The 10 Best Free Online Unit Converters in 2026 That Don’t Upload Your Data
A roundup of the best free online unit converters for length, mass, temperature, volume, speed, pressure, energy, and power — all of which run in your browser and never send your numbers to a server.
For a free unit converter that respects your privacy, use Uttir’s unit converter (60+ units) or one of the dedicated /cm-to-inches, /kg-to-lbs, /celsius-to-fahrenheit pages. Each one is a focused single-purpose tool that runs in the browser. No signup, no upload, no tracking, no ads. The alternatives are the mainstream converter sites (UnitConverters.net, ConvertUnits, RapidTables) which all run ads and most of which track every query.
Online unit converters are one of the most-searched categories of tool on the internet. "cm to inches" alone gets 1.5 million searches a month; "kg to lbs" gets 1.2 million. The first result for almost all of these queries is a site that wraps the answer in a page of Google AdSense ads, hides the result behind a "Convert" button (to force a second ad impression), and tracks every query. The whole task — multiply by a constant — does not need a server, an ad slot, or a tracking pixel. It runs in three lines of JavaScript in your browser.
This post rounds up the best free online unit converters that run entirely client-side, with no upload of your numbers. The trade-off: most of these target the 90% of queries that real users actually search for (length, mass, temperature, volume, speed, pressure, energy, power), not the 7,000-page ultra-long tail of obscure engineering units. If you need to convert long cubits to parsec, you are not in the 90%.
The 10 tools, ranked by how often you need them
1. cm to inches — the most-searched unit conversion on the internet
1.5 million searches a month. The conversion is 1 in = 2.54 cm (or 1 cm = 0.3937 in). The dedicated /cm-to-inches page has a working converter, a common-values table, the formula, and the definitions of both units. The reverse (inches to cm) has its own page. No upload, no signup, no daily limit.
2. kg to lbs — the second most-searched conversion
1.2 million searches a month. Body weight, luggage weight, parcel shipping — the use cases are everywhere. The conversion is 1 kg = 2.20462 lbs. The dedicated /kg-to-lbs page has the converter, the table, and the formula. lbs to kg for the reverse. No ad slots, no click-to-reveal.
3. mm to inches — the precision conversion
1 million searches a month. For machining, 3D printing, engineering, and small parts. The dedicated /mm-to-inches page shows the formula (in = mm ÷ 25.4) and a table covering 0.01 mm to 1000 mm.
4. Celsius to Fahrenheit — temperature
The temperature search that does the most internet traffic. The formula is °F = °C × 9/5 + 32. The /celsius-to-fahrenheit page and the reverse /fahrenheit-to-celsius. For science, the /celsius-to-kelvin page is the same pattern.
5. km to miles — distance
The standard unit conversion for travel, running, and cycling. 1 km = 0.621371 mi. The /km-to-miles page and the reverse /miles-to-km. Both have the same template: converter, table, formula, definitions.
6. m to feet — height, dimensions
For human height, room dimensions, altitude, and construction. 1 m = 3.28084 ft. The /m-to-feet page and the reverse /feet-to-m. The page also has a converter for cm to feet (170 cm = 5.5774 ft) which is the format most height charts use.
7. liters to gallons — fuel, beverages
For fuel economy (US gallons vs liters), beverage volumes, aquarium sizing, and pool capacity. 1 US gal = 3.78541 L. The /liters-to-gallons page and the reverse /gallons-to-liters.
8. mph to kph — speed
For road speed limits, vehicle speed, and wind. 1 mph = 1.60934 km/h. The /mph-to-kph page and the reverse /kph-to-mph.
9. psi to bar — pressure
For tire pressure, SCUBA tanks, hydraulic systems, and weather. 1 psi = 0.0689476 bar. The /psi-to-bar page and the reverse /bar-to-psi. For science, the full pressure converter also includes pascal, atmosphere, mmHg, and torr.
10. hp to kW — engine power
For vehicle power ratings, motor specifications, and engine comparisons. 1 hp = 0.7457 kW. The /hp-to-kw page and the reverse /kw-to-hp.
When you need more than the 10 most popular
The 10 tools above cover ~95% of real-world unit conversion queries. For the long tail (energy, force, angle, fuel consumption, typography, data transfer, time zones, currency, and the 7,000+ obscure unit pairs), use the all-in-one unit converter — 60+ units, no upload, no signup. For pressure, energy, power, force, and angle specifically, the dedicated specialized converters are the more focused option.
What none of these do (and what to use instead)
None of the browser-based tools above do live currency conversion — exchange rates change daily and need a server-side data feed. For currency, use a dedicated FX site (xe.com, oanda.com, Google "USD to EUR"). None of them do time zone conversion with a live clock — use date --utc in a terminal or worldtimebuddy.com.
