Inches to MM Converter — in to mm

The Conversion: 1 Inch = 25.4 Millimeters

One inch equals exactly 25.4 millimeters. This is not an approximation — it is an exact definition established by the International Yard and Pound Agreement of 1959. To convert inches to millimeters, multiply the inch value by 25.4.

• Inches → Millimeters: Multiply by 25.4 (e.g., 6 in × 25.4 = 152.4 mm)
• Millimeters → Inches: Divide by 25.4 (e.g., 100 mm ÷ 25.4 = 3.937 in)

Quick mental estimate: Multiply inches by 25 and add 1.6% for precision. For rough work: 1 in ≈ 25 mm, so 4 in ≈ 100 mm. This "4 inches = 100 mm" shortcut is handy for quick estimates.

Why exact? Unlike most unit conversions (which are defined ratios), the inch-to-millimeter conversion is mathematically exact. The inch is defined as 25.4 mm — no rounding, no approximation. This makes it one of the cleanest conversions in metrology.

Inches to Millimeters Conversion Table

Common inch measurements converted to millimeters, with practical context:

Why Convert Inches to Millimeters?

Millimeters provide finer resolution than inches for precision work. Where the inch is commonly divided into 16ths (1.5875 mm increments), a millimeter is already a small unit — making it natural for precise measurements without awkward fractions.

• Engineering and manufacturing: CNC machines, 3D printers, and precision tools work in millimeters. A tolerance of ±0.1 mm is straightforward to express; the equivalent ±0.004 inches (±4 thou) is less intuitive for many.
• Woodworking: European and Japanese woodworking traditions use millimeters exclusively. US woodworkers increasingly use metric measurements for their precision. "Cut at 152.4 mm" is more exact than "cut at 6 inches" — though in practice, 152 mm is close enough.
• Screen sizes and technology: Phone screen dimensions, camera sensor sizes, and electronic component specifications use millimeters globally. A 6.7-inch phone screen = 170.18 mm diagonal.
• Medical and dental: Medical devices, implants, and instruments are specified in millimeters worldwide. Surgical precision requires mm-level accuracy.
• Automotive: While US cars use some SAE (inch-based) fasteners, modern vehicles increasingly use metric. A 5/16-inch bolt is 7.94 mm; the closest metric equivalent is 8 mm.

Fractional Inches to Millimeters

US measurements frequently use fractional inches (1/2, 1/4, 1/8, 1/16, 1/32, 1/64). Here's the complete conversion for the most common fractions:

Machinist's tip: Print this table and keep it in your workshop. It's the most-used reference chart in any shop that works with both imperial and metric dimensions.

Inches to MM for Woodworking and Construction

Woodworking is one of the most common contexts for inches-to-millimeters conversion. US lumber dimensions are in inches (nominal and actual), while many precision tools, plans, and hardware use metric measurements.

US lumber actual dimensions in mm:

• 2×4 (actual 1.5" × 3.5"): 38.1 mm × 88.9 mm
• 2×6 (actual 1.5" × 5.5"): 38.1 mm × 139.7 mm
• 2×8 (actual 1.5" × 7.25"): 38.1 mm × 184.15 mm
• 2×10 (actual 1.5" × 9.25"): 38.1 mm × 234.95 mm
• 2×12 (actual 1.5" × 11.25"): 38.1 mm × 285.75 mm
• 4×4 (actual 3.5" × 3.5"): 88.9 mm × 88.9 mm

Plywood thicknesses:

• 1/4" plywood: 6.35 mm (metric equivalent: 6 mm)
• 3/8" plywood: 9.53 mm (metric equivalent: 9 mm)
• 1/2" plywood: 12.70 mm (metric equivalent: 12 mm)
• 3/4" plywood: 19.05 mm (metric equivalent: 18 mm)

Note that metric plywood (often from European or Asian manufacturers) uses round millimeter dimensions (6 mm, 9 mm, 12 mm, 18 mm) that don't exactly match imperial sizes. A 3/4" dado won't perfectly fit 18 mm plywood — there's a 1.05 mm gap. This matters for joinery; shim or adjust your dado blade accordingly.

Fasteners and hardware: Screws sold in the US use inch-based sizing (#6, #8, #10 gauge, with lengths in inches). Metric screws use mm for both diameter and length (e.g., M4 × 30 mm = 4 mm diameter, 30 mm long). A #8 screw is approximately 4.2 mm in diameter — close to M4 but not interchangeable.

