About ZapUnits

Why we built it

Unit converters are among the most used tools on the internet and among the worst served. Most are buried under advertising, most give you a number with no context, and almost none show you the relationship between the two units — only a single result that you have to trust.

We wanted three things that were hard to find in one place: complete coverage, an honest presentation of the maths, and a memory. ZapUnits has 632 units across 64 categories, shows the formula alongside every result, plots the conversion on an X/Y graph, and — if you want an account — keeps a history of everything you have converted.

How the conversions work

Every category has a base unit. Length converts through the metre, mass through the kilogram, energy through the joule. Converting from A to B means converting A to the base, then the base to B. This keeps the data set small and consistent: adding a new unit means adding one factor, not one factor per pair.

Most units are a simple multiplication. Two families are not, and we handle them explicitly:

• Temperature needs an offset as well as a factor, because Celsius, Fahrenheit and Kelvin disagree about where zero sits.
• Fuel economy and running pace are reciprocal — more miles per gallon is better, fewer litres per 100 km is better. On those pages the graph is a curve, and that curve carries real information about where efficiency gains actually matter.

Where the factors come from

Conversion factors follow the SI definitions and the NIST Special Publication 811 guide. Where a unit is defined exactly — the inch as 25.4 mm, the pound as 0.45359237 kg — we use the exact value rather than a rounded one, and only round for display at the end.

Some units are genuinely ambiguous, and we say so on the page rather than pretending otherwise. The bigha varies by Indian state. Three different tons are in daily use. A US gallon and an imperial gallon differ by twenty percent. Where a choice had to be made, the page tells you which one we made.

If a number matters legally, commercially or clinically, verify it against the primary standard for your field. ZapUnits is built carefully, but it is a reference tool, not a certification. See accuracy of results.

Accounts and your data

You can use every converter without an account. Creating one adds a single feature: your conversions are saved, so you can pull them up later, filter them, and plot them on a chart.

We store your name, email address, a description of what you do, and a hashed password — never the password itself. We do not sell data and we do not send marketing email. You can clear your conversion history, or delete your account and everything attached to it, from the dashboard at any time. See the privacy page for the full detail.

Light and dark

The whole site ships in both themes. Dark mode is not an inverted copy — the charts use a separately chosen palette, validated for contrast and colour-vision deficiency against the dark surface rather than the light one. The toggle sits in the header, and your choice is remembered on your device.

What is next

Currency conversion, number-base conversion, and ingredient-aware cooking conversions (grams of flour, not just millilitres) are the three most requested additions. If there is a unit you need, tell us — regional and trade units are exactly what we want to cover.