Convert any unit, in an instant.
Length, mass, temperature, area, volume, speed, data and more — 12 categories — plus currency, percentage, time-zone and business calculators. Everything runs privately in your browser.
Why UnitBook
12 unit categories
Length, mass, temperature, area, volume, speed, time, data, pressure, energy, power and angle — with the exact units you need in each.
Instant, accurate results
Type a value and the answer updates as you go. Temperature uses proper affine formulas, and results are precise to ten significant figures.
Private & offline
Every calculation runs on your device. No accounts, no uploads, no tracking — and it keeps working once the page has loaded.
English & Russian
The whole site — converter, guides and help — is fully bilingual. Switch languages anytime from the header.
Every unit, converted correctly
UnitBook is a free online unit converter and calculator for the measurements that come up every day: how many centimetres are in an inch, what 100 °F is in Celsius, how many megabytes fit in a gigabyte, how fast 60 mph is in km/h, and dozens more. Twelve categories — length, mass, temperature, area, volume, speed, time, data, pressure, energy, power and angle — each carry the exact units you need, from millimetres to nautical miles and bytes to petabytes.
Beyond units, UnitBook bundles handy calculators for currency, percentages, time zones and business math — VAT, discounts, margin, compound interest, loans, break-even and ROI. Everything works client-side in your browser, so it is fast, private and available offline once loaded. There is nothing to install and nothing to sign up for, and every conversion is backed by a plain-language guide that explains the math behind it.
Guides & articles
Plain-language explainers on units, measurement systems and the math behind the conversions.
Metric vs Imperial: A Practical Guide
Why most of the world uses metres and kilograms while a few countries still count in feet and pounds — and how to move between the two without mistakes.
Temperature Scales: Celsius, Fahrenheit, Kelvin
Temperature does not convert like length. Learn why it needs an offset, not just a multiplier, and how the three common scales relate.
Data Units: KB vs KiB, and Why Disks Look Smaller
A "1 TB" drive shows up as roughly 931 GB — because there are two competing definitions of the kilobyte. How binary and decimal units differ.
Frequently asked questions
Which unit categories does UnitBook support?+
The converter covers 12 categories: length, mass/weight, temperature, area, volume, speed, time, data/storage, pressure, energy, power and angle. Each category has its own set of units — from millimetres to nautical miles, bytes to petabytes, and Celsius to Rankine. There are also built-in calculators for currency, percentages, time zones and business math.
Are the conversions accurate?+
Yes. Most units convert through a fixed factor relative to a base unit, so results are exact to ten significant figures. Temperature is handled differently — it is an affine conversion (with an offset, not just a multiplier), so Celsius, Fahrenheit, Kelvin and Rankine are calculated with their proper formulas rather than a single ratio.
Does it work offline?+
Once the page has loaded, every calculation runs locally in your browser with JavaScript — no server round-trips. You can convert units, run the calculators and switch languages with no connection. Nothing you type is uploaded or stored anywhere.
Why is KB shown as 1024 bytes, not 1000?+
UnitBook uses binary (power-of-two) sizes for data, so 1 KB = 1024 bytes, 1 MB = 1024 KB, and so on. That matches how operating systems report file and disk sizes. Storage manufacturers often use decimal units (1 KB = 1000 bytes), which is why a "500 GB" drive shows as roughly 465 GB in your OS. Our guide on data units explains the KB-vs-KiB distinction in full.
Are the currency rates live?+
No — the currency converter uses fixed reference rates (as of March 2025) so it can run fully offline without calling any API. It is great for quick estimates and understanding relative value, but for financial decisions always check a live rate from your bank or a market source.