Calculators
Math, finance, health, and everyday calculators — 24 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 math 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.
All calculators (24)
BMI Calculator
Work out your BMI, and the measurement current NICE guidance asks for alongside it: waist-to-height ratio, which…
Age Calculator
Your exact age in years, months and days plus every other unit, the weekday you were born, your next birthday, and…
Date Calculator
Find the days, weeks, weekdays and exact years/months/days between two dates, or add or subtract days, weeks…
Percentage Calculator
A percentage calculator with the working shown: percent of a number, percent change, percent difference, reverse…
Tip Calculator
A tip calculator that works on the pre-tax or post-tax amount and shows what the choice costs, splits any number…
Unit Converter
A unit converter across 12 categories and 90+ units that shows the factor used, and whether it is exact by…
Standard Deviation & Variance Calculator
A standard deviation calculator for sample and population, with the full working, relative standard deviation, and…
Correlation Coefficient Calculator
Paste paired X and Y data for Pearson r, r², Spearman ρ, the t statistic and two-tailed p-value, a strength rating…
Quadratic Equation Formula Solver
A quadratic equation solver for ax² + bx + c = 0: the discriminant step by step, real or complex roots, and the…
Fraction Arithmetic & Simplifier Calculator
Add, subtract, multiply and divide fractions, including mixed numbers and an optional third fraction chained left…
Mean, Median, Mode & Range Calculator
Get the mean, median, mode and range of any list, with honest multimodal and no-mode results other calculators…
Weighted Average Calculator
Calculate a weighted average from any list of values and weights (raw counts or percentages), plus a reverse…
Cooking Measurement Converter
A cooking measurement converter that accounts for ingredient density, using King Arthur Baking's weight chart, so…
Scientific Calculator
A free scientific calculator with sin, cos, tan, logs, roots, powers, factorials and degree/radian modes.
Concrete Calculator
Work out how much concrete you need for a slab, footing, post hole, column or stairs, in cubic yards, cubic feet…
Deck Calculator
Work out deck boards (with the gap and a ripped last row), joists at 12, 16 or 24 in on center, deck screws and…
Square Footage Calculator
Add up the square footage of rooms and odd shapes (rectangles, circles, triangles, trapezoids), subtract cut-outs…
Time Calculator
Add or subtract hours, minutes and seconds, add time to a clock time, or find the time between two times, with…
Area Calculator
Find the area of a rectangle, triangle, circle, trapezoid, ellipse, sector, polygon and more, with the formula…
Paint Calculator
Work out how many gallons of paint you need for a room: walls minus doors and windows, number of coats, ceiling…
Electricity Cost Calculator
Find how much your appliances cost to run per day, month and year. Add devices by watts and hours, use your own…
Ratio Calculator
Simplify ratios like 12:18 to 2:3 (decimals and three-part ratios too), solve A:B = C:D for the missing value, or…
Probability Calculator
Probability of A and B, A or B, neither, at least once in n tries, or exactly k successes in n trials.
Tile Calculator
Tiles and boxes for a floor or wall: several areas with cut-outs, any tile size and grout joint, waste by layout…
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: the answer comes from 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 math 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.