How many mm is 1/4 inch?

1/4″ = 6.350mm. One inch is exactly 25.4mm, so multiply a fractional inch by 25.4 to get millimeters.

How many mm are 1/2 inch and 3/8 inch?

1/2″ = 12.700mm, 3/8″ = 9.525mm, 5/8″ = 15.875mm, 3/4″ = 19.050mm.

Is there any error when converting inch to mm?

No. Since 1959 the international inch has been defined as exactly 25.4mm, so the only error comes from rounding the decimal places.

FREE CNC TOOL · Unit conversion

Inch ↔ mm Converter (with Fractional-Inch Table)

Two-way, instant conversion with a complete 1/64″–1″ fractional-inch table. 1 in = 25.4 mm exactly, so the math is error-free.

Inch in

Millimeter mm

Type in either field and the other converts instantly (1 in = 25.4 mm exactly).

Fractional inch → millimeter table (1/64″–1″)

Fractional inch	Decimal inch	Millimeters (mm)
1/64″	0.0156	0.397
1/32″	0.0312	0.794
3/64″	0.0469	1.191
1/16″	0.0625	1.587
5/64″	0.0781	1.984
3/32″	0.0938	2.381
7/64″	0.1094	2.778
1/8″	0.1250	3.175
9/64″	0.1406	3.572
5/32″	0.1562	3.969
11/64″	0.1719	4.366
3/16″	0.1875	4.762
13/64″	0.2031	5.159
7/32″	0.2188	5.556
15/64″	0.2344	5.953
1/4″	0.2500	6.350
17/64″	0.2656	6.747
9/32″	0.2812	7.144
19/64″	0.2969	7.541
5/16″	0.3125	7.938
21/64″	0.3281	8.334
11/32″	0.3438	8.731
23/64″	0.3594	9.128
3/8″	0.3750	9.525
25/64″	0.3906	9.922
13/32″	0.4062	10.319
27/64″	0.4219	10.716
7/16″	0.4375	11.112
29/64″	0.4531	11.509
15/32″	0.4688	11.906
31/64″	0.4844	12.303
1/2″	0.5000	12.700
33/64″	0.5156	13.097
17/32″	0.5312	13.494
35/64″	0.5469	13.891
9/16″	0.5625	14.287
37/64″	0.5781	14.684
19/32″	0.5938	15.081
39/64″	0.6094	15.478
5/8″	0.6250	15.875
41/64″	0.6406	16.272
21/32″	0.6562	16.669
43/64″	0.6719	17.066
11/16″	0.6875	17.462
45/64″	0.7031	17.859
23/32″	0.7188	18.256
47/64″	0.7344	18.653
3/4″	0.7500	19.050
49/64″	0.7656	19.447
25/32″	0.7812	19.844
51/64″	0.7969	20.241
13/16″	0.8125	20.637
53/64″	0.8281	21.034
27/32″	0.8438	21.431
55/64″	0.8594	21.828
7/8″	0.8750	22.225
57/64″	0.8906	22.622
29/32″	0.9062	23.019
59/64″	0.9219	23.416
15/16″	0.9375	23.812
61/64″	0.9531	24.209
31/32″	0.9688	24.606
63/64″	0.9844	25.003
1″	1.0000	25.400

FAQ

How many mm is 1/4″?

1/4″ = 0.25″ = 6.350mm. Common approximate references: 1/8″≈3.175, 3/8″≈9.525, 1/2″≈12.700, 3/4″≈19.050mm.

Why 25.4?

Since 1959 the international inch has been defined as exactly 25.4mm, so converting inch to millimeter has no rounding error—the only error comes from how many decimal places you keep.

What if a drawing has both inches and millimeters?

First confirm the primary unit (make sure G20/G21 matches), and on mixed-unit drawings it's best to convert everything to a single unit before programming, to avoid a 25.4× dimension accident.

Further reading: Complete G-code primer (G20/G21 unit setting) · RFQ drawing checklist | More tools: Tools hub

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