Instant Inches ↔ Millimetres Conversion – Decimals, Fractions & Feet
Enter a value in any field below and click Convert. You can enter decimal inches (e.g. 2.5), fractional inches (e.g. 3/8), or feet and inches (e.g. 5 feet 6 inches). Results are shown to 4 decimal places where relevant.
The conversion between inches and millimetres is exact and defined by international agreement. Since 1959, the international inch has been defined as exactly 25.4 millimetres, making the conversion between the two units perfectly precise with no rounding error. This means that any number of inches can be converted to millimetres simply by multiplying by 25.4, and any number of millimetres can be converted to inches by dividing by 25.4.
This exact definition is important in engineering and manufacturing, where measurements must be converted between imperial and metric systems without introducing errors. A measurement of, say, 3/8 inch is exactly 9.525 mm — not approximately 9.5mm or approximately 10mm. Using the exact conversion factor ensures precision is maintained across unit systems.
The table below shows every standard imperial fraction of an inch (in 1/64 inch increments up to 1 inch) converted to its exact millimetre equivalent. This chart is widely used in engineering, woodworking, metalworking, and construction where imperial measurements must be cross-referenced with metric specifications.
The table below covers commonly used inch measurements from 1/16 inch up to 72 inches (6 feet), with exact millimetre equivalents and centimetre equivalents for quick reference.
| Inches | Fraction | Exact mm | Rounded mm | cm | Metres |
|---|---|---|---|---|---|
| 1/16″ | 0.0625″ | 1.5875 | 2 mm | 0.159 cm | 0.002 m |
| 1/8″ | 0.125″ | 3.175 | 3 mm | 0.318 cm | 0.003 m |
| 1/4″ | 0.25″ | 6.350 | 6 mm | 0.635 cm | 0.006 m |
| 3/8″ | 0.375″ | 9.525 | 10 mm | 0.953 cm | 0.010 m |
| 1/2″ | 0.5″ | 12.700 | 13 mm | 1.270 cm | 0.013 m |
| 5/8″ | 0.625″ | 15.875 | 16 mm | 1.588 cm | 0.016 m |
| 3/4″ | 0.75″ | 19.050 | 19 mm | 1.905 cm | 0.019 m |
| 7/8″ | 0.875″ | 22.225 | 22 mm | 2.223 cm | 0.022 m |
| 1″ | 1.0″ | 25.400 | 25 mm | 2.540 cm | 0.025 m |
| 1.5″ | 1½″ | 38.100 | 38 mm | 3.810 cm | 0.038 m |
| 2″ | 2″ | 50.800 | 51 mm | 5.080 cm | 0.051 m |
| 2.5″ | 2½″ | 63.500 | 64 mm | 6.350 cm | 0.064 m |
| 3″ | 3′ | 76.200 | 76 mm | 7.620 cm | 0.076 m |
| 4″ | 4″ | 101.600 | 102 mm | 10.160 cm | 0.102 m |
| 5″ | 5″ | 127.000 | 127 mm | 12.700 cm | 0.127 m |
| 6″ | 6″ | 152.400 | 152 mm | 15.240 cm | 0.152 m |
| 7″ | 7″ | 177.800 | 178 mm | 17.780 cm | 0.178 m |
| 8″ | 8″ | 203.200 | 203 mm | 20.320 cm | 0.203 m |
| 9″ | 9″ | 228.600 | 229 mm | 22.860 cm | 0.229 m |
| 10″ | 10″ | 254.000 | 254 mm | 25.400 cm | 0.254 m |
| 11″ | 11″ | 279.400 | 279 mm | 27.940 cm | 0.279 m |
| 12″ (1ft) | 12″ | 304.800 | 305 mm | 30.480 cm | 0.305 m |
| 18″ (1½ft) | 18″ | 457.200 | 457 mm | 45.720 cm | 0.457 m |
| 24″ (2ft) | 24″ | 609.600 | 610 mm | 60.960 cm | 0.610 m |
| 30″ (2½ft) | 30″ | 762.000 | 762 mm | 76.200 cm | 0.762 m |
| 36″ (3ft / 1yd) | 36″ | 914.400 | 914 mm | 91.440 cm | 0.914 m |
| 39.37″ | ≈39⅓″ | 1000.000 | 1000 mm | 100.000 cm | 1.000 m |
| 48″ (4ft) | 48″ | 1219.200 | 1219 mm | 121.920 cm | 1.219 m |
| 60″ (5ft) | 60″ | 1524.000 | 1524 mm | 152.400 cm | 1.524 m |
| 72″ (6ft) | 72″ | 1828.800 | 1829 mm | 182.880 cm | 1.829 m |
Since 1959, the international inch is defined as exactly 25.4 millimetres by international agreement between the UK, USA, Canada, Australia, New Zealand, and South Africa. This makes the conversion perfectly precise — there is no rounding or approximation in the conversion factor itself.
