The 10 Best Free Online Unit Converters in 2026 That Don’t Upload Your Data

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