Fractions, decimals, and millimetres: the metal buyer's conversion chart

How do you convert inch fractions to decimals and millimetres?

Divide the top of the fraction by the bottom and you have the decimal: 3/8″ is 3 divided by 8, so 0.375″. Multiply inches by 25.4 and you have millimetres: 0.375″ becomes 9.53 mm. That 25.4 isn't an approximation, either. Canada and five other countries signed the 1959 agreement that defines the inch as exactly 25.4 mm, so this chart can't go stale.

Going the other way, divide millimetres by 25.4: 10 mm is 0.3937″. The chart below covers every 16th and 32nd up to 1″, plus every 64th up to 1/2″. That's where 64ths earn their keep; below 1/2″ they're drill sizes, and above it nobody dimensions in 64ths. Print it and tape it to the saw.

Four anchors are worth memorizing: 1/8″ is 3.175 mm, 1/4″ is 6.35 mm, 1/2″ is 12.7 mm, and 1″ is 25.4 mm. All four are exact. The chart rounds everything else to two decimal places, so the exact 3.175 shows as 3.18. A hundredth of a millimetre is already finer than anything a saw or a tape can hold.

How do you read the marks on a tape measure?

Tick length is the whole code. The longest lines carry the numbers: whole inches. The next longest, halfway between, is the half inch. Then come the quarters, then the eighths, and the shortest ticks on a standard tape are sixteenths. Sixteen small steps per inch, and every length on the tape is some count of them.

To read an odd mark, start at the last numbered line and count the smallest ticks past it: three small ticks past 5″ is 5-3/16″. The tick you land on names the denominator: half marks are /2, the next size down /4, then /8, then /16. Most tapes stop at sixteenths. If a drawing calls out 64ths or thousandths, that's caliper work, not tape work.

For a cut list headed our way, sixteenths are plenty. Our cuts hold ±1/8″, so chasing a 64th with a tape is precision you can't use. Write each fraction down the moment you read it; a measurement trusted to memory is how a 15-3/8″ leg comes back as 15-5/8″.

What if your drawing is in millimetres and your stock is imperial?

Divide by 25.4 and shop in inches. For brackets, frames, jigs, and anything getting welded or painted, buy the closest imperial size and move on; half a millimetre of diameter won't change how a gate swings. Here's how the common metric callouts land on an imperial rack:

The swap stops working when the number is a fit. A shaft sliding into a 10 mm bearing, a pin in a metric clevis, a rod running in a metric seal: those need true metric stock. A 3/8″ stand-in runs half a millimetre slim, and the part will rattle or leak.

We stock some true metric for exactly those jobs. Our induction-hardened chrome rod (1045 steel under the plating) runs 20 mm to 120 mm on the chrome rack, the stock hydraulic cylinder rebuilds want. A 20 mm rod is $20.30 CAD for a 12″ piece as of June 2026. O1 drill rod comes metric too, 2 mm up to 14 mm in fixed 36″ bars, with the 3 mm bar at $3.95 CAD as of June 2026. Metric sheet, tube, and anything else we don't rack is a sourcing job: 2-21 days, priced before you commit. Sheet thickness runs on a third system entirely, gauge numbers, and our sheet metal gauge chart untangles those.

Why do drill charts mix fractions, numbers, and letters?

Four sizing families grew up separately, and all four stuck. Fractional bits step in 64ths. Number bits run #80 to #1, covering 0.0135″ to 0.228″, with the numbering backwards: bigger number, smaller bit. Letter bits pick up from there, A (0.234″) to Z (0.413″). Metric bits thread through the lot.

The families exist to fill each other's gaps. A fractional set jumps from 15/64″ straight to 1/4″; letter bits B, C, and D (0.238″, 0.242″, 0.246″) live inside that jump. It's why tap charts call out odd sizes like a #7 bit for a 1/4-20 tap. Don't memorize any of it. The tap-drill chart in our drilling and tapping guide names the right bit for every common tap and lists fractional substitutes when you don't own number or letter bits.

How do you add fractions without a calculator?

Convert everything to 16ths, add the 16ths, then add the whole inches. Say one bracket leg is 3-5/8″ and the other is 2-13/16″. 5/8 is 10/16, so the fractions add to 10 plus 13, which is 23/16, which is 1-7/16″. Wholes: 3 plus 2 plus 1 makes 6. Total: 6-7/16″. No calculator, no decimal round trip.

Two helpers speed it up. Getting to 16ths is just doubling: eighths double once (5/8 is 10/16), quarters double twice (3/4 is 12/16), a half is 8/16. And halving any fraction is doubling its bottom: half of 5/8″ is 5/16″, half of 3/4″ is 3/8″. Centre lines and symmetric layouts fall out of that one trick.

How do you order an odd length like 17-1/2 inches?

Type 17.5. Our cut-to-length bar pages have a box labelled Cut to size (inches): enter your length as a decimal, the price updates, you pick how many pieces, and that's the order. The box works in quarter-inch steps from 1″ up to 96″, so the only conversions you'll ever need at the order screen are .25, .5, and .75. Here's how buying by the inch works end to end.

Working from a 16ths cut list? Round each length to the nearest quarter inch when you order; cuts hold ±1/8″ either way. If one dimension truly matters, order the next quarter up and trim it yourself. Metric lengths convert like everything else here: 500 mm is 19.69″, so type 19.75.

Need a cut past 96″, a metric length held exact, or a size that isn't on the rack? Send a quote request with the numbers in whatever units your drawing uses. We cut up to 21 ft by special request, source odd stock in 2-21 days, and you'll see the price before you commit.