The single most important conversion to remember: 1 inch = 25.4 mm exactly. From this, all other inch-to-mm conversions follow by multiplication. Common approximation: 1 inch ≈ 25 mm (0.4 mm error, acceptable for rough estimates). For precision work, always use 25.4.
One foot (12 inches) equals exactly 304.8 mm or approximately 30.5 cm. This is an essential reference for construction, carpentry, and plumbing work in the UK where imperial and metric measurements are frequently used side by side.
One metre equals exactly 1000 ÷ 25.4 = 39.3701 inches, or approximately 39 and 3/8 inches. Equivalently, 1 metre = 3 feet 3.37 inches. This is useful for converting metric room dimensions to approximate imperial equivalents.
Despite officially adopting the metric system, the UK still uses imperial measurements in many contexts in 2026: road distances (miles), vehicle speeds (mph), beer and cider (pints), body height (feet and inches), body weight (stones and pounds), and in many construction trades where pipe sizes, timber, and fixings are still quoted in inches.
In the UK plumbing and heating industry, pipe sizes are still commonly referred to in inches even though the actual measured diameter is in millimetres. A ¾″ copper pipe has an actual outside diameter of 22mm; a ½″ pipe has an OD of 15mm. The nominal inch sizes are historical and do not directly correspond to the 25.4mm conversion.
Television, monitor, and phone screen sizes are universally quoted in inches (the diagonal measurement) even in countries that otherwise use metric. A 55-inch TV has a screen diagonal of 55 × 25.4 = 1,397 mm (139.7 cm). Similarly, laptop screens (13″, 15″, 17″) and phone screens are specified in inches.
UK timber merchants commonly quote timber sections in millimetres for sawn timber (e.g. 100×50mm) but in inches for planed/regularised timber derived from imperial stock (e.g. 4×2, 6×2). Sheet materials (plywood, OSB, plasterboard) are typically 2400×1200mm but are also described as 8×4 sheets (8 feet × 4 feet = 2438×1219mm — the metric sheet is slightly smaller).
UK drill bits, screws, and fixings are sold in both metric (mm) and imperial (inch/fraction) sizes, and the two systems do not align precisely. Common imperial drill sizes such as 1/16″ (1.59mm), 3/32″ (2.38mm), 1/8″ (3.18mm), and 3/16″ (4.76mm) do not correspond to standard metric drill sizes (1.5mm, 2mm, 2.5mm, 3mm, etc.).
Car tyre sizes use a mixed metric/imperial notation: e.g. 205/55 R16 means 205mm wide, 55% aspect ratio, on a 16-inch rim diameter. The rim diameter is still specified in inches universally in the automotive industry, even in countries that otherwise use metric.
Print sizes, photo frame sizes, and sensor sizes are commonly quoted in inches: 4×6″, 5×7″, 8×10″ prints; 1/1.7″ or 1″ camera sensors. Printer resolution is specified in DPI (dots per inch) and image resolution in PPI (pixels per inch) universally, making inch-to-mm conversion important for print layout work.
