Unit conversions: length is not area, and litres are not kilograms
If a conversion produces a surprising number, check the kind of quantity first. Metres to centimetres is a length question. Square metres to square centimetres is an area question, so the same multiplier will not work.
Choose the quantity before the unit
Length, area, volume, mass and temperature are different quantities. Metric prefixes express powers of ten, but dimensions matter: 1 m equals 100 cm, while 1 m² equals 10,000 cm².
References: NIST — SI Units
Three traps worth checking
The international inch is exactly 2.54 cm. Converting millilitres to grams, however, requires the material’s density. Equal volumes of flour and oil need not have equal masses.
Labels such as “ton” and “gallon” need an identified unit system. Absolute temperature and temperature differences also convert differently: a rise of 10°C is a rise of 18°F, not a temperature of 50°F.
References: NIST — SI Units: Length · NIST SP 811 — Conversion Factors by Quantity
Keep useful precision, not decorative digits
An exact conversion factor cannot improve an approximate measurement. Keep precision during the calculation, then round appropriately for measuring or purchasing. For paint and building materials, separate geometric quantity from waste, coats and product coverage.
Traditional measures require a stated definition or calibration. They do not imply one fixed mass for every material. An unknown calibration should not become an assumed fact.
Worked example: a rectangular panel and a box
A panel 30 cm wide and 20 cm high has an area of 600 cm². In metres, 0.30 × 0.20 = 0.06 m², not 6 m².
A 40 × 30 × 20 cm box has a geometric volume of 24,000 cm³ = 24 L. That does not establish the mass of its contents or the usable capacity after allowing for wall thickness.