Unit conversion is the kind of task you do hundreds of times a year — recipes, travel, school, work, hobbies — and yet most people just type "5 feet 9 inches in meters" into Google and accept whatever comes back. The big stand-alone unit converter sites still exist for a reason, though. They handle more categories, more unit variants, and edge cases (US vs UK gallons, decimal vs binary data, US survey vs international foot) that Google's box quietly gets wrong. We tested five of the most-used free unit converters on a 30-item conversion battery covering every category they all support.
Our pick
The fastest interface in our test (no page reload between conversions), correct on every edge case we tried, with a full table showing your value in every unit at once. No ads, no tracking, runs entirely in your browser.
How we tested
We picked 30 conversions across the nine common categories: 4 length, 4 mass, 3 temperature, 4 volume, 3 area, 3 speed, 3 time, 3 data, and 3 energy. Each included at least one edge-case (US gallon vs UK gallon, KB vs KiB, kcal vs kJ). We graded each tool on:
- Accuracy — did the answer match official conversion factors?
- Coverage — how many of the 30 conversions did the tool support?
- Speed — time from entering a value to seeing the answer
- Distractions — ads, popups, redirects, signup walls
- Privacy — does it run locally or send data to a server?
How we scored
Each tool was scored out of 10: accuracy (3 points), coverage (3 points), speed (2 points), and absence of ads/friction (2 points). Privacy is noted but not scored — converting feet to meters is not sensitive data, so the privacy impact is small.
The full ranking
Rank #1
ToolHub Unit Converter
Best overall. Nine categories, every unit correct, smart formatting that uses scientific notation only for genuinely huge or tiny numbers. The full-table view (your value in every other unit at once) is faster than typing into a one-to-one converter.
Pros
- All nine common categories covered
- Correct US vs UK gallon, KB vs KiB, kcal vs kJ
- Both decimal and binary data units
- Live update — no page reload between conversions
- Common-pair chips (ft↔m, °F↔°C, mph↔km/h)
- Full conversion table showing every output unit
- No ads, no tracking, no signup
- Works offline once loaded
Cons
- No currency conversion (would need live API)
- No custom unit definitions
Rank #2
Google calculator (search box)
Convenient because it is always one URL bar away, accurate for the conversions it supports, but limited coverage and prone to interpreting your input as a different query when you mistype. Ads above the answer on most queries.
Pros
- Always available — no need to remember a site
- Accurate on the conversions it supports
- Voice input works on mobile
- Handles natural language like '5 ft 9 in to cm'
Cons
- No data category (KB, GB, MiB) — at all
- No nautical mile, knot, BTU, or kcal in the box
- Quietly defaults to US units (US gallon, US ton) without warning
- Ads above the answer reduce visibility
- Each conversion is a page load
Rank #3
RapidTables Unit Converter
Comprehensive coverage but the UX shows its age. Each conversion is on its own page, navigation is clunky, and the site is heavy with display ads. Accurate on the math when you finally get to the converter you need.
Pros
- Good coverage of obscure units
- Stable, well-known reference for years
- Includes engineering units beyond the common nine
Cons
- Old-style multi-page UI
- Heavy ads on most pages
- Slow to navigate between categories
- No live update — you click 'convert' for every change
Rank #4
ConvertWorld
Multi-language and extensive unit coverage, especially regional measures. The UX is dense and dated, with significant ad clutter. Accuracy is good but the interface slows you down.
Pros
- 30+ language interface
- Regional units (Chinese, Indian, Japanese)
- Currency conversion included (via live API)
Cons
- Crowded UI with ads competing for attention
- Slow first load
- Some unit definitions inconsistent across pages
Rank #5
OnlineConversion.com
Functional but feels frozen in 2010. Form-based UI, no live update, heavy ads, some pages with broken layouts on mobile. Still works for the basic conversions but there is no reason to use it over the alternatives.
Pros
- Reasonable category coverage
- Accurate for basic conversions
Cons
- Form-submit UI for every conversion
- Heavy banner ads on every page
- Mobile layout broken on some pages
- No keyboard shortcuts or quick presets
Feature matrix
| Feature | ToolHub | RapidTables | ConvertWorld | OnlineConv | |
|---|---|---|---|---|---|
| Length, mass, temperature | Yes | Yes | Yes | Yes | Yes |
| Volume (US + UK distinct) | Yes | Partial | Yes | Yes | Yes |
| Area + acres + hectares | Yes | Yes | Yes | Yes | Yes |
| Speed including knots | Yes | Partial | Yes | Yes | Partial |
| Data (KB, MB, GB, TB) | Yes | No | Yes | Yes | Yes |
| Data binary (KiB, MiB, GiB) | Yes | No | Partial | No | No |
| Energy (J, cal, kcal, kWh, BTU) | Yes | Partial | Yes | Yes | Yes |
| Live update (no page reload) | Yes | Yes | No | No | No |
| Full conversion table view | Yes | No | No | No | No |
| Mobile-first design | Yes | Yes | No | No | No |
| No ads, no tracking | Yes | No | No | No | No |
| Works offline (PWA-style) | Yes | No | No | No | No |
Edge cases the rankings hinge on
US gallons vs UK gallons
A US gallon (3.785 L) and a UK gallon (4.546 L) differ by 20%. Google quietly defaults to US gallons unless you specify "UK gallon" in the query, which is easy to forget. ToolHub, RapidTables, and ConvertWorld all give you both as distinct unit options. OnlineConversion has them but on different pages.
KB vs KiB (decimal vs binary)
A kilobyte is 1,000 bytes. A kibibyte is 1,024 bytes. The difference matters: a "500 GB" hard drive shows as ~465 GiB in Windows because Windows uses binary while the manufacturer labels in decimal. ToolHub is the only test entrant that supports both with clear labelling. Google does neither.
Calorie vs kilocalorie
The "calorie" on a nutrition label is actually a kilocalorie. Most converters mishandle this — including some entries in our test. ToolHub explicitly labels both ("Calorie (small)" and "Kilocalorie / food Calorie") to avoid the trap.
The privacy footnote
Converting "5 feet 9 inches" to meters is not sensitive data; we are not worried about state-actor surveillance of unit conversion. But a tool that runs locally is faster anyway (no round-trip to a server), and it works offline. The privacy upside is a nice bonus on top of the speed and reliability benefit.
The bottom line