Unit Converter

Four inputs, in this order:

1. Pick the category. Length, weight, temperature, volume, or speed. This filters the unit dropdowns so you don’t accidentally try to convert miles to grams (which would be a fun calculator bug but a useless answer).
2. Pick the “from” unit. The unit your number is currently in. Reading a recipe in metric? Pick milliliters or grams. Looking at a hiking trail map in miles? Pick miles.
3. Pick the “to” unit. The unit you want to end up with. Same category as the from-unit; the dropdown will show only valid options.
4. Enter the value. Decimals are fine. Negative numbers work for temperature.

Hit calculate and the result appears immediately, along with the conversion factor used and a reverse conversion as a sanity check (converting the answer back should land on your original number, give or take floating-point rounding).

One pro move: when you’re unsure of a unit, run a check on a value you already know. If you’re converting body weight from pounds to kilograms, plug in 220 lb. If the answer is ≈100 kg, the calculator is healthy. If it says 220 kg, something is wrong — you may have selected the same unit on both sides, or selected pounds-force vs. pounds-mass in some advanced converter. (This calculator only does pound-mass, the everyday kind.)

The world cannot agree on how to measure things. The U.S. uses inches and pounds; the rest of the planet uses meters and kilograms. Within each system there are sub-tribes — ounces (volume) vs. ounces (weight), short tons vs. long tons vs. metric tons, U.S. fluid cups vs. imperial cups vs. metric cups. A unit converter is the tool you reach for when one system collides with another, which happens every time someone reads a recipe in metric, eyeballs a hardware-store ruler, or follows a road sign while driving abroad.

The good news: most conversions are just multiplication. 1 mile = 1.60934 kilometers, exactly. 1 kilogram = 2.20462 pounds, exactly. 1 inch = 0.0254 meters, exactly — that one is built into the international definition of the inch and is not approximate. Multiply the number you have by the right factor, and you have the same physical measurement expressed in different vocabulary.

The bad news: temperature breaks the pattern. You can’t just multiply °F by some factor to get °C, because the two scales don’t share a zero point. Water freezes at 0°C and 32°F. Same temperature, two different numbers, no clean ratio. Temperature conversion needs a shift and a scale: °C = (°F − 32) × 5/9. The calculator handles this for you, but it’s the one place students reliably trip up.

Why does any of this matter? Because unit mistakes have, no exaggeration, crashed spacecraft. The Mars Climate Orbiter burned up in the Martian atmosphere in 1999 because Lockheed Martin’s navigation software output thrust in pound-seconds and NASA’s receiving software expected newton-seconds. The mismatch was a factor of 4.45. Total bill: $327 million and one missing spacecraft. On the more terrestrial scale, getting drug doses, baking ratios, or 2-by-4 lengths wrong because of a unit slip is also a bad day. A converter that always gives you the same exact answer takes that whole class of mistakes off the table.

This calculator handles five categories: length (everything from millimeters to miles), weight (grams through tons), temperature (Celsius, Fahrenheit, Kelvin), volume (milliliters, liters, cups, gallons), and speed (km/h, mph, m/s). Pick a category, pick a from-unit, pick a to-unit, type a number, and you’re done.

The calculator uses a base-unit pivot: for each category there is one canonical unit, and every conversion goes through it. Length pivots on the meter, weight on the gram (or kilogram), temperature on Kelvin, volume on the liter, speed on m/s. Going from feet to inches works in two short hops — feet → meters → inches — rather than hard-coding a feet-to-inches factor. This keeps the conversion table linear in the number of units instead of quadratic.

The general formula for any non-temperature conversion: v′ = v × f_from ÷ f_to. For temperature, see the next section.

Length conversion factors (everything to meters):

Temperature is the special case. The three formulas you need: °C → °F: F = C × 9/5 + 32. °F → °C: C = (F − 32) × 5/9. °C → K: K = C + 273.15. The +32 (or −32) is the offset between freezing points; the 9/5 (or 5/9) is the ratio of degree sizes (a Fahrenheit degree is smaller than a Celsius degree, so the same temperature change spans more F-degrees than C-degrees).

Example 1 — Hiking trail (length). A trail map says 5 miles. How far in kilometers?

• Conversion factor: 1 mile = 1.60934 km
• v′ = 5 × 1.60934 = 8.05 km

Sanity check: a 5K race is 3.1 miles, so 5 miles should be a bit more than 8 km. 8.05 lines up.

Example 2 — Recipe rescue (volume). A British recipe calls for 2 cups of flour. Your kitchen scale only shows milliliters. (And the British and U.S. cup are slightly different sizes, which is its own conspiracy.) Using the U.S. cup (236.588 mL):

• v′ = 2 × 236.588 = 473.18 mL (round to ~473–480 mL when measuring)

If the recipe is from the UK, use the imperial cup (284.131 mL): 2 cups = 568.26 mL. The 95 mL difference is enough to ruin a delicate cake. Always check whose cup the recipe means.

Example 3 — Driving in Europe (speed). A road sign reads 100 km/h. The U.S. driver sees this and panics — that sounds like a lot. Converting:

• 1 km/h = 0.621371 mph
• v′ = 100 × 0.621371 = 62.14 mph

Roughly the same as a U.S. interstate. Crisis averted. The reverse direction is useful too: 65 mph in U.S. signage equals 65 × 1.60934 = 104.6 km/h, which lines up with European autostrada limits in the 100–110 km/h range.