Building drawings are almost always dimensioned in millimetres, yet older and imported units turn up constantly: inches on pipes and threads, feet on sheet materials, bar on pipework pressures. Mixing them up is an expensive mistake.
Length
- 1 inch = 25.4 mm
- 1 foot = 12 inches = 304.8 mm
- 1 yard = 3 feet = 0.9144 m
- Old local units — ells, rods, regional feet — varied from town to town before metrication. Never convert them without knowing the source
Note that a “half inch” water pipe is a nominal size. The actual outside diameter differs; always work from the stated diameter in millimetres.
Area and volume
- 1 are = 100 m²
- 1 hectare = 100 ares = 10,000 m²
- 1 m³ = 1,000 litres
Weight per cubic metre (rules of thumb for transport and loading)
- Concrete: about 2,400 kg/m³
- Building and fill sand: about 1,500 to 1,700 kg/m³
- Gravel: about 1,500 to 1,800 kg/m³
- Water: 1,000 kg/m³
A trailer holding 1 m³ of sand is therefore already well over 1.5 tonnes — often more than the trailer or the licence allows.
Pressure, temperature and energy
- 1 bar = 100,000 pascal = 100 kPa
- 1 atmosphere = 101,325 pascal
- Fahrenheit to Celsius: (°F − 32) ÷ 1.8
- 1 kWh = 3.6 MJ
- 1 kcal = 4.1868 kJ
Building physics units
- λ (lambda) in W/(m·K): how readily a material conducts heat — the lower, the better it insulates
- R-value in m²·K/W: thickness in metres divided by lambda. 120 mm of PIR at λ 0.022 gives 0.12 ÷ 0.022 ≈ 3.6 (excluding surface resistances)
- U-value in W/(m²·K): the heat transfer of a complete element, all layers included. The lower, the better
Related terms
- Setting out
- Are
- Hectare
- Inch
- Folding rule
- Lambda value
- U-value
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.