Unit Converter

Unit Conversion: Exact Factors and Famous Failures

Built and verified by Andrius R. · Updated June 2026

Most conversion mistakes aren't math errors — they're using the wrong factor, the wrong variant of a unit, or treating an exact definition as an approximation (or vice versa). This guide covers the conversions people actually get wrong.

Which factors are exact?

Since the 1959 international agreement, the inch is defined as exactly 2.54 cm — not measured, defined. That makes these exact:

The gallon trap (and the pint, and the fluid ounce)

A US gallon is 3.785 L; an imperial (UK) gallon is 4.546 L — about 20% bigger. The pint splits the same way (473 mL vs 568 mL), and the fluid ounce flips direction: a US fl oz (29.57 mL) is slightly larger than an imperial one (28.41 mL), even though the US pint is smaller, because the US pint contains 16 fl oz and the imperial pint 20. Recipes, fuel economy figures and brewing instructions all break on this. When you see "gallon" or "pint", always establish which system first.

Temperature is different: offset, not just scale

Length and mass conversions multiply by a factor. Temperature also shifts: °F = °C × 9/5 + 32. Consequence: you can't convert temperature differences with the full formula. A rise of 10 °C is a rise of 18 °F (×9/5 only, no +32). Useful anchors: −40° is the same in both scales, 0 °C = 32 °F, 37 °C ≈ 98.6 °F, 100 °C = 212 °F. For science, Kelvin = °C + 273.15, with no degree symbol and no negative values. More in our temperature converter.

Mass vs weight, and the kilogram-force confusion

Kilograms measure mass; newtons measure force (weight). On Earth's surface, a 1 kg mass weighs about 9.81 N. Bathroom scales display kg but actually measure force and assume Earth gravity — on the Moon they'd read ~16.5% of your true mass. The distinction matters in engineering: confusing mass and force units contributes to real structural and aerospace errors.

When conversion errors get expensive

• NASA's Mars Climate Orbiter (1999) — destroyed because one team's software produced thruster data in pound-force-seconds while another expected newton-seconds. Cost: a $327 million mission.
• The "Gimli Glider" (1983) — an Air Canada 767 ran out of fuel mid-flight after fuel was loaded in pounds instead of kilograms during Canada's metric transition. The crew glided it to a safe landing on a disused runway.
• Medication dosing — mg vs mcg (µg) confusion is a recognized, recurring source of thousand-fold drug errors in healthcare.

Quick sanity checks that catch most mistakes

1. Direction check: converting to a smaller unit must produce a bigger number. 5 km → 5,000 m, never 0.005.
2. Anchor check: memorize a few landmarks — a meter is ~3.3 feet, a kg ~2.2 lb, 100 km/h ~62 mph, a liter ~a quart. If a result strays far from the anchor's prediction, recheck.
3. Unit algebra: write units into the calculation and cancel them like factors. If the leftover unit isn't the one you wanted, the setup is wrong — this single habit catches nearly everything.