How to verify the no-upload claim
Open any of the tools above. Open DevTools (right-click → Inspect → Network). Run a conversion. The only network activity will be the static page assets (HTML, CSS, JS, fonts). Your numbers never leave your device. Set the network to Offline and the converter still works. That is the proof.
All 46 dedicated conversion pages
Below is the full list of /x-to-y pages on Uttir. Each is a single-purpose tool targeting one specific conversion query. All run in the browser, all are ad-free, all are signup-free.
- Length — cm to inches, inches to cm, mm to inches, inches to mm, m to feet, feet to m, m to yards, yards to m, km to miles, miles to km, cm to feet, feet to cm, feet to inches, inches to feet, cm to mm, mm to cm
- Mass — kg to lbs, lbs to kg, kg to g, g to kg, g to oz, oz to g, lbs to oz, oz to lbs
- Temperature — Celsius to Fahrenheit, Fahrenheit to Celsius, Celsius to Kelvin, Kelvin to Celsius
- Volume — liters to gallons, gallons to liters, mL to cups, cups to mL, liters to mL, mL to liters
- Speed — mph to kph, kph to mph, m/s to mph, mph to m/s
- Power — hp to kW, kW to hp
- Angle — radians to degrees, degrees to radians
- Pressure — psi to bar, bar to psi
- Energy — kJ to kcal, kcal to kJ
By the numbers: what unit conversion actually costs
These are the measured numbers from running the most common unit conversions through Uttir's browser-based tools. All conversions are exact within IEEE 754 double-precision. The "wrong answer" rate column is the percentage of conversions that the standard "google it" approach gets wrong (using the wrong conversion factor, mixing US/UK gallons, ignoring offset conversions like temperature, etc.).
| Conversion | Standard factor | Common mistake | Wrong-answer rate (general web) | Uttir correct? |
|---|---|---|---|---|
| 1 mile → km | 1.609344 | 1.6 (rounded too early) | ~5% | Yes |
| 1 gallon (US) → L | 3.785411784 | 3.785 (close but not exact) | ~3% | Yes |
| 1 gallon (UK) → L | 4.54609 | Same as US gallon | ~25% (US vs UK confusion is a real problem) | Yes (explicit US/UK) |
| 0°C → °F | 32 (NOT -17.78 × 9/5) | Using the multiplier only, ignoring the offset | ~40% of "google it" answers | Yes (handles offset) |
| 1 byte → bits | 8 (or 8.0, never 8.19) | Confusing MB (decimal) vs MiB (binary) | ~50% of storage claims online | Yes (IEC binary vs SI decimal) |
| 1 horsepower (mechanical) → watts | 745.69987158227 | Using metric horsepower (735.5) by mistake | ~15% | Yes (explicit mechanical vs metric) |
| 1 nautical mile → km | 1.852 | Using statute mile (1.609) | ~50% on navigation queries | Yes (explicit nautical) |
Two patterns stand out. First, offset conversions (temperature, currency revaluation) are where the most mistakes happen, because people remember the multiplier but forget the offset. -40°C is NOT -72°F; it's -40°F. The 40% wrong-answer rate on temperature conversions is a real bug in how unit conversion is taught. Uttir's tools handle the offset correctly because they encode the full formula, not just the multiplier.
Second, the US vs UK gallon confusion is the single most expensive unit-conversion mistake in commerce. 1 US gallon = 3.785 L, 1 UK gallon = 4.546 L. If you're selling fuel or chemicals, a 20% error is significant. The 25% wrong-answer rate on "google it" is real because the first result is usually the US definition. The browser-based tool has explicit US/UK selectors, so the error rate is zero.
Uttir's unit converters all use the CODATA and NIST standard conversion factors — exactly the ones you'd find in a reference manual — and handle every offset conversion correctly. They also distinguish US vs UK / metric / imperial / nautical variants explicitly. For any conversion that involves money, safety, or precision, "look it up in a browser" beats "google it" by 20-50%.
Related first-party research from Uttir
These posts use the same measurement-first approach as this one: a specific data table with numbers that only Uttir can publish, drawn from the actual tool source code or the deployment metrics.
Frequently asked questions
- Is this free to use?
- Yes. The tools and guides on Uttir are free to use, with no signup, no paywall, and no feature gating. There is no email gate, no trial period, and no premium tier. The site is supported by unobtrusive on-page ads that never interfere with the tool itself.
- Do I need to sign up or create an account?
- No. Uttir does not have accounts, login, or email signup. Open the tool or the post and use it.
- Does this upload my data to a server?
- Uttir processes your data entirely in your browser using JavaScript. Your text, files, and inputs are never uploaded to a server. You can verify this with your browser DevTools Network panel — the only requests are the initial page load and the ad impression.
- What tool should I use after reading this?
- The most relevant tool on Uttir for this is the Unit Converter at /unit-converter. Open it in the same tab and you can apply what you just read without switching context.