Length Tool

Inches to Millimeters
(in to mm) Converter

Enter whole or fractional inches — get an exact mm result instantly, with formula, worked examples, and a complete reference table

From — Enter Value

To — Result

Formula will appear here

Your result will appear here

⚡ Common Fractions — tap to convert instantly

⅛ inch

¼ inch

⅜ inch

½ inch

⅝ inch

¾ inch

⅞ inch

1 inch

1 in → mm

2 in → mm

6 in → mm

12 in → mm

24 in → mm

36 in → mm

100 mm → in

1 ft → mm

📖 Complete Guide

Inches to Millimeters (in to mm): The Complete Conversion Guide

One number separates two measurement worlds: 25.4. That is the exact number of millimeters in one inch — not an approximation, not a rounded value, but an internationally agreed definition that has been exact since 1959. Whether you are converting a screen diagonal, a pipe diameter, a wood thickness, or a bolt specification, this guide gives you everything you need to convert inches to millimeters accurately, instantly, and with confidence.

Why Convert Inches to Millimeters?

Inches are the primary length unit in the United States customary system and are still widely used in the UK and Canada for everyday measurements. Millimeters are part of the International System of Units (SI), used by science, engineering, manufacturing, and medicine worldwide.

The two systems constantly collide. A US-made drill bit is labelled in inches; the CNC machine program expects millimeters. A screen size is advertised in inches; the engineering drawing is in mm. A patient's height is recorded in feet and inches; the medical chart uses centimeters and millimeters. Every day, in millions of contexts, someone needs to bridge these two systems — and the bridge is a single multiplication by 25.4.

  The key fact: 1 inch = exactly 25.4 mm. This is not rounded. It is the internationally agreed exact definition. Every inch-to-mm conversion is simply multiplication by 25.4 — no rounding, no approximation.

The Exact Formula: in to mm

There is only one formula to remember, and it has been exact since 1959:

Official Conversion Formula

mm = inches × 25.4

Reverse: inches = mm ÷ 25.4 | 1 inch = exactly 25.4 millimeters

To go the other way — millimeters back to inches — simply divide by 25.4. There are no other constants, no lookup tables, no approximations. Just one number: 25.4.

Why 25.4 exactly?

Before 1959, countries used slightly different definitions of the inch. The British inch was 25.399978 mm. The US survey inch was 25.400050 mm. Scientists and engineers working across borders constantly ran into tiny but compounding errors. In 1959, six nations — the USA, UK, Canada, Australia, New Zealand, and South Africa — signed the International Yard and Pound Agreement, defining the international inch as exactly 25.4 mm. The choice of 25.4 was a compromise that minimized the difference from all existing national definitions. Since then, the conversion has been exact by definition.

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Mental shortcut: Memorize 25.4 as "two and a half centimeters plus a tiny bit." One inch ≈ 2.54 cm = 25.4 mm. For quick estimates: 4 inches ≈ 100 mm (exact: 101.6 mm). That's close enough for rough checks.

Worked Examples: in to mm Step by Step

Let's work through the most common inch values you'll actually encounter — from screen sizes to pipe fittings to standard stock sizes.

Example 1

1 in × 25.4

= 25.4 mm

Example 2

2 in × 25.4

= 50.8 mm

Example 3

3.5 in × 25.4

= 88.9 mm

Example 4

6 in × 25.4

= 152.4 mm

Example 5

12 in × 25.4

= 304.8 mm

Example 6

0.5 in × 25.4

= 12.7 mm

How to multiply by 25.4 in your head

25.4 is awkward to multiply mentally — unlike multiplying by 1,000. Here's a practical mental method that works without a calculator:

1Multiply your inch value by 25 (easier: multiply by 100, then halve it, then halve again)
2Multiply your inch value by 0.4 (multiply by 4, then divide by 10)
3Add the two results. Example: 7 inches → 7 × 25 = 175, then 7 × 0.4 = 2.8, total = 177.8 mm

Fractional Inches to mm — The Complete Reference

Imperial measurements use fractions constantly: ½ inch bolts, ¾ inch pipe, 1⅝ inch crown molding. Here's how fractions map to millimeters — the values every carpenter, plumber, and machinist needs.

How to convert fractional inches

1Convert the fraction to a decimal: divide the top number by the bottom. Example: 3/8 → 3 ÷ 8 = 0.375
2Multiply by 25.4: 0.375 × 25.4 = 9.525 mm
3For mixed numbers like 2⅜: convert the fraction first (3/8 = 0.375), add the whole number (2.375), then multiply: 2.375 × 25.4 = 60.325 mm

⅛ inch

0.125 in

3.175 mm

¼ inch

0.250 in

6.350 mm

⅜ inch

0.375 in

9.525 mm

½ inch

0.500 in

12.700 mm

⅝ inch

0.625 in

15.875 mm

¾ inch

0.750 in

19.050 mm

⅞ inch

0.875 in

22.225 mm

1 inch

1.000 in

25.400 mm

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Important for plumbing and piping: "½ inch pipe" does NOT have an outside diameter of 12.7 mm. Pipe sizes are nominal — historical labels that don't match actual dimensions. A nominal ½ inch pipe has an OD of approximately 21.3 mm. Always verify actual pipe dimensions from a technical data sheet, not by converting the nominal size.

