How close is a 64th to a millimetre value?

Usually within two tenths of a millimetre — a 64th of an inch is 0.396875 mm wide, so the nearest mark can miss by at most half of that. The difference column states each miss exactly, signed: + means the fraction is oversize.

Is 10 mm the same as 3/8 inch?

No — close, not the same. 3/8″ is 9.525 mm, nearly half a millimetre under 10 mm. The nearest 64th to 10 mm is 25/64″ at 9.921875 mm, still 0.078 mm under. Whether that matters is exactly what the difference column is for.

Why does the chart stop at 25.5 mm?

Because 25.4 mm is one inch exactly, and past an inch the fractional pattern repeats: 30 mm is 1 inch plus the 4.6 mm row. Subtract 25.4 and read the remainder.

Millimetres → inches chart

Half a millimetre at a time to just past one inch. Each row gives the exact decimal inches (25.4 mm to the inch, by definition) and the nearest 64th with its exact miss — signed, so + means the fraction runs oversize.

mm	Inches	Nearest 64th	Difference
0.5	0.01969	1/64″	−0.103 mm
1	0.03937	3/64″	+0.191 mm
1.5	0.05906	1/16″	+0.088 mm
2	0.07874	5/64″	−0.016 mm
2.5	0.09843	3/32″	−0.119 mm
3	0.11811	1/8″	+0.175 mm
3.5	0.1378	9/64″	+0.072 mm
4	0.15748	5/32″	−0.031 mm
4.5	0.17717	11/64″	−0.134 mm
5	0.19685	13/64″	+0.159 mm
5.5	0.21654	7/32″	+0.056 mm
6	0.23622	15/64″	−0.047 mm
6.5	0.25591	1/4″	−0.15 mm
7	0.27559	9/32″	+0.144 mm
7.5	0.29528	19/64″	+0.041 mm
8	0.31496	5/16″	−0.063 mm
8.5	0.33465	21/64″	−0.166 mm
9	0.35433	23/64″	+0.128 mm
9.5	0.37402	3/8″	+0.025 mm
10	0.3937	25/64″	−0.078 mm
10.5	0.41339	13/32″	−0.181 mm
11	0.43307	7/16″	+0.113 mm
11.5	0.45276	29/64″	+0.009 mm
12	0.47244	15/32″	−0.094 mm
12.5	0.49213	31/64″	−0.197 mm
13	0.51181	33/64″	+0.097 mm

mm	Inches	Nearest 64th	Difference
13.5	0.5315	17/32″	−0.006 mm
14	0.55118	35/64″	−0.109 mm
14.5	0.57087	37/64″	+0.184 mm
15	0.59055	19/32″	+0.081 mm
15.5	0.61024	39/64″	−0.022 mm
16	0.62992	5/8″	−0.125 mm
16.5	0.64961	21/32″	+0.169 mm
17	0.66929	43/64″	+0.066 mm
17.5	0.68898	11/16″	−0.038 mm
18	0.70866	45/64″	−0.141 mm
18.5	0.72835	47/64″	+0.153 mm
19	0.74803	3/4″	+0.05 mm
19.5	0.76772	49/64″	−0.053 mm
20	0.7874	25/32″	−0.156 mm
20.5	0.80709	13/16″	+0.138 mm
21	0.82677	53/64″	+0.034 mm
21.5	0.84646	27/32″	−0.069 mm
22	0.86614	55/64″	−0.172 mm
22.5	0.88583	57/64″	+0.122 mm
23	0.90551	29/32″	+0.019 mm
23.5	0.9252	59/64″	−0.084 mm
24	0.94488	15/16″	−0.188 mm
24.5	0.96457	31/32″	+0.106 mm
25	0.98425	63/64″	+0.003 mm
25.5	1.00394	1″	−0.1 mm

Between the rows, or need a different rule? Metric → fraction takes any value; within tolerance finds the simplest fraction inside a stated window.

Common questions

How close is a 64th to a millimetre value?: Usually within two tenths of a millimetre — a 64th of an inch is 0.396875 mm wide, so the nearest mark can miss by at most half of that. The difference column states each miss exactly, signed: + means the fraction is oversize.
Is 10 mm the same as 3/8 inch?: No — close, not the same. 3/8″ is 9.525 mm, nearly half a millimetre under 10 mm. The nearest 64th to 10 mm is 25/64″ at 9.921875 mm, still 0.078 mm under. Whether that matters is exactly what the difference column is for.
Why does the chart stop at 25.5 mm?: Because 25.4 mm is one inch exactly, and past an inch the fractional pattern repeats: 30 mm is 1 inch plus the 4.6 mm row. Subtract 25.4 and read the remainder.

Where this comes from. Exact rational arithmetic on the values shown — fractions are pairs of integers until display, and inch–metric conversion uses 25.4 mm to the inch, exact by definition since 1959. See the full methodology.