🧮 Calculators
Math, finance, health, and everyday calculators — 72 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.
- 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 (72)
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…
Frequency Unit Converter
A frequency converter across Hz, kHz, MHz, GHz, RPM and angular frequency, plus the period — all from one value…
Standard Deviation & Variance Calculator
A standard deviation calculator for sample and population, with the full working, relative standard deviation, and…
Standard Normal Z-Score & Percentile Calculator
A z score calculator both ways — score to percentile and back — with the step most skip: for a sample mean the…
Scientific & Engineering Notation Converter
A scientific notation converter for decimal, E-notation and engineering form without losing digits — and a warning…
Quadratic Equation Formula Solver
A quadratic equation solver for ax² + bx + c = 0: the discriminant step by step, real or complex roots, and the…
Right Triangle Trigonometry & Pythagorean Calculator
A right triangle trig calculator solving from any two values: sides, angles, area, altitude, inradius and…
Circle Area, Circumference & Diameter Calculator
Solve a circle from radius, diameter, circumference or area — plus arc length, sector, chord and segment, which…
Fraction Arithmetic & Simplifier Calculator
Add, subtract, multiply and divide fractions — including mixed numbers and a third fraction, which the top-ranking…
Significant Figures Counter & Rounding Calculator
Count significant figures and round correctly — including the ambiguous cases like 100 and 1000, and the…
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 integer inputs, so a…
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 working…
Prime Number Tester & Factorization Generator
Prime factors, divisor count and primality — plus the square root in simplest radical form, which no other…
Permutations (nPr) & Combinations (nCr) Calculator
nPr and nCr with and without repetition — all four cases, in exact arbitrary precision, where most calculators…
Recipe Portion & Serving Size Scaler
Scale recipe ingredient quantities up or down automatically by serving size or party count.
Baking Yeast Ratio Converter (Instant, Active & Fresh)
Convert between Instant Dry Yeast (IDY), Active Dry Yeast (ADY), and Fresh Compressed Cake Yeast with precision…
Specialty Coffee Brew Ratio Calculator
Calculate golden coffee-to-water brewing ratios (1:15 to 1:18) for Pour Over (V60/Chemex), French Press…
Meat & Poultry Roasting Temperature Timer
Calculate oven roasting times, temperatures, and USDA-recommended internal meat thermometer temperatures for Beef…
Sourdough Hydration & Baker’s Percentage Calculator
Calculate artisan sourdough dough hydration percentages, main flour, water, 100% hydration starter/levain, and…
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-temperature…
Airline Luggage Weight (KG to LBS) & Dimension Converter
Convert luggage weights between kilograms (kg) and pounds (lbs). Checks standard airline baggage allowances (23kg…
Camera Sensor Crop Factor & Focal Length Equivalent
Calculate 35mm full-frame equivalent focal length (mm), field of view classification, and equivalent depth of…
Photography Exposure Value (EV) & Lux Calculator
Calculate photography Exposure Value (EV100), ambient scene illuminance in Lux, and aperture/shutter/ISO exposure…
Golden Hour & Blue Hour Photography Lighting Calculator
Calculate morning and evening Golden Hour and Blue Hour natural light windows from local sunrise and sunset times…
Mean, Median, Mode & Range Calculator
Enter any list of numbers to get the mean, median, mode and range — including honest multimodal and no-mode…
Weighted Average Calculator
Calculate a weighted average from any list of values and weights — raw counts or percentages — plus a reverse…
GCF & LCM Calculator
Find the greatest common factor and least common multiple of 2 or more numbers, with the full Euclidean algorithm…
Mulch Calculator
A mulch calculator for rectangle, circle, or triangle beds that converts area and depth into cubic yards, cubic…
Cooking Measurement Converter
A cooking measurement converter that accounts for ingredient density, using King Arthur Baking's weight chart, so…
Button Size Calculator
A sewing button size calculator that converts between ligne (L), millimeters, and inches, with typical garment use…
Roofing Calculator
A roofing calculator that converts building footprint and roof pitch into true roof area, squares, shingle…
Leap Year Calculator
A leap year calculator that checks any year against the Gregorian calendar rule and lists every leap year in a…
Random Number Generator
Pick random numbers in any range: one or thousands at once, with or without repeats, sorted or in draw order.
