Inches to Millimeters Converter
Common Conversions
| in | mm |
|---|---|
| 0.01 | 0.254 |
| 0.1 | 2.54 |
| 0.25 | 6.35 |
| 0.5 | 12.7 |
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 5 | 127 |
| 10 | 254 |
| 25 | 635 |
| 50 | 1270 |
| 100 | 2540 |
Why this conversion matters in chemistry
US-built lab equipment carries inch-based fittings — quarter-inch tubing, half-inch pipe unions, NPT threaded connections — but the glassware and column hardware those fittings have to mate with are usually specified in millimeters. The conversion factor is exact: 1 inch equals 25.4 mm by the 1959 international yard and pound agreement. A 0.25-inch fitting is 6.35 mm. A 6-inch petri dish is 152.4 mm across. The arithmetic is what lets a metric-spec part actually thread into a US-spec fitting on a chromatography or vacuum line without needing an adapter.
Formula
Where the factor comes from
The factor is exact because a standards agreement made it so, not because anyone measured it. The 1959 international yard and pound agreement fixed the yard at exactly 0.9144 m; divide by 36 and the inch falls out at exactly 0.0254 m, or 25.4 mm. Before that, the US and imperial inches were tied to separate national prototypes and disagreed by roughly three parts per million. The United States then kept a second inch alive for land surveying, built on a yard of 3600/3937 m, which works out to 25.400 0508 mm — two parts per million longer than the international inch, and enough to matter over a survey baseline even though it is invisible on any bench. That survey definition was retired at the end of 2022. For laboratory hardware, 25.4 is the only inch in play.
Precision and significant figures
An exact factor printed with three digits still tempts people into treating it as three significant figures. It is not: 25.4 means 25.400 000… and it limits nothing. Every figure comes from the inch value itself. Where the fraction has a power of two underneath, the millimeter result terminates exactly — 1/4 in is 6.35 mm, 1/16 in is 1.5875 mm, 1/64 in is 0.396875 mm — so binary fractional hardware sizes convert with no rounding at all. Decimal inch specifications behave differently, since 0.250 in carries three figures and 6.35 mm is the honest answer rather than 6.350. What the shop floor delivers is separate again: a micrometer reads 0.001 mm, a caliper 0.01 mm, and a nominal size is a name.
Worked Examples
The defining identity — one inch is exactly 25.4 mm by international agreement.
Half-inch tubing diameter — a common spec for laboratory drain lines and vacuum manifolds.
Quarter-inch fitting — the standard tubing size for many gas-line and instrument connections.
A six-inch Petri dish, expressed in the millimeter units the rest of the lab inventory uses.
Common mistakes
Nominal pipe size read as a diameter
A 1/4 in NPT fitting is not 6.35 mm anywhere a caliper can reach it. Nominal thread and pipe designations are names inherited from wall thicknesses that stopped applying long ago, so converting the label to millimeters produces a number that will not match the hardware in your hand. Work from the manufacturer's stated outer diameter and thread pitch and treat the fraction as an identifier.
Fraction rounded before multiplying
Tubing, drill and wire sizes arrive as fractions, and turning 5/32 in into 0.16 in before multiplying loses more than the conversion ever could: the exact answer is 3.96875 mm while the rounded route gives 4.06 mm. Keep the fraction intact through the multiplication and round once at the end, where whatever tolerance you actually need is visible on the page.
Converted threads assumed to mate
Expressing a thread's dimensions in millimeters does not make it a metric thread. A 1/4-28 UNF and an M6 × 1 are close enough in outer diameter to start by hand and different enough in pitch to bind or strip. The arithmetic tells you the diameters are similar and says nothing about whether the two will mate, which turns on pitch, flank angle and class of fit.