Inches to mm Converter: Fractions, Lumber Sizes and Plywood

Most inches to mm converters stop at 25.4. This one is built for the lumber aisle. Type a measurement the way a plan writes it, 15-1/2, 8′ 6″, 2×4, and get the number you can cut to, not just the exact one. Paste a whole cut list and it converts every line. Below the converter there is an inches to millimeters chart in 1/16 steps, a fraction to mm calculator that also runs backwards (mm to the nearest fraction of an inch), and the actual size of every board from a 1×2 to a 6×6 in inches and millimetres.

Everything is free and nothing is gated. If you want it printed for the shop wall, the same tables are on our Lumber Sizes & Metric Chart (PDF).

Inches to mm chart: fractions, decimals and millimetres

The chart most people are looking for: 1/4 inch is 6.35 mm, 3/8 inch is 9.53 mm, 1/2 inch is 12.7 mm, 3/4 inch is 19.05 mm, and 1 inch is 25.4 mm exactly. The full table in 1/16 steps is under “Fraction → mm chart & stock lengths” in the tool above, with the decimal inch beside each fraction so it doubles as a fraction to decimal chart. For a printable inches to mm chart, the same table is on the back of the Lumber Sizes & Metric wall chart, free as a PDF.

Two rules that save cuts: convert to the nearest millimetre for anything that has to fit (part lengths, shelf widths, hole spacing), and round to 5 mm only for gaps and overhangs that nothing else depends on. The converter shows both numbers so you can choose.

mm to inches in fractions: reading a metric number on an imperial tape

A metric plan says 394 mm and your tape says sixteenths. 394 mm is 15.51 inches, which is 15-1/2″ to the nearest 1/16 with 0.3 mm left over, close enough for any cut. Type any millimetre value above and the converter gives the nearest 1/8, 1/16 or 1/32 fraction and tells you how far off that mark is. When the error is over about 0.5 mm, switch to 1/32 or cut to the mm; a joint will feel it.

The reverse works the same way: fractions to mm, mixed numbers to mm, feet and inches to mm. 8′ 6-1/2″ is 2604 mm.

2×4 actual size, in inches and mm (and every other board)

A 2×4 is 1-1/2 by 3-1/2 inches, 38 by 89 mm. A 2×6 is 1-1/2 by 5-1/2 (38 by 140 mm), a 4×4 is 3-1/2 square (89 mm), a 1×4 is 3/4 by 3-1/2 (19 by 89 mm). The nominal size is the rough-sawn size before drying and planing; the actual size is what you buy and what every cut list assumes. The lumber table in the tool lists nominal and actual for 18 common sizes, in inches and millimetres, from 1×2 through 6×6 and 5/4 decking.

Boards still move. A “3-1/2 inch” 2×4 measures 3-3/8 to 3-5/8 depending on moisture, so cut parts to the boards in front of you, not to the drawing.

What replaces a 2×4 outside the US

Multiply 1-1/2 by 3-1/2 by 25.4 and you have the right number and nothing to buy. UK CLS happens to be 38 x 89, so that swap is clean. Australian MGP10 framing is 90 x 45: the same width but 7 mm thicker, which matters anywhere two boards stack. Most of Europe stocks 45 x 95, wider and thicker. South Africa’s SANS sizes skip 89 mm entirely, so a 2×4 becomes a choice between 38 x 76 and 38 x 114. Japan is the easy case: home centres stock SPF two-by at exactly US sizes.

That is why the table above shows a width delta for every size. Any dimension in a plan that is really “one board width” or “two boards plus a gap”, a filler block, a stretcher between two legs, the depth of a bench seat, will come out wrong if you convert the number. Measure it off the boards you bought. Everything else converts straight through.

Plywood: 3/4 inch is not 19 mm

US “3/4 inch” sanded plywood is really 23/32 inch, about 17.9 mm. Metric sheets are sold as 18 mm in the UK and Europe and 17 mm in Australia, so the swap is close but not exact; measure the sheet before you cut a dado to fit it. Sheet sizes differ too: a US 4 x 8 is 1219 x 2438 mm, a UK sheet is 1220 x 2440, an EU sheet 1250 x 2500, Australian 1200 x 2400, and Japanese plywood is 910 x 1820, far smaller than a 4 x 8, so sheet-goods plans need their layouts re-checked there.

Inches to mm FAQ

What is 1 mm in fractions of an inch?

1 mm is 0.039 inches, which is just under 3/64″ and a hair over 1/32″ (0.031″). On a 1/16″ tape it rounds to the nearest mark, 1/16″ (1.59 mm), so treat 1 mm as “about a 32nd” and read it as a rounding tolerance, not a cut.

Does 5 mm equal 1/4 inch?

Not quite. 1/4″ is 6.35 mm; 5 mm is 0.197″, closest to 3/16″ (4.76 mm). The 1.35 mm difference matters for a dado or a drill bit, so use 6 mm or 6.5 mm stock where a plan says 1/4″, and a 6.5 mm bit for a 1/4″ hole.

What is 3/4 inch in mm?

3/4″ is exactly 19.05 mm. Cut to 19 mm. For plywood the label lies both ways: US “3/4” sanded ply is really 23/32″ (17.9 mm), and metric sheets are sold as 18 mm (UK, EU) or 17 mm (Australia).

How do I convert fractions to mm?

Divide the fraction, add the whole inches, multiply by 25.4. 15-1/2″ is 15.5 × 25.4 = 393.7 mm, so cut 394 mm. The converter above does this for any format, including feet and inches, and shows the exact value, the cut-to-the-mm value, and a 5 mm rounding for gaps and overhangs.

Is a 2×4 actually 3.5 inches?

Yes. A US 2×4 is 1-1/2″ × 3-1/2″ (38 × 89 mm) once it is dried and planed. The nominal name comes from the rough-sawn size. Every plan cut list assumes the actual size, and boards still vary about 1/8″ with moisture, so cut to fit.

What size is a 2×4 in mm?

38 × 89 mm. In the UK that is exactly a CLS 38 × 89 stud. In Australia the stocked size is 90 × 45 MGP10 (7 mm thicker). In most of Europe it is 45 × 95. South Africa has no 89 mm width; builders use 38 × 114 or 38 × 76. Pick your region above and the table shows the swap for every size.

Is a 2x4x8 actually 8 feet long?

Usually yes, and often a touch over: length is not shortened the way width and thickness are, and studs are sold in trimmed lengths like 92-5/8″. The metric catch is the other way round: the 2.4 m board that replaces an 8-footer is 2438 mm minus 2400 mm = 38 mm shorter. Check your longest cut before you buy.

Why has a 2×4 gotten smaller?

The 2 × 4 name dates from rough-sawn green lumber. Modern boards are dried and planed after sawing, which takes off about 1/4″ each way, and the standard (PS 20) fixed the dressed size at 1-1/2″ × 3-1/2″ in the 1960s. Plans are written to that dressed size, which is why the mm column here is 38 × 89 and not 51 × 102.

Skip the converting: plans with a metric edition

Our newer plans ship with a metric edition already worked out: cut lists rounded to sizes you can buy, the board substitutions made, and the dependent cuts flagged. Every plan page says whether one is included. Browse the plans. And once a cut list is converted, the free Cut-List Optimizer lays it out on your boards and sheets.