Free Online Calculators

180+ tools across Finance, Health & Fitness, Math, Time & Date, and Unit Conversions — with the formula and source behind every result.

Popular Calculators

Finance Calculators

Health & Fitness Calculators

Time & Date Calculators

Unit Conversion Tools

Table of Contents

Calculators that show their working

Most calculator sites give you a number and leave you guessing where it came from. Calculator Suite shows the formula on every page, names the source it draws from — the Mifflin-St Jeor equation for BMR, Naegele’s rule for pregnancy due dates, the 1959 International Yard and Pound Agreement for length conversions — and walks through a worked example using the default inputs. Every tool runs in your browser, with no account, no email signup, and no paid tier.

180+ calculators across five areas: finance (mortgage, loan, retirement, tax), health and fitness (BMI, BMR, calorie, body composition), math (geometry, algebra, statistics, scientific), time and date (countdowns, age, day-counting), and unit conversions (length, weight, temperature, volume, speed).

Finance calculators

The financial tools cover the math behind the major decisions: what a mortgage actually costs over its term, how compound interest changes a savings outcome, how long it takes to clear a credit card at a given monthly amount, and what take-home pay looks like at a given salary. The mortgage calculator includes a full amortization schedule, so you can see how the principal-versus-interest split shifts month by month over a 15- or 30-year term. The compound interest calculator handles monthly, quarterly, or annual compounding with side-by-side comparisons for different contribution rates. The debt payoff calculator models the avalanche and snowball strategies on the same debts so you can choose a method instead of guessing at one.

Health and fitness calculators

The health tools include BMI, BMR (basal metabolic rate), daily calorie needs (TDEE), body fat percentage, fat-free mass index, and 18 activity-specific calorie-burn estimates. Every formula is cited — the BMR calculator uses the Mifflin-St Jeor equation (more accurate for modern populations than the older Harris-Benedict), the BMI calculator applies the WHO category cutoffs, and the activity calculators (running, walking, cycling, swimming, weight-lifting, and more) use MET values from the 2011 Compendium of Physical Activities. Where a calculation has a known limitation — BMI does not distinguish muscle from fat; MET values assume average effort — the limitation is stated on the page rather than hidden.

Online calculators with cited formulas and worked examples

Math and scientific calculators

The math section covers geometry (area of a circle, volume, Pythagorean theorem), algebra (quadratic equations, slope, ratio), statistics (mean, median, mode, standard deviation), and a full scientific calculator with trigonometric, logarithmic, and exponential functions. Each page shows the formula being applied alongside the answer, so the calculator can be used to learn the method as well as to get a result — useful for homework, exam prep, or just checking your own working.

Time and date calculators

The time and date tools include a days calculator for the gap between any two dates, an age calculator that returns years, months, and days at any reference date, an hours calculator for elapsed time including overnight shifts, and 50+ countdown timers for holidays and personal events. The countdowns handle floating dates correctly — Easter Sunday via the Computus algorithm, Ramadan from the Islamic lunar calendar, Diwali from the Hindu Amanta calendar — with upcoming dates verified against authoritative sources rather than estimated.

Unit conversion tools

Length, weight, volume, temperature, speed, and Roman numerals — using the precise factors defined by NIST and the 1959 International Yard and Pound Agreement, not the rounded shortcuts. The kilogram-to-pound conversion uses the exact factor 0.45359237 (not 2.205), and the calculator is built on the 2019 Planck-constant redefinition of the kilogram rather than the obsolete International Prototype. The Celsius-to-Fahrenheit conversion preserves the offset correctly across negative temperatures. CM to feet and inches returns whole-inch precision in mixed units — which is what people actually want when looking up a height.

How the calculations are sourced

Every calculator on the site cites the formula and underlying source on its own page. The references are weighted toward government and institutional sources where they exist — the Federal Reserve, Consumer Financial Protection Bureau, NIST, NIH, WHO, BLS, CDC — with peer-reviewed research papers where they don’t. When a calculator’s accuracy is bounded by an assumption (Mifflin-St Jeor over-estimates BMR for highly trained athletes; BMI does not distinguish lean from fat mass), the limitation is stated on the page, not buried in a tooltip. This is the difference between a calculator you can trust for a real decision and one that hands you a number with no context.

FAQs

No. Every calculator runs entirely in your browser — no signup, no email collection, no paid tier, and no usage limit. The numbers you enter stay on your device; the math runs client-side.

Most differences come down to which formula or constant is used. For BMR we use the Mifflin-St Jeor equation, which has been shown more accurate for modern populations than the older Harris-Benedict equation that some sites still default to. For kilogram-to-pound conversion we use the exact 1959 International Yard and Pound Agreement factor (0.45359237) rather than the rounded 2.205. Every calculator page documents the formula it applies, so you can verify the math against any other source.

Treat them as planning tools, not a substitute for professional advice. The math is correct, but the inputs and assumptions matter. A mortgage calculator gives the payment for a given rate and term, but it does not know which rate you will actually qualify for or how lender fees will affect your APR. A BMI or BMR result is a screening number; a clinician will look at body composition, history, and goals. Use these to prepare for the conversation, not to replace it.

It depends what you are working out. For housing decisions, the mortgage calculator with its amortization table is the most-used tool on the site. For weight or fitness goals, the calorie calculator pairs with the BMR calculator to give a daily target. For time-based planning, the days calculator handles “how long until” or “how old will I be” questions without manual date math. Each category index page surfaces the most popular tools in its area.

Whenever an underlying source changes. The unit conversion calculators were last reviewed in May 2026 after the 2019 redefinition of the kilogram by the Planck constant. The health calculators were reviewed against current WHO, CDC, and ACOG references in the same window. Each page shows a Last Updated date in its metadata, and any change to the formula, constant, or category boundaries is reflected on the page itself.