Inches to MM Conversion Guide: Precise Metric Tables

In precision engineering, 3D printing, mechanical prototyping, CNC machining, and CAD design, millimeters (mm) are the preferred unit because they eliminate the ambiguity of fractional inches and enable tighter tolerances. This guide provides comprehensive conversion tables and formulas for converting any inch measurement to millimeters with absolute precision.

The Core Conversion Standard

Under the International Yard and Pound Agreement of 1959:

1 inch = exactly 25.4 millimeters

This is an exact definition, not an approximation. Every conversion between inches and millimeters derives from this single standard.

Conversion Formulas

Millimeters = Inches × 25.4
Inches = Millimeters ÷ 25.4

Complete Fractional Inch to MM Lookup Table

This table covers every standard fractional division from 1/64 through 1 inch. This is the reference chart used by machinists, engineers, and fabricators worldwide.

Sixteenths of an Inch

Fraction	Decimal (in)	Millimeters (mm)
1/16	0.0625	1.5875
1/8 (2/16)	0.1250	3.1750
3/16	0.1875	4.7625
1/4 (4/16)	0.2500	6.3500
5/16	0.3125	7.9375
3/8 (6/16)	0.3750	9.5250
7/16	0.4375	11.1125
1/2 (8/16)	0.5000	12.7000
9/16	0.5625	14.2875
5/8 (10/16)	0.6250	15.8750
11/16	0.6875	17.4625
3/4 (12/16)	0.7500	19.0500
13/16	0.8125	20.6375
7/8 (14/16)	0.8750	22.2250
15/16	0.9375	23.8125
1 (16/16)	1.0000	25.4000

Thirty-Seconds of an Inch (Machinist Precision)

Fraction	Decimal (in)	Millimeters (mm)
1/32	0.03125	0.7938
3/32	0.09375	2.3813
5/32	0.15625	3.9688
7/32	0.21875	5.5563
9/32	0.28125	7.1438
11/32	0.34375	8.7313
13/32	0.40625	10.3188
15/32	0.46875	11.9063
17/32	0.53125	13.4938
19/32	0.59375	15.0813
21/32	0.65625	16.6688
23/32	0.71875	18.2563
25/32	0.78125	19.8438
27/32	0.84375	21.4313
29/32	0.90625	23.0188
31/32	0.96875	24.6063

Whole Inch to MM Conversion Table

Inches	Millimeters	Centimeters
1”	25.40 mm	2.540 cm
2”	50.80 mm	5.080 cm
3”	76.20 mm	7.620 cm
4”	101.60 mm	10.160 cm
5”	127.00 mm	12.700 cm
6”	152.40 mm	15.240 cm
7”	177.80 mm	17.780 cm
8”	203.20 mm	20.320 cm
9”	228.60 mm	22.860 cm
10”	254.00 mm	25.400 cm
12” (1 ft)	304.80 mm	30.480 cm
18”	457.20 mm	45.720 cm
24” (2 ft)	609.60 mm	60.960 cm
36” (3 ft / 1 yd)	914.40 mm	91.440 cm

Practical Applications

3D Printing and CAD

STL files and slicer software typically use millimeters as the default unit. If your design reference is in inches (common in US-origin projects), converting to mm before modeling prevents scaling disasters. A bracket designed at “2 inches” must be modeled as 50.80 mm — not 50 mm (which would be 1.2% short).

CNC Machining

Machine tools in metric countries operate in millimeters. When working from imperial drawings, machinists convert every dimension. Tolerance callouts like ±0.005” become ±0.127 mm.

Hardware and Fasteners

Many fastener standards straddle both systems:

• M6 bolt: 6.0 mm diameter = 0.2362”
• 1/4-20 bolt: 0.2500” diameter = 6.35 mm
• These are NOT interchangeable despite being close in size!

Pipe and Tubing

Plumbing pipe sizes in North America use nominal inch designations (1/2”, 3/4”, 1”) that don’t correspond to actual physical dimensions. A “1/2-inch” copper pipe has an outside diameter of 0.625” (15.88 mm). Always verify actual dimensions rather than assuming the nominal size.

Common Metric Fastener / Hardware Sizes in Inches

Metric Size	Inches (Decimal)	Nearest Imperial Fraction
1 mm	0.0394”	~1/32” (loose)
2 mm	0.0787”	~5/64”
3 mm	0.1181”	~1/8” (close)
4 mm	0.1575”	~5/32”
5 mm	0.1969”	~13/64”
6 mm	0.2362”	~15/64”
8 mm	0.3150”	~5/16” (close)
10 mm	0.3937”	~25/64”
12 mm	0.4724”	~15/32”
16 mm	0.6299”	~5/8” (close)
20 mm	0.7874”	~25/32”
25 mm	0.9843”	~63/64” ≈ 1”

Quick Mental Approximation

For fast rough estimates without a calculator:

• 1 inch ≈ 25 mm (actual: 25.4 — only 1.6% error)
• 1 mm ≈ 0.04 inches (actual: 0.03937)
• 10 mm = 1 cm (exact)
• 4 inches ≈ 100 mm (actual: 101.6 — close enough for casual use)

Why Millimeters Are Preferred Over Fractional Inches

In professional engineering, millimeters offer several advantages:

1. No fractions: “12.7 mm” is unambiguous; “1/2 inch” requires recognizing the fraction
2. Easier math: Adding 12.7 + 9.5 = 22.2 mm is simpler than adding 1/2” + 3/8” = 7/8”
3. Finer default precision: Whole millimeters give ~0.04” precision by default
4. Global standard: Metric drawings are understood worldwide without conversion

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Related Guides

• CM to Inches Conversion Guide
• How to Read an Inch Ruler