Unit converters

Converters that show their working

A conversion result on its own is easy to mistrust. Every converter here shows the exact factor it used, the formula in both directions, and a table of nearby values — so you can check the answer rather than take it on faith.

What is covered

• Units — length, weight, temperature, volume, area, speed, data, time, pressure, energy, power, angle and frequency, with every pair of units in each category.
• Time zones — conversions between fixed-offset abbreviations, each with a 24-hour table and the working-hours overlap between the two zones.
• CSS units — px, rem, em, pt and the rest, with a root font size you can set to match your project.
• Cooking — cups to grams per ingredient, because a cup of flour is 125 g and a cup of honey is 340 g.
• Colors — HEX, RGB, HSL and CMYK with WCAG contrast ratios.
• Paper sizes — A4, Letter and the rest in millimetres, inches and pixels at any DPI.
• Screen resolutions — 1080p to 8K with aspect ratios, megapixels and pixel density at any screen size.
• Screen sizes — how wide a 55-inch television actually is, in inches and centimetres, at every aspect ratio.

Where conversions get subtle

Most unit conversions are a single fixed factor and cannot go wrong. A few genuinely can:

• Temperature needs an offset as well as a factor, because the scales have different zero points.
• Digital storage has two competing definitions — a manufacturer's "1 TB" is 10¹² bytes, while your operating system displays 2⁴⁰, which is why a new drive shows as 931 GB.
• US and imperial volumes differ. A US gallon is 3.785 L; an imperial gallon is 4.546 L. Recipes and fuel figures cross this boundary constantly.
• Cooking measures are not a unit conversion at all — they depend on the density of the ingredient, which is why they have a table of their own, by ingredient.

The factors worth knowing by heart

Most day-to-day conversions come down to a dozen numbers. These are exact unless marked otherwise — an inch has been defined as exactly 25.4 mm since 1959, and a pound as exactly 0.45359237 kg, so those are definitions rather than measurements.

Two of these are worth a second look. The calorie has several competing definitions; the 4.184 above is the thermochemical calorie, which is the one food labelling uses. And the gallon entry is the single most common source of wrong answers on this list — a US gallon is about 17% smaller than an imperial one, so a fuel-economy figure quoted in mpg means two different things either side of the Atlantic.

Doing it in your head

Approximations that are close enough to be useful and easy to remember:

• Kilometres to miles: multiply by 0.6 — a slight underestimate, about 3% low. More precisely, consecutive Fibonacci numbers are near-perfect km/mile pairs — 5 km ≈ 3 miles, 8 km ≈ 5 miles, 13 km ≈ 8 miles. The ratio converges on the golden ratio, 1.618, which happens to sit within 0.6% of the true factor 1.609.
• Celsius to Fahrenheit: double it and add 30. Exact at 10°C and within 4° anywhere between −10°C and 30°C, which covers most weather. It drifts further at the extremes — at 100°C it is out by 18°.
• Kilograms to pounds: double it and add 10%. 80 kg → 160 + 16 = 176 lb, against a true 176.4.
• Litres to US gallons: divide by 4, then add 5%. 40 L → 10 + 0.5 = 10.5, against a true 10.57.