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About

Why we build calculators this way

A library of free single-purpose calculators, built one category at a time — each one shipping with its formula in plain English, a worked example verified against the code, and a source for every figure that isn't pure arithmetic.

Calculators live
87
Calculators live
Categories
18
Categories
Formulas documented
100%
Formulas documented
Articles published
2
Articles published

What we're building

CalculatorPlanet, also known as Calculator Planet, is a growing library of free, single-purpose calculators. It's the kind of tool you land on to answer one specific question (what's 12% of 340? what's my exact age in days? how much gravel do I need for a 200-square-foot path?) and then move on with your day. No account, no paywall, no forced sign-up.

The site is being built out one category at a time, in a deliberate order based on real search-demand research rather than guesswork — Academic and Science & Math tools came first, with Finance, Health, Construction, and the rest following as each category's content is researched, built, and checked.

How a calculator gets built

Every calculator follows the same four steps before it ships. Steps 2 and 4 are enforced by the build itself, not just by intent.

  1. 01

    Formula stated in plain English

    Before any code is written, the formula is described in a sentence a non-specialist can follow, with every variable named and given a unit.

  2. 02

    Worked example written and verified

    A realistic example runs through the formula by hand, and its result must match the live calculator exactly. If they disagree, one of them is wrong — and the build fails until they don't.

  3. 03

    Sources cited for any non-arithmetic figure

    Thresholds, bands, tax rates, and clinical cut-offs all get a named source and a retrieval date. Pure arithmetic needs no citation; everything else does.

  4. 04

    Sensitive categories get a disclaimer and reviewer

    Health, Finance, Tax, and Professional pages carry a standardized disclaimer and a named editorial reviewer. The content schema refuses to build these pages without both.

Who's behind it

CalculatorPlanet is currently maintained by a small editorial team rather than a large newsroom. We'd rather credit content honestly to "the editorial team" than invent individual bylines with credentials we can't back up — as the site grows, real named contributors and their credentials will be added here and to each page they work on, not before.

Write for us

What we get wrong (and how we fix it)

  • Corrections are made on the page itself, never in a separate errata list nobody reads.
  • The "last updated" date changes with every substantive edit, so you can judge how current a figure is.
  • Reported formula bugs are reproduced from your inputs — send the numbers and we can usually confirm the same day.
Report a problem

What stays true

  • Free forever

    Every calculator, no paywall, no usage limit.

  • No account

    No login, no profile, no email required to calculate.

  • Formulas documented

    The method is printed on the page, not hidden in code.

  • Sources cited

    Any figure that isn't arithmetic names where it came from.

Meanwhile

Start with one of these

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