How many millimetres in an inch?

There are exactly 25.4mm in one inch. This is the fundamental conversion factor for all metric/imperial length conversions. 1 foot = 304.8mm, 1 yard = 914.4mm, 1 metre = 39.3701 inches. In UK construction, materials are manufactured to metric dimensions but many tradespeople still think in feet and inches — particularly for room sizes, timber lengths, and pipe runs.

What are the most common construction lengths I need to know?

The most commonly encountered construction lengths are: 2440mm = 8 feet (standard sheet size length), 1220mm = 4 feet (standard sheet width), 2400mm = common stud wall height, 600mm ≈ 2 feet (joist/stud spacing), 300mm ≈ 1 foot, 100mm ≈ 4 inches. Plasterboard comes in 2400 x 1200mm sheets (close to 8 x 4 feet), timber is sold in 300mm increments from 1.8m to 6.0m.

Why are UK sheet materials in metric sizes?

The UK construction industry metricated in the 1970s. Sheet materials (plywood, plasterboard, OSB) are manufactured to metric sizes — 2440 x 1220mm is not exactly 8 x 4 feet (which would be 2438.4 x 1219.2mm) but is close enough to be interchangeable. Some imported products retain exact imperial sizes. Timber is sold by metric cross-section (e.g., 47 x 100mm) but often in imperial-derived lengths (2.4m ≈ 8ft, 3.0m ≈ 10ft).

Metric to Imperial Length Conversions

Quick-reference conversion table for millimetres, centimetres and metres to inches, feet and yards. Common UK construction lengths in both units.

Fundamental Conversion Factors

From	To	Multiply By
Inches	Millimetres	25.4
Millimetres	Inches	0.03937
Feet	Metres	0.3048
Metres	Feet	3.2808
Yards	Metres	0.9144
Metres	Yards	1.0936

Common Construction Lengths

Metric	Imperial	Common Use
6mm	1/4"	Plywood thickness, tile spacer
10mm	3/8"	Mortar joint thickness
12.5mm	1/2"	Plasterboard thickness
15mm	~5/8"	Copper pipe OD
22mm	~7/8"	Copper pipe OD
25mm	~1"	Timber batten thickness
50mm	~2"	Insulation board, timber thickness
100mm	~4"	Block width, concrete slab
150mm	~6"	DPC height, joist depth
215mm	~8.5"	Brick length (standard UK)
300mm	~12" (1ft)	Rule of thumb: 300mm ≈ 1 foot
600mm	~24" (2ft)	Standard joist/stud spacing
900mm	~36" (3ft)	Door width, worktop height
1200mm	~4ft	Sheet material width
1800mm	~6ft	Fence panel height, shortest timber
2100mm	~7ft	Standard door height
2400mm	~8ft	Sheet material length, ceiling height
3000mm	~10ft	Common timber length
3600mm	~12ft	Common timber length
4800mm	~16ft	Long timber length