Unit Converter

Using the Quick Unit Converter

1. Pick a category tab at the top: Length, Weight, Temperature, Speed, Volume or Area. This converter is tuned for quick jumps between the most common measurements in each of those six domains.
2. Pick the "From" unit inside the active tab. Each category ships a curated short list rather than every obscure variant.
3. Type a number. The output recomputes on every keystroke via a Preact onInput handler - no submit button to press.
4. Pick the "To" unit.
5. Swap direction with the swap button, or Copy the result to your clipboard through the Clipboard API.

Quick vs Full Catalog - What This Tool Optimizes For

This page exposes a tight set of units per category - the ones 95% of users need. The sibling "All-in-One Unit Converter" (/tools/unit-converter-all/) carries a broader catalog with extras like astronomical units, nautical miles, Imperial gallons and acres. Pick this page when you want speed and a small, non-confusing dropdown; pick the full-catalog page when you need an exotic unit. Under the hood both tools use the same strategy: every unit is stored as a factor relative to an SI base (meter, kilogram, second, kelvin), and conversion is one multiplication and one division in IEEE 754 double precision. Temperature is the exception - it uses affine formulas (slope + offset) because the Celsius, Fahrenheit and Kelvin scales do not share a zero point. Everything runs inside the Preact component in your browser; nothing is sent to a server for the math itself.

When to Reach For It

• Converting a recipe ingredient amount between cups and milliliters on the fly in the kitchen.
• Translating a single dimension on a shipping label between cm and inches.
• Checking an oven temperature from a European cookbook in Celsius against a US dial in Fahrenheit.
• Comparing a car\'s top speed in mph against the km/h figure on a European spec sheet.
• Sanity-checking an apartment listing\'s floor area quoted in square meters against square feet.
• Filling out a customs or medical form that wants weight in kilograms when you only know the pound figure.

Pitfalls

Three worth highlighting. First, temperature is not linear between Celsius and Fahrenheit - it is affine. That means a 10 C interval equals an 18 F interval (slope only), whereas 10 C equals 50 F (the offset matters). The tool applies the right affine formula automatically for readings; if you are converting an interval, work it out manually. Second, US vs Imperial ambiguity: this page uses US gallons, US tons and US cups by default because US traffic dominates. If you are in the UK or a commonwealth country, cross-check with the all-in-one converter, which exposes Imperial variants. Third, category boundaries: you cannot convert a length to a volume. If you want to know "how many liters fit in a 1 m cube", use the length tool to get the cube\'s edge, cube it to get cubic meters, then use the volume tool - the converter will not do it in one shot.

Where the Conversion Factors Come From

Length, mass, time, temperature: SI Brochure (BIPM, 9th edition, 2019). US customary definitions (inch, foot, mile, pound, ounce): the 1959 international yard and pound agreement, codified in NIST Special Publication 811. Volume: US gallon = 3.785411784 L exactly, US cup = 236.5882365 mL exactly (NIST), US fluid ounce = 1/128 gallon = 29.5735295625 mL exactly. Area: acre = 4046.8564224 m^2 exactly (64 / 10000 of a square mile). Speed factors inherit from length and time definitions: mph, km/h, m/s, knot. Since the 2019 SI redefinition, every base unit is fixed by a defining constant (Planck h, elementary charge e, Boltzmann k, Avogadro N_A), making every derived conversion mathematically exact at the definition level. The only rounding you see is the 10-significant-digit output cap.

Alternatives

Spotlight on macOS and the Windows calculator both handle common conversions inline. Google\'s search box handles "10 miles in km" with zero clicks but sends your query through a server. The GNU units CLI handles every unit imaginable (barleycorns, furlongs, hands, cubits, Planck length) plus unit algebra like cc/min versus L/s. Scientific stacks - Python\'s pint, MATLAB unit types, Julia\'s Unitful.jl - propagate units through formulas and catch mismatches at compile or run time. Choose this page when you want six-category speed with no installation; choose the all-in-one sibling for broader unit coverage; choose a language library for scripted arithmetic.