Metric vs imperial in construction: conversions, lumber sizes and gallons

What are the exact conversion factors for building work?

The table gives the factors our calculators use, each from NIST or UK legislation. Most are exact by definition, so there is no rounding error until you round the answer. Multiply by the factor to go from imperial to metric; divide to go back.

Which units have the same name but different sizes?

The gallon and the ton. The UK imperial gallon is about 20% larger than the US gallon, and the UK long ton is 2,240 pounds against the US short ton of 2,000 pounds, per NIST Handbook 44 (2026) Appendix C. A UK spread rate in square metres per litre and a US one in square feet per gallon can't be compared until both are in the same units, and a "ton" of gravel from a US quarry is lighter than a UK long ton and a little lighter than a metric tonne. When a supplier quotes in tons, ask which one.

The foot used to have a twin as well. The US survey foot was retired after 31 December 2022, leaving the international foot (exactly 0.3048 m) as the single US foot except for legacy uses, per NIST Handbook 44 (2026) Appendix C. The two differed only slightly, which mattered on long survey distances rather than on a building plot.

Why is a 2×4 not 2 inches by 4 inches?

Because 2×4 is a nominal size. A 2x4 is 1-1/2 in by 3-1/2 in when dry and dressed (38 x 89 mm). The rough-sawn board is planed and dried down to its dressed size, and PS 20 sets the minimum dressed size for each nominal size. So a wall of 2×4 studs is 3-1/2 in thick before sheathing, not 4 in.

UK studwork is sold in metric target sizes instead. CLS/ALS studwork is 38 mm thick in widths 63, 89, 114, 140, 184, 235 and 285 mm, with rounded arrises. A US 2×4 and a UK 38 × 89 mm CLS stud are the same section. A UK 47 × 100 mm sawn timber is not; it is thicker and wider than a 2×4. Lengths step differently too: US softwood lumber comes in 1 ft or 2 ft length multiples; 2 ft (even) lengths are usual for construction lumber. UK/Irish structural softwood lengths step in 300 mm increments.

How do sheet, bag and roll sizes differ between the US and UK?

Many products are made to round numbers in each system, so a converted size is close but not equal. That difference changes counts. The table pairs the products our calculators handle.

How do you convert R-values and roof pitch?

Multiply an imperial R-value by 0.1761 to get RSI (m2·K/W); multiply RSI by 5.678 for imperial R. So R-38 is about RSI 6.7. A UK U-value is different: it is the reciprocal of the whole wall or roof's resistance, surface films included, so 1 ÷ U is not the R-value of the insulation alone. That is a common mistake when comparing a US attic target with a UK Building Regulations limit.

Roof pitch converts with trigonometry, not a factor: the angle is the arctangent of rise over run. The table gives the common steps. A 6:12 roof is 26.6°, not 6° or 50°.

What happens when units get mixed up?

The best-documented case is not a building. NASA's Mars Climate Orbiter was lost in 1999 because ground software reported thruster impulse in pound-force seconds instead of newton-seconds, understating effects by a factor of 4.45; the probe reached about 57 km altitude against an 80 km survival minimum (NASA Mishap Investigation Board, 1999; old but still the standard unit-error case). The error was a missing conversion between two teams' software, the same kind of slip that happens when one supplier quotes in metric and the drawing is in feet.

On a building site the damage is smaller but real: a slab ordered in cubic metres from a volume worked in cubic yards, a paint order sized with UK spread rates and US gallons, or a timber list mixing nominal and actual sizes. The lesson for site work is simple: one unit system per calculation, with conversions done once, at a stated point. Our concrete calculator and brick calculator keep the whole sum in the unit you choose and convert only at the end.

Illustration sources

• NASA — Mars Climate Orbiter Mishap Investigation Board Phase I Report (10 Nov 1999)
• NIST — SP 811 Appendix B: Conversion Factors

Why does the US still build in feet and inches?

Not because the law requires it. US law has named the metric system the preferred system for US trade and commerce since 1988 and required federal agencies to use it in procurements, grants and business activities by the end of fiscal year 1992 where economically feasible (15 U.S.C. 205b). The law sets a preference for trade, not a rule for private building, and US lumber standards such as PS 20 still define sizes in inches.

The UK construction industry planned its changeover more than half a century ago. The British construction industry's metric programme, published by BSI in February 1967, aimed for metric design by the end of 1971 and metric building on site by the end of 1972, per a 1973 US National Bureau of Standards report (historic, still the main primary account). The pace in private practice was slower. Private-sector UK building projects reaching production drawings in metric rose from 11.5% of design work by September 1970 to 28% in the second quarter of 1971, while public bodies had designed in metric since 1969, per a 1973 US National Bureau of Standards report. Both figures come from a single 1973 report, still the main primary account of that changeover. In law, UK law defines the yard as exactly 0.9144 metre and the pound as exactly 0.45359237 kilogram, and lists imperial units such as the foot, inch and acre as usable for trade only as supplementary indications (Weights and Measures Act 1985, Schedule 1).

Across the EU, imperial figures may still appear on labels. EU Directive 2009/3/EC (11 March 2009) removed the 31 December 2009 cut-off, so imperial 'supplementary indications' may still appear beside metric units, provided the metric value predominates. That is why some European products show inches in small print beside the metric size.

What is the difference between a soft and a hard conversion?

A soft conversion relabels the same product in the other unit, usually rounded. A hard conversion changes the product to a round size in the new unit. Both are common in building, and confusing them causes miscounts.

• Tile: Custom lists a 12 in tile as 300 mm, a soft conversion; a true 12 in is 304.8 mm.
• Plasterboard: UK boards are hard-converted to 1200 mm wide; a US sheet is 48 in, which is 1219 mm.
• Studs: 38 × 89 mm CLS matches the US 2×4's dressed size, so it is effectively the same section.

Whenever a product is described in both systems, work from the size the maker actually produces.

Illustration sources

• NIST — SP 811 Appendix B: Conversion Factors

How do you use these conversions with the calculators?

Pick one system for the whole job and let the tool convert. Use the area in square feet or square metres tool for floors and walls, and the volume in cubic yards or cubic metres tool for anything poured or spread. For loose materials sold by weight, the tons of gravel for your area tool asks which ton you mean. A printable concrete bag chart keeps US and UK bag counts on one sheet for site use. More method pages are in the measuring guides and references.

When is a straight conversion not enough?

• The product is different. A UK concrete bag or plasterboard sheet is not a converted US one; count with its own size.
• The code is different. US IECC R-values and UK U-values measure different things, and stair or pitch limits come from different rules, so convert to compare, not to comply.
• The size is nominal. Convert the actual dressed size of lumber, not the name.
• The ton or gallon is unstated. Ask before converting.

Summary

The factors are exact and easy: 2.54 cm per in to the inch and 0.3048 m per ft to the foot cover nearly everything. The mistakes come from names that mean different things in each country, from nominal sizes, and from products made to round numbers in their own system. Convert once, at a stated point, and count with the product's own size.

Unit conversion questions