About & methodology
Accurate by design.
Calculator Kong is a precision toolkit of 41+ free calculators for finance, health, math, construction and more. Every result is grounded in a published formula or an industry-standard library — never guesswork.
Standard formulas
Each tool implements a published, widely-accepted formula — and shows it to you.
Tested libraries
Money and date math run on battle-tested libraries (Formula.js, date-fns), not hand-rolled code.
Cross-referenced
Constants and coefficients are checked against authorities like WHO and NASA (see below).
Formulas & sources
The standards and authoritative references behind our calculators. Links open the source.
Health & fitness
- World Health Organization ↗
BMI
BMI = weight (kg) ÷ height (m)²
- Mifflin MD et al., PubMed ↗
Calories / TDEE
Mifflin–St Jeor basal metabolic rate (1990)
- Body fat percentage — reference ↗
Body fat
U.S. Navy circumference method (Hodgdon–Beckett, 1984)
- Ideal body weight — reference ↗
Ideal weight
Devine, Robinson, Miller & Hamwi formulas
- One-repetition maximum — reference ↗
One-rep max
Epley and Brzycki equations (averaged)
- Estimated date of delivery — reference ↗
Due date
Naegele's rule (LMP + 280 days)
Money & finance
- Formula.js (Excel-compatible financial functions) ↗
Loan / mortgage / auto
Standard fixed-rate amortization (PMT)
- Formula.js ↗
Compound interest / retirement
Future value with periodic contributions (FV)
- Standard definition ↗
Simple interest
I = P × r × t
Science & space
- NASA Planetary Fact Sheet ↗
Weight on planets
weight = mass × surface-gravity ratio
- NASA Planetary Fact Sheet ↗
Age on planets
age ÷ orbital period (Earth years)
Units & conversions
- BIPM — The International System of Units ↗
Length, mass, area, speed, temperature
Exact SI conversion factors (e.g. 1 in = 0.0254 m, 1 lb = 453.59237 g)
- NIST — Metric (SI) ↗
Unit definitions
Metric ↔ imperial reference values
Dates & math
- date-fns ↗
Age, date & time durations
Calendar-accurate date arithmetic (leap years, month lengths)
- Standard definition ↗
Standard deviation
Population (÷N) and sample (÷N−1, Bessel’s correction)
- Standard definition ↗
Quadratic formula
x = (−b ± √(b²−4ac)) ÷ 2a
Our principles
Free forever
No accounts, no paywalls, no limits.
Private
Every calculation runs in your browser — your inputs never leave your device.
Fast
Lightweight static pages that load instantly.
Frequently asked questions
Is Calculator Kong really free? +
Yes — every calculator is free forever, with no account, no sign-up and no paywall.
How accurate are the calculators? +
Each tool uses a published, widely-accepted formula or a battle-tested library (Formula.js for money, date-fns for dates), and the physical constants are cross-referenced against authorities such as the WHO and NASA — see the Formulas & sources list above.
Is my data private? +
Completely. Every calculation runs locally in your browser, so the numbers you enter never leave your device or reach a server.
Can I switch between metric and imperial units? +
Yes. Calculators that use measurements have a Metric/Imperial toggle in the tool bar, and each field lets you pick its own unit (e.g. cm, m, ft & in) and type values directly.
Can I change the currency? +
Yes — money calculators include a currency selector in the tool bar, and the input ranges scale sensibly to the currency you choose.
Can I share or embed a calculator? +
Yes. The Share button gives you a link that preserves your entered values, a downloadable image, and an embed snippet to place the calculator on your own site.
How do I request a new calculator or report a problem? +
Head to the contact page and send us a note — we add and refine tools based on what people ask for.
Results are estimates provided for informational purposes only and are not a substitute for professional financial, medical, or engineering advice. Spotted an issue or want a new calculator? Get in touch.