Complete Inches to mm Reference Table

A full lookup table from 1/16 inch through 36 inches, with real-world references so each value is concrete.

Inches (in)	Millimeters (mm)	Real-World Reference
1/16 in (0.0625)	1.588 mm	Thickness of a credit card (0.76 mm) doubled
1/8 in (0.125)	3.175 mm	Standard PCB board thickness
3/16 in (0.1875)	4.763 mm	Common drywall screw shank diameter
1/4 in (0.25)	6.350 mm	Standard headphone jack diameter
3/8 in (0.375)	9.525 mm	Common bolt size in machinery
1/2 in (0.5)	12.700 mm	Standard wall outlet hole width
5/8 in (0.625)	15.875 mm	Standard drywall thickness
3/4 in (0.75)	19.050 mm	Standard plywood facing thickness
1 in	25.400 mm	Width of a standard USB-A port area
1.5 in	38.100 mm	Standard 1×2 lumber actual width
2 in	50.800 mm	Standard 2×4 lumber actual width
3 in	76.200 mm	Standard PVC drain pipe diameter
4 in	101.600 mm	Standard floor drain opening
6 in	152.400 mm	Standard ruler half-length
8 in	203.200 mm	Letter paper short side (8.5 in = 215.9 mm)
10 in	254.000 mm	Small tablet screen diagonal
12 in (1 ft)	304.800 mm	Standard 12-inch ruler full length
15 in	381.000 mm	Standard laptop screen diagonal (common)
18 in	457.200 mm	Standard kitchen cabinet depth
24 in (2 ft)	609.600 mm	Standard desk keyboard clearance depth
36 in (3 ft / 1 yd)	914.400 mm	Standard door width (US)
48 in (4 ft)	1,219.200 mm	Standard plywood sheet width
72 in (6 ft)	1,828.800 mm	Standard interior door height
96 in (8 ft)	2,438.400 mm	Standard ceiling height (US construction)

Common Screen Sizes in Inches and Millimeters

Screen size is one of the most common everyday reasons people need to convert inches to millimeters. Display diagonals are universally quoted in inches, but when comparing dimensions physically — fitting a monitor into a cabinet, choosing a phone for a case, or specifying a display in a technical document — millimeters are what you need.

Screen Size	Diagonal (mm)	Common Device
4.7 inches	119.4 mm	iPhone SE (3rd gen)
5.4 inches	137.2 mm	iPhone 12 mini
6.1 inches	154.9 mm	iPhone 15, Galaxy S23
6.7 inches	170.2 mm	iPhone Plus / Pro Max
7.6 inches	193.0 mm	Galaxy Z Fold (unfolded)
10.9 inches	276.9 mm	iPad (10th gen)
13 inches	330.2 mm	MacBook Air / Pro 13"
15.3 inches	388.6 mm	MacBook Air 15"
24 inches	609.6 mm	Standard desktop monitor
27 inches	685.8 mm	Common gaming / design monitor
32 inches	812.8 mm	Large desktop / small TV
55 inches	1,397.0 mm	Mid-size living room TV
65 inches	1,651.0 mm	Large living room TV

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Note: Screen diagonal in mm is the advertised inch × 25.4. But the actual physical screen width and height depend on the aspect ratio. A 27-inch 16:9 monitor has a width of about 597 mm and height of 336 mm — not 685.8 mm. The diagonal is just one measurement.

Where Is in to mm Conversion Actually Used?

The inch-to-millimeter conversion crosses more industries than almost any other unit conversion — because the inch is deeply embedded in US product manufacturing while the rest of the technical world uses millimeters.

🔧

Woodworking & Carpentry

US lumber is specified in inches (2×4, 1×6), but European and professional woodworking tools use millimeter scales. Converting board thickness and dado widths correctly determines whether joints fit.

🔩

Fasteners & Hardware

US bolts, screws, and nuts are in fractional inches. Metric equivalents are in millimeters. A ¼-20 bolt has a 6.35 mm shank — but the nearest metric equivalent (M6) is 6.0 mm. They are not interchangeable.

🖥️

Electronics & Displays

Screen diagonals, PCB dimensions, connector sizes, and enclosure cutouts mix inches and mm constantly. A standard 3.5-inch hard drive bay is 101.6 mm wide — critical for case compatibility.

🏗️

Construction & Renovation

US building materials (drywall, lumber, pipe) are in inches. European and imported fixtures are in mm. Renovating with mixed-source materials means converting constantly.