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…
Square Footage Calculator
Add up the square footage of rooms and odd shapes — rectangles, circles, triangles, trapezoids — subtract…
Long Division Calculator
Divide any two numbers and see the full written long division, step by step — with remainders, decimal answers…
Slope Calculator
Find the slope of a line from two points or rise and run: slope as a fraction, angle, percent grade, ratio, y = mx…
Time Calculator
Add or subtract hours, minutes and seconds, add time to a clock time, or find the time between two times — with…
Dog Age Calculator
Convert your dog’s age to human years with the 2020 epigenetic-clock formula and the veterinary rule of thumb…
Square Root Calculator
Find the square root, cube root or any nth root of a number, simplify radicals like √72 = 6√2, handle negatives…
Decimal to Fraction Calculator
Convert any decimal to a fraction in simplest form with steps — including repeating decimals, mixed numbers and…
Tire Size Calculator
Enter a tire size like 225/45R17 for diameter, sidewall and revs per mile, then compare a new size to see…
Area Calculator
Find the area of a rectangle, triangle, circle, trapezoid, ellipse, sector, polygon and more, with the formula…
Gravel Calculator
Work out how much gravel or crushed stone you need in cubic yards and tons, plus bags and cost, for a rectangle…
Cubic Yard Calculator
Calculate cubic yards for soil, mulch, gravel or fill. Add several rectangles, circles or triangles with their own…
Golf Handicap Calculator
Estimate your World Handicap System Handicap Index from recent scores, course rating and slope, then get your…
BTU Calculator
Find the right air conditioner size in BTU for any room from its square feet, sun exposure, people and kitchen…
Paint Calculator
Work out how many gallons of paint you need for a room: walls minus doors and windows, number of coats, ceiling…
Sample Size Calculator
Find the sample size you need for a survey from confidence level, margin of error and population, or the margin of…
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…
Cat Age Calculator
Convert your cat’s age to human years with the veterinary chart (15, then 24, then +4 a year) and see its life…
Exponent Calculator
Raise any number to any power: exact results for whole-number exponents (2^100 in full), negative exponents as…
Rounding Calculator
Round numbers to decimal places, the nearest 0.25, 5, 10 or 100, or significant figures — half up, banker’s…
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…
Midpoint Calculator
Find the midpoint of two points in 2D or 3D, a missing endpoint, or the point dividing a segment in a ratio — with…
Volume Calculator
Volume of a cube, box, cylinder, cone, sphere, pyramid, prism, capsule, tube and more, with the formula shown and…
Probability Calculator
Probability of A and B, A or B, neither, at least once in n tries, or exactly k successes in n trials.
Watts to Amps Calculator
Convert watts to amps and amps to watts for DC, AC single-phase and three-phase circuits, with power factor, kVA…
Voltage Drop Calculator
Voltage drop for copper or aluminum wire by AWG or kcmil, current and length — single phase, three phase or DC…
Stair Calculator
Plan stairs from the total rise: number of risers and treads, riser height, total run, stringer length and angle…
P-Value Calculator
Find the p-value from a z, t, chi-square or F statistic, or a correlation r — one- or two-tailed, with critical…
Confidence Interval Calculator
Confidence intervals for a mean (t or z, from summary stats or raw data) or a proportion (Wilson, Wald, exact)…
Triangle Calculator
Solve any triangle from three values — SSS, SAS, ASA, AAS or the ambiguous SSA case — with angles, area…
Board Foot Calculator
Calculate board feet for one board or a whole lumber list, with 4/4–16/4 thicknesses, price per board foot, a…
Pool Volume Calculator
How many gallons is your pool? Rectangle, round, oval or kidney, flat or sloped floor, in feet or meters — plus…
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 — 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.