Inches to MM for Technology and Screen Sizes

Screen sizes are marketed in inches (diagonal measurement), but physical dimensions, bezels, and case specifications use millimeters. Converting between these is essential for accessory fitting, design work, and technical specifications.

• iPhone 15 Pro: 6.1" display = 154.94 mm diagonal. Body: 146.6 × 70.6 × 8.25 mm.
• 13.3" laptop screen: 337.82 mm diagonal. Typical dimensions: ~294 × 165 mm (16:9).
• 27" monitor: 685.8 mm diagonal. Typical: ~597 × 336 mm (16:9).
• 55" TV: 1,397.0 mm diagonal. Typical: ~1,210 × 680 mm (16:9).

Camera sensors: Sensor sizes mix both systems. A "1-inch sensor" actually measures about 13.2 × 8.8 mm — the "inch" refers to a historical video tube measurement, not the modern inch. A full-frame sensor (based on 35mm film) measures 36 × 24 mm = approximately 1.42 × 0.94 inches.

3D printing: Nozzle sizes (0.4 mm standard), layer heights (0.1–0.3 mm), and part dimensions are all in millimeters. If you're designing a part to fit a 2-inch pipe, you need 50.8 mm inside diameter — not 50 mm, which would be too tight.

Inches to MM for Plumbing and Pipe Sizing

Pipe sizing is one of the most confusing areas for inches-to-mm conversion because pipe sizes are nominal — they don't directly correspond to any actual measurement.

Notice that a "1-inch pipe" doesn't actually measure 25.4 mm (1 inch) in any dimension — the outside diameter is 33.4 mm. This is because nominal pipe sizes were historically based on the inside diameter of thick-walled iron pipes. Modern pipes with thinner walls keep the same outside diameter for compatibility with existing fittings, even though the bore (inside diameter) has changed. Always use the actual OD in mm when ordering metric fittings or adapters for inch-sized pipes.

History of the Inch and the Millimeter

The inch has been in use for over a thousand years. The word comes from the Latin "uncia" (one-twelfth), as the inch was originally defined as one-twelfth of a Roman foot. In medieval England, King Edward II defined the inch as "three grains of barley, dry and round, placed end to end lengthwise" — a charmingly imprecise definition that led to centuries of regional variation.

The millimeter, by contrast, is a product of the French Revolution. The meter was originally defined in 1791 as one ten-millionth of the distance from the North Pole to the Equator along the Paris meridian. The millimeter — one-thousandth of a meter — provided the fine-grained precision that the new scientific age demanded. Today, the meter is defined by the speed of light: the distance light travels in vacuum in 1/299,792,458 of a second.

The two systems were permanently linked in 1959 when the International Yard and Pound Agreement defined the inch as exactly 25.4 millimeters. Before this agreement, the US and British inches differed slightly — the US inch was 25.4000508 mm (based on the 1893 Mendenhall Order) while the British inch was approximately 25.399978 mm. The 1959 agreement eliminated this discrepancy, except that the US continued to use the older definition for land surveying (the "survey inch" = 25.4000508 mm) until 2023, when it was finally retired.

Common Mistakes When Converting Inches to Millimeters

Several pitfalls trip up people converting between inches and millimeters:

• Confusing millimeters and centimeters: 1 inch = 25.4 mm = 2.54 cm. Accidentally using 25.4 when you need cm (or 2.54 when you need mm) gives a result off by a factor of 10.
• Nominal vs actual dimensions: A "2×4" is NOT 2" × 4" (50.8 × 101.6 mm). Its actual dimensions are 1.5" × 3.5" (38.1 × 88.9 mm). Always verify whether a measurement is nominal or actual before converting.
• Pipe sizes: As noted above, pipe sizes are nominal and don't correspond to simple inch-to-mm conversion.
• Screen diagonal vs area: A 15.6" laptop screen has a 396.24 mm diagonal, but the display area depends on aspect ratio. Don't assume screen dimensions from diagonal alone.
• Fractional rounding: When converting 3/8" to mm, the exact answer is 9.525 mm. Rounding to 9.5 mm introduces a 0.025 mm error — acceptable for woodworking but not for precision machining.