 to 5×5)
Determinants for 2×2, 3×3, 4×4 and 5×5 matrices with the cofactor expansion shown — exact for intege...
Logarithm & Natural Log (ln) Calculator
Solve for any part of log₆(x) = y — the logarithm, the antilog or the base — with the change-of-base...
Prime Number Tester & Factorization Generator
Prime factors, divisor count and primality — plus the square root in simplest radical form, which no...
Permutations (nPr) & Combinations (nCr) Calculator
nPr and nCr with and without repetition — all four cases, in exact arbitrary precision, where most c...
Recipe Portion & Serving Size Scaler
Scale recipe ingredient quantities up or down automatically by serving size or party count. Handles ...
Baking Yeast Ratio Converter (Instant, Active & Fresh)
Convert between Instant Dry Yeast (IDY), Active Dry Yeast (ADY), and Fresh Compressed Cake Yeast wit...
Specialty Coffee Brew Ratio Calculator
Calculate golden coffee-to-water brewing ratios (1:15 to 1:18) for Pour Over (V60/Chemex), French Pr...
Meat & Poultry Roasting Temperature Timer
Calculate oven roasting times, temperatures, and USDA-recommended internal meat thermometer temperat...
Sourdough Hydration & Baker’s Percentage Calculator
Calculate artisan sourdough dough hydration percentages, main flour, water, 100% hydration starter/l...
Boiled Egg Cooking Timer (Soft, Jammy & Hard)
Exact boiling times for soft, jammy and hard-boiled eggs. Adjusts for egg size, fridge-cold or room-...
Airline Luggage Weight (KG to LBS) & Dimension Converter
Convert luggage weights between kilograms (kg) and pounds (lbs). Checks standard airline baggage all...
Camera Sensor Crop Factor & Focal Length Equivalent
Calculate 35mm full-frame equivalent focal length (mm), field of view classification, and equivalent...
Photography Exposure Value (EV) & Lux Calculator
Calculate photography Exposure Value (EV100), ambient scene illuminance in Lux, and aperture/shutter...
Golden Hour & Blue Hour Photography Lighting Calculator
Calculate morning and evening Golden Hour and Blue Hour natural light windows from local sunrise and...
Percentages are three different problems
Almost every percentage mistake comes from solving the wrong one of three. "What is 15% of 80" is a straightforward multiplication. "80 is what percent of 200" divides first. "A price rose 20% to £120, what was it before" is the reverse case, and it is the one people get wrong most often — dividing by 1.2 rather than subtracting 20%, because a 20% rise followed by a 20% fall does not return you to the start. A percentage increase and the decrease that undoes it are never the same number, and any tool that treats them as such is wrong.
Sample or population changes the answer
Standard deviation and variance have two formulas that differ only in whether you divide by n or by n − 1. The n − 1 version, Bessel's correction, applies when your numbers are a sample drawn from a larger group and you are estimating that group's spread. Dividing by n applies when your numbers are the entire group. For small datasets the gap is substantial — with ten values the two differ by about 5% — and reporting the wrong one is a routine source of lost marks. Our calculator returns both and labels which is which.
Kitchen and camera maths has non-obvious units
Several calculators here exist because the arithmetic is easy and the units are not. Baker's percentages express every ingredient as a share of flour weight, not of the total, so a 75% hydration dough is not 75% water. Camera crop factor multiplies focal length but leaves the aperture's light-gathering unchanged while altering depth of field. Exposure value combines aperture, shutter and ISO into a single number where each step is a doubling. In each case the formula is trivial once you know which quantity is the denominator, which is exactly what these pages spell out.
Frequently asked questions
Are these calculators free to use?
Yes, all of them, with no account, no signup and no usage limit. They are supported by advertising.
Is my data sent to a server?
No. Every calculator in this category runs in your browser using JavaScript. The numbers you enter never leave your device.
Can I see the formula behind a result?
Yes. Every calculator page shows the formula it applies, defines each symbol, and works through a numeric example so you can verify the result independently.
Do these work on a phone?
Yes. Every tool is responsive and works in any modern mobile browser with no app to install.
Can I use these for schoolwork?
They are built to be checkable — the formula and worked example are on every page, which is more useful for learning than an answer alone. Check your institution's rules on calculator use for assessed work.