The United States remains the only major industrialised nation that has not fully adopted metric measurements for everyday use. All US construction, building codes, and everyday measurements use inches, feet, and yards. This makes inch-to-mm conversion particularly important when working with US-sourced drawings, materials, or products.
| Imperial Measurement | Exact mm | Practical Use / Notes |
|---|---|---|
| 1/16 inch | 1.588 mm | Smallest common imperial fraction; thin sheet metal, fine tolerances |
| 1/8 inch | 3.175 mm | Common gap / clearance; timber biscuit joiner slots; grout joint |
| 3/16 inch | 4.763 mm | Common bolt and drill size; welding gap; carpet tack strip |
| 1/4 inch | 6.350 mm | Common bolt, dowel, and drill size; 1/4″ plywood; quarter-turn fittings |
| 5/16 inch | 7.938 mm | 5/16″ bolts, nuts, and fixings; deck screws; roofing nails |
| 3/8 inch | 9.525 mm | Common bolt, pipe, and drill size; 3/8″ copper pipe OD = 9.52mm |
| 1/2 inch | 12.700 mm | Half-inch pipe, bolts, OSB; common across plumbing and construction |
| 5/8 inch | 15.875 mm | 5/8″ plywood; standard drywall screw length reference; conduit |
| 3/4 inch | 19.050 mm | 3/4″ plywood; common copper pipe; standard timber flooring thickness |
| 1 inch | 25.400 mm | The fundamental conversion reference — 1″ = exactly 25.4mm |
| 1-1/4 inch | 31.750 mm | Waste pipe diameter (nominal 32mm actual); common fitting size |
| 1-1/2 inch | 38.100 mm | Waste pipe (nominal 40mm); door/window rebate; timber PAR thickness |
| 2 inches | 50.800 mm | Nominal 2″ pipe; 50mm EPS insulation; standard plinth block |
| 3 inches | 76.200 mm | Nominal 3″ soil pipe (75mm); roof tile batten gauge; standard door thickness |
| 4 inches | 101.600 mm | 4″ soil pipe (110mm nominal); brick + mortar course = 75mm; 4″ duct |
| 6 inches | 152.400 mm | 6″ half-brick wall; soil and vent pipe; standard concrete block length ÷ 2 |
| 8 inches | 203.200 mm | Standard US CMU block width; 8″ fan/duct diameter; stair riser reference |
| 12 inches (1 foot) | 304.800 mm | One foot; floor tile reference size; standard ceiling tile |
| 18 inches | 457.200 mm | Standard kitchen base unit depth; 18″ scaffold board width |
| 24 inches (2 feet) | 609.600 mm | Standard wall unit depth; stud spacing (US framing); fascia width |
| 36 inches (3 feet / 1 yard) | 914.400 mm | Standard door width reference; worktop depth; 1 yard |
| 48 inches (4 feet) | 1219.200 mm | Half of 8ft sheet; standard pallet height; worktop length module |
| 60 inches (5 feet) | 1524.000 mm | Standard bath length; fence post height above ground; 5ft rule |
| 72 inches (6 feet) | 1828.800 mm | Standard door height; 6ft fence panel; person’s height reference |
| 96 inches (8 feet) | 2438.400 mm | Standard sheet material length (plywood, OSB, plasterboard = 2440mm) |
There are exactly 25.4 millimetres in one inch. This is an exact, defined conversion — not an approximation. Since 1959 the international inch has been defined as exactly 25.4mm by international agreement. So 1 inch = 25.4mm, 2 inches = 50.8mm, 3 inches = 76.2mm, and so on. To convert any number of inches to millimetres, simply multiply by 25.4. To convert millimetres to inches, divide by 25.4.
Multiply the number of inches by 25.4. For example: 6 inches × 25.4 = 152.4mm. For fractional inches, first convert the fraction to a decimal (e.g. 3/8 = 0.375), then multiply by 25.4: 0.375 × 25.4 = 9.525mm. For a measurement given in feet and inches, first convert to total inches (multiply feet by 12 and add the remaining inches), then multiply by 25.4. For example, 5 feet 6 inches = (5 × 12) + 6 = 66 inches × 25.4 = 1,676.4mm.
