Category
Calculators Tools & Calculators
Practical everyday calculators with clear formulas shown for every result: percentages, percentage change, discounts, averages, fractions, ratios, and rule of three. Deterministic math you can verify.
Everyday calculators with the formula shown alongside every result: percentages, percentage change, discounts, averages, fractions, ratios, and rule of three. The math is deterministic — the value added is showing which formula is applied so you can verify and reuse it.
Most used in Calculators
Percentage Calculator
Three percentage calculations in one place: X% of a number, what percentage X is of Y, and the percentage change between two values.
Percentage Change Calculator
Measure the percentage increase or decrease between two numbers, or work backwards: what was the original value before a known change?
Discount Calculator
Compute the final sale price after one or two stacked percentage discounts, showing the total saved at every step.
Average, Median & Mode Calculator
Paste a list of numbers and get mean, median, mode, range, count, sum, and standard deviation with the formulas shown.
Fraction Calculator
Add, subtract, multiply, and divide fractions — results are simplified automatically and shown as improper fractions and mixed numbers.
Odds Ratio Calculator
Compute the odds ratio from a 2×2 contingency table with 95% confidence interval and p-value — the standard measure of association for case-control studies.
Relative Risk Calculator
Compute relative risk from a 2×2 table with 95% confidence interval and p-value — the natural measure of association for cohort studies and randomized trials.
Sample Size Calculator
Find the minimum sample size for a survey or experiment: one-proportion studies with a margin of error, or two-group comparisons powered at 80% with α = 0.05.
Confidence Interval Calculator (Proportion)
Build a confidence interval for a proportion from successes and trials — with the Wilson interval that stays accurate for small samples and 0% or 100% results.
Confidence Interval Calculator (Mean)
Compute a confidence interval for a population mean from raw data or summary statistics, using the t-distribution — correct for the small samples the normal formula gets wrong.
RMSE & MAE Calculator
Paste paired predictions and observations to get RMSE, MAE, MAPE, bias, and R² — the standard error metrics for evaluating forecasts, models, and measurements.
Z-Score Calculator
Turn a raw value into a z-score — how many standard deviations it sits from the mean — and convert any z into a percentile rank and one/two-tailed probability.
P-Value Calculator
Convert a test statistic (z, t, or χ²) into its p-value — one- and two-sided — with plain-language guidance on what the number does and does not mean.
BMI Calculator
Calculate Body Mass Index from height and weight in metric or imperial units, with the WHO category and the weight range that would land you in the healthy band.
All Calculators
Tip Calculator
Calculate the tip for any bill at any percentage and split the total evenly between any number of people, with per-person amounts.
Ratio Calculator & Simplifier
Simplify ratios to lowest terms, scale one side of a ratio to a new value, and solve proportions for the missing number.
Rule of Three Calculator
Solve direct and inverse proportion problems — the classic rule of three — with every step shown so you can verify the reasoning.
Random Number Generator
Generate one or many random numbers in any range using the browser’s cryptographic random source — with unique-only and sort options.
Macronutrient Calculator
Turn a daily calorie target into gram targets for protein, carbs, and fat — using goal-based presets or a custom split, with the math shown so you can adjust it.
Percentage questions are three different questions
- What is X% of Y — the direct share (15% of 80 = 12).
- X is what percent of Y — the ratio (12 of 80 = 15%).
- Percentage change from X to Y — the relative difference ((Y−X)/X × 100), which is directional: +25% followed by −25% does not return to the start.
Discounts and stacked offers
A 20% discount on 50 is 40; a further 20% on 40 is 32 — stacked percentages multiply, they do not add to 40% off. The discount calculator applies sequential discounts in the order entered, matching how stores actually apply offers at checkout.
Common mistakes
- Averaging percentages of different-sized groups without weighting (a true average needs the underlying totals).
- Adding percentage changes instead of compounding them over multiple periods.
- Interpreting a ratio result in the wrong direction (3:1 diluted vs concentrated).