⚙️

Machining & Manufacturing

Drill bits, end mills, and tooling come in both inch and metric sizes. CNC programs may need both — a G-code file in millimeters but tooling specified in inches. Exact conversion prevents scrap.

👟

Footwear & Apparel

US shoe sizes relate to foot length in inches. European sizes (Mondopoint) use millimeters. A US men's size 10 corresponds to approximately 279 mm foot length. Understanding the conversion prevents sizing errors.

Inches vs Millimeters: Key Differences

Property	Inch (in)	Millimeter (mm)
System	US customary / Imperial	SI (metric)
Used primarily in	USA, some UK/Canada uses	Worldwide (science, engineering, medicine)
Subdivision	Fractions (½, ¼, ⅛…)	Decimal (0.5, 0.25, 0.1…)
Engineering drawings	US-only drawings	✓ ISO/international standard
Consumer electronics specs	✓ Diagonal screens	Internal dimensions
Medical use	Rarely (some US clinical)	✓ Global standard
Decimal math	Awkward (fractions)	✓ Clean decimals

Common Mistakes When Converting Inches to mm

Mistake 1: Using 25 instead of 25.4

A common approximation is "1 inch ≈ 25 mm." For rough estimates this is fine — the error is only 1.57%. But for engineering, manufacturing, or any precision work, the error compounds fast. At 12 inches, using 25 instead of 25.4 gives 300 mm instead of the correct 304.8 mm — a 4.8 mm error, enough to ruin a machined part or a furniture joint.

Mistake 2: Not converting fractional inches first

People often type "3/4 inch" into a calculator and get a division result of 0.75, then forget to multiply by 25.4. The correct workflow: convert fraction to decimal first (3/4 = 0.75), then multiply (0.75 × 25.4 = 19.05 mm). Our tool handles this automatically — just enter decimals like 0.75.

Mistake 3: Confusing nominal and actual pipe sizes

As mentioned earlier, pipe sizes in inches are nominal — they do not equal the actual diameter in any direct calculation. A "1 inch" pipe has an actual outside diameter of 33.4 mm (not 25.4 mm). Always check the actual dimension from a spec sheet for pipe, conduit, and tubing.

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Never use 25 as a shortcut for precision work. The correct factor is 25.4. The 1.6% error from using 25 seems small but adds up: at 100 inches, you're off by 40 mm — nearly 4 cm. Use the exact value, every time.

Frequently Asked Questions

How many millimeters are in 1 inch?▾

There are exactly 25.4 millimeters in 1 inch. This is the internationally defined exact value — not an approximation — established in 1959 by the International Yard and Pound Agreement between the USA, UK, Canada, Australia, New Zealand, and South Africa.

What is the formula to convert inches to millimeters?▾

The formula is: mm = inches × 25.4. For example, 5 inches = 5 × 25.4 = 127 mm. To convert back, divide by 25.4: mm ÷ 25.4 = inches. Example: 127 mm ÷ 25.4 = 5 inches.

What is 6 inches in millimeters?▾

6 inches = 6 × 25.4 = 152.4 mm. This is half a foot — equal to exactly 15.24 cm. It's close to the width of an A5 sheet of paper (148 mm wide) and is a common measurement in woodworking and layout work.

What is 12 inches (1 foot) in millimeters?▾

12 inches = 12 × 25.4 = 304.8 mm. This is exactly 1 foot. It's also 30.48 cm or 0.3048 m. A standard 12-inch ruler is 304.8 mm long exactly.

Why is 1 inch equal to 25.4 mm exactly?▾

The value of 25.4 mm per inch was defined exactly in 1959 under the International Yard and Pound Agreement. Before 1959, different countries had slightly different inch definitions (the US and British inches differed by about 2 parts per million). The 25.4 mm value was chosen as the internationally agreed exact conversion, unifying measurement systems worldwide. It has been exact by definition ever since.

What is 1/4 inch in millimeters?▾

¼ inch = 0.25 × 25.4 = 6.35 mm. Common fractional conversions: ⅛ in = 3.175 mm, ¼ in = 6.35 mm, ⅜ in = 9.525 mm, ½ in = 12.7 mm, ¾ in = 19.05 mm, 1 in = 25.4 mm.

How do I convert fractional inches to mm?▾

First convert the fraction to a decimal by dividing the top number by the bottom: 3/8 → 3 ÷ 8 = 0.375. Then multiply by 25.4: 0.375 × 25.4 = 9.525 mm. For mixed numbers like 2⅜: add 2 + 0.375 = 2.375, then 2.375 × 25.4 = 60.325 mm. Use the fraction quick-select buttons in our tool above for the most common fractions.

What is 2.5 inches in mm?▾

2.5 inches = 2.5 × 25.4 = 63.5 mm. This is exactly 6.35 cm. A common real-world reference: 2.5 inches is close to the diameter of a standard tennis ball (approximately 65–68 mm diameter).

Inches to Millimeters(in to mm) Converter