Imperial ↔ Metric
Converter
Convert construction quantities between imperial and metric — length, area, mass, and volume. Pick a category, enter one value, and see every unit at once using exact NIST conversion factors.
Who it's for: architects, engineers, and contractors converting room dimensions, slab areas, concrete volumes, or material weights between imperial and metric units on the fly, from schematic design through construction documents.
How to use this calculator
The formula
Converted value = input × (from-unit factor ÷ to-unit factor)
Each factor is the unit's exact ratio to the base unit: metres for length, m² for area, kg for mass, litres for volume.
For example:
1 ft = 0.3048 m (exact, by international definition since 1959)
1 lb = 0.453 592 37 kg (exact)
1 US gal = 3.785 411 784 L (exact)
Worked example
When this estimate will be off
- Assumes a pure unit conversion — no material-specific adjustments for density, compaction, or moisture content.
- Cannot convert between different physical quantities (e.g., cubic metres to kilograms): that requires a material density value not captured here.
- US gallons and UK (imperial) gallons are different units — about 20% apart. Confirm which your specification requires before converting.
- Does not account for tolerance stacking in fabrication; always verify final dimensions against the project specification.
Frequently asked questions
A common mistake: Assumes a pure unit conversion — no material-specific adjustments for density, compaction, or moisture content.
The inch-to-metre ratio (0.0254) has been exact by international definition since 1959, making all length and area conversions mathematically exact. The pound (0.453 592 37 kg) and US gallon (3.785 411 784 L) are also defined exact values under NIST SP 811. No rounding is introduced by the base factors themselves.
They are different units. One US gallon equals 3.785 411 784 litres; one UK imperial gallon equals 4.54609 litres — roughly 20% larger. US construction specs use the US gallon; UK and Commonwealth projects use the imperial gallon. Always confirm which your contract documents specify before converting volumes.
No. Converting volume to mass requires knowing the material density, which varies by material. This converter handles unit changes within the same physical quantity. To find the mass of a concrete volume, multiply cubic metres by the material density — roughly 2,400 kg/m³ for normal-weight concrete.
In 1959, English-speaking countries agreed to define the international inch as exactly 25.4 mm. Since 1 foot equals 12 inches, 1 ft = 12 × 0.0254 m = 0.3048 m exactly. This is codified in NIST SP 811 and eliminates any rounding error in foot-to-metre conversions.
Sources
- NIST SP 811 (2008) — U.S. National Institute of Standards and Technology guide defining exact conversion factors: 1 inch = 25.4 mm, 1 lb = 0.453 592 37 kg, 1 US gal = 3.785 411 784 L.
This free Imperial to Metric Converter is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.