Divide the number of millimetres by 25.4. For example: 100mm ÷ 25.4 = 3.937 inches (approximately 3 and 15/16 inches). 500mm ÷ 25.4 = 19.685 inches (approximately 19 and 11/16 inches). To express the result as feet and inches: divide by 25.4 to get total inches, then divide by 12 to get feet (whole number), and the remainder is the inches. For example, 1800mm ÷ 25.4 = 70.866 inches = 5 feet 10.866 inches ≈ 5 feet 10 and 7/8 inches.
One quarter inch (1/4″) is exactly 6.35mm (0.25 × 25.4 = 6.35). In practice this is often rounded to 6mm for metric specifications, but the exact value is 6.35mm. Other common quarter-inch multiples: 1/2″ = 12.7mm, 3/4″ = 19.05mm, 1″ = 25.4mm.
Half an inch (1/2″) is exactly 12.7mm (0.5 × 25.4 = 12.7). This is commonly approximated as 13mm in metric specifications. A half-inch is one of the most frequently encountered imperial measurements in UK plumbing, joinery, and fixings.
Three quarters of an inch (3/4″) is exactly 19.05mm (0.75 × 25.4 = 19.05). Commonly approximated as 19mm in metric specifications. Three-quarter inch is a very common size for timber boards (typically sold as 19mm PAR in the UK), copper pipe OD, and many fixings and fittings.
One inch is exactly 25.4mm. This is the fundamental conversion factor from which all other inch-to-mm conversions are derived. The value 25.4 is exact — defined, not measured — so there is no rounding error in this conversion.
Two inches is exactly 50.8mm (2 × 25.4 = 50.8). Sometimes approximated as 50mm in metric specifications, with a 0.8mm error. In plumbing, a “2-inch” pipe refers to a nominal bore size — the actual outside diameter will differ from 50.8mm.
Six inches is exactly 152.4mm (6 × 25.4 = 152.4). Often rounded to 152mm in practice. Six inches is also half of one foot (304.8mm). In UK construction, 150mm is a commonly used metric dimension that is close to (but not exactly) 6 inches.
First convert 5 feet 10 inches to total inches: (5 × 12) + 10 = 70 inches. Then multiply by 25.4: 70 × 25.4 = 1,778mm exactly (or 1.778 metres). This is approximately the average height of an adult male in the UK. As a check: 6 feet = 1,828.8mm, and 5 feet 10 inches is 2 inches (50.8mm) less: 1,828.8 − 50.8 = 1,778mm. ✓
This is a common source of confusion in UK plumbing. Pipe sizes are nominal designations inherited from historical pipe manufacturing standards — they do not represent the actual measured diameter of the pipe, and they do not correspond to the exact 25.4mm/inch conversion. For example, a ¾″ (three-quarter inch) copper pipe has an actual outside diameter of 22mm, not 19.05mm (which is the exact metric equivalent of 3/4″). Similarly, a ½″ copper pipe has an OD of 15mm, not 12.7mm. These nominal sizes are historical artefacts from the era when pipe dimensions were measured differently. Always verify the actual physical dimensions of pipes and fittings rather than relying on the nominal conversion.
The easiest memory aid is: “One inch is roughly the width of your thumb — about 25mm.” More precisely, remember that 4 inches ≈ 100mm (exact: 101.6mm) and 12 inches = 304.8mm. From these anchors you can estimate other values quickly. For calculations, always use the exact factor of 25.4. A useful check: if your answer is roughly 25 times the number of inches, you are in the right ballpark. If converting mm to inches and your answer is roughly 1/25th of the mm value, that is approximately correct (the true factor is 1/25.4).
Related conversion tools and references: National Physical Laboratory (NPL) – UK Measurement Standards | BIPM – International Bureau of Weights and Measures | GOV.UK – UK Weights and Measures