From this site

One project, every trade

Each calculator adds its lines to a single estimate — consolidated BOM, schedule and cash-flow included.

Open My Project →

Millimetres to Inches Calculator

Convert millimetres to decimal inches for working from metric drawings on imperial tools.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Length

11.81 in

High confidence

1 mm = 1/25.4 inch, the exact inverse of the defined inch. Round only at the end — rounding each dimension before calculating compounds error through a build-up.

Conversion factor applied
0.04 in per mm

Metric drawings reaching an imperial site create this need daily, and the useful skill is converting the decimal result into something readable on a tape. Multiply the decimal part by sixteen and round: 0.42 inches becomes roughly seven-sixteenths. Two things are worth internalising. Three hundred millimetres is not a foot but 11.81 inches, because the metric module was chosen for convenience rather than equivalence, and that 1.6 percent gap accumulates conspicuously across a room's worth of setting out. And rounding belongs at the end of the calculation, never partway through: rounding every dimension to the nearest sixteenth before adding a build-up together produces a cumulative error far larger than any single rounding.

Add the equipment this sizes

This result is a specification — 11.811 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • 1 in = 25.4 mm exactly, so 1 mm = 0.0393700787... in

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

How do I turn the decimal into a tape fraction?
Multiply the fractional part by sixteen and round. 0.42 inches times sixteen is 6.7, so about seven-sixteenths. For finer work multiply by thirty-two instead, and remember that most site tapes do not resolve below a sixteenth reliably.
Why is 300 mm not exactly a foot?
Because a foot is 304.8 mm. The 300 mm module is a metric convenience size used throughout European construction, and it is about 1.6% smaller than a foot — enough to accumulate visibly over a long run of setting out.
Should I round before or after calculating?
After, always. Rounding each dimension to the nearest sixteenth before multiplying compounds the error across a build-up. Carry the decimals through the arithmetic and round only the final figure you are going to mark out.