Free Online Unit & Temperature Converters

Most unit conversion searches are not idle curiosity, they arrive with a deadline. The recipe is on the counter and the oven wants 175 degrees while the card says 350. The freight quote is due and the pallet is listed in cubic feet while the form wants cubic meters. The homework asks for milliliters and the measuring cup prints ounces. This page exists for exactly those moments: one input, one answer, zero navigation. Every converter below remembers your last value, so a quick double-check does not mean retyping.

Accuracy in conversion is mostly about knowing which convention the source uses. A US gallon and an imperial gallon differ by twenty percent, and recipes, fuel economy figures, and paint coverage charts pick one without labeling it. Miles and kilometers are safe to round for travel planning but not for pacing a race. Temperature conversions are exact, but oven behavior is not, so a converted recipe may still need local calibration. Where a convention matters, the dedicated pages call it out instead of silently assuming.

Roman numerals, still everywhere

Copyright years, book chapters, clock faces, movie sequels, stadium names, and outlines all use Roman numerals. Reading them is a five minute skill: repeats add, a smaller value before a larger one subtracts, and you never see the same letter more than three times in a row. Writing them is where mistakes creep in, the fours and nines carry the subtraction pairs, and a hand-converted 2024 is one slip away from being wrong on a poster. The Roman numeral converter handles both directions instantly and shows the breakdown so you can see which pairs were used.

How the two systems drifted apart

France formalized the metric system in 1795, after the revolutionaries got tired of a country where a "pinte" could mean different volumes in Paris and Bordeaux. The meter was defined as one ten-millionth of the pole-to-equator distance through Paris, and everything else — gram, liter — was derived from it. That interlocking design is the whole trick: 1 cubic centimeter of water is 1 milliliter and, at standard conditions, 1 gram. Conversions inside metric are decimal moves; conversions at the metric boundary are where the real work happens.

Meanwhile the British Empire kept its older measures, and the US kept an even older fork of them. Britain redefined the imperial gallon in 1824 as the volume of 10 pounds of water — 4.546 liters. The US stuck with the older wine gallon, 3.785 liters. Same name, twenty percent apart. A Canadian fuel-economy figure and an American one do not compare directly, and a UK recipe calling for half a pint means 284 ml while a US recipe means 237. The weight side split too: gold and silver are still traded in troy ounces at 31.103 grams, while the avoirdupois ounce in your kitchen is 28.35 grams. Weigh a gold chain on a kitchen scale in regular ounces and you've understated its metal content by about 9.7%.

When units fail, they fail expensively

The clearest argument for taking conversions seriously is a spacecraft. In September 1999, NASA lost the Mars Climate Orbiter — $327 million of hardware — because Lockheed Martin's ground software sent thrust data in pound-force seconds while the navigation team's software expected newton-seconds, a factor of 4.45. Nobody caught it in time, the trajectory corrections came out wrong, and the orbiter entered Mars' atmosphere about 57 km lower than planned and broke apart. No exotic physics, no bad luck — one unlabeled unit pair. It is the standing reminder that a conversion is only safe when both sides agree on which convention the number is in, which is why our converter pages name the system instead of assuming it.

Temperature across kitchens and labs

Fahrenheit survives in American kitchens, weather reports, and a surprising amount of industrial documentation, while Celsius owns science and most of the world. The formulas are exact and bidirectional: F = C × 9/5 + 32, and C = (F − 32) × 5/9. They cross at exactly −40, the one point where both scales read the same — a genuinely useful checksum for winter weather reports. The conversion itself is the easy part; the context is what the Fahrenheit to Celsius page documents: 350°F for baking is 175°C in a real oven because no domestic oven holds 176.67, and the fever thresholds — 37.5°C rising, 38°C confirmed — matter more than the raw arithmetic when the question is a sick kid at 2 a.m.

Aviation: feet, flight levels, and the cost of a wrong digit

Altitude is one of the few domains where the whole planet standardized on one unit — feet — and it happened through inertia rather than agreement. Postwar commercial aviation was dominated by American aircraft and procedures, so cruising altitude became flight levels: FL350 is 35,000 feet, always, regardless of what's on the ground below. Russia and China run meter-based altitude in their domestic airspace, so crews crossing those borders restate their altitude on the way in — 10,000 meters is about 32,808 feet. Vertical separation minimums are measured in hundreds of feet, so an altitude conversion that rounds the wrong way is an airspace problem, not a homework problem. It's also why the aviation convention of reading 18,000 feet as "flight level one-eight-zero" exists: fewer digits, fewer transpositions.

Kitchen measures by ingredient

Volume to weight conversion changes with the ingredient: a US cup is 236.6 ml, but what that weighs depends entirely on what's in it — a cup of flour is about 120 grams, a cup of butter is 227, a cup of honey is 340. Treating them the same is the single most common baking failure in converted recipes. Tablespoons and teaspoons carry their own traps: 1 tablespoon is 3 teaspoons and about 14.8 ml in the US, but an Australian tablespoon is 20 ml — four teaspoons' worth. For serious baking, weighing beats measuring every time, and scale-based recipes sidestep the whole question.

Running paces and the mile trap

Runners live in a two-unit world: most of the planet trains in min/km, American races post paces in min/mile, and race reports mix them freely. The factor is 1.609 — a 5:00 min/km pace is an 8:03 min/mile pace, and a 6:00 min/mile marathoner is running 3:44 per kilometer. The trap is comparing the raw numbers, because smaller means faster here: 5:00/km is far quicker than 5:00/mile. Treadmills make it worse by labeling speed in mph while your watch logs min/km. Convert once, write your race pace on your hand, and don't do arithmetic at kilometer 30. Thekm to miles converter covers the distance side; pace is just distance divided the other way.

Body weight is the conversion most people actually repeat, which is why it deserves its own reference: the kg to lbs page keeps the exact factor (1 kg = 2.20462 lbs) alongside the rounded one people use in conversation, and cm to feet handles heights, where the feet-and-inches split format causes more errors than the factor itself.

Everything runs in the browser, works offline after the first load, and stores nothing. No account, no history synced anywhere, no ads between you and the answer. If a conversion you need is missing, the category pages list the full set, and the common requests are linked below.

Length, area, and volume for real work

Renovation math mixes feet and meters in the same afternoon: the tile is sold in square meters, the room is measured in feet, the grout coverage is in square yards. The length and area converters keep three decimals by default, which is more precision than a tape measure but enough to catch material quantity errors before ordering. Volume conversions for shipping default to the pallet-level units, cubic feet to cubic meters, because that is the pair freight quotes actually use.

Everything runs in the browser, works offline after the first load, and stores nothing. No account, no history synced anywhere, no ads between you and the answer. If a conversion you need is missing, the category pages list the full set, and the common requests are linked below.