What is 1/8 as a decimal?

0.1250 in, or 3.175 mm. Divide 1 by 8 to get the decimal, then multiply by 25.4 for millimetres.

What is 3/8 as a decimal?

0.3750 in, or 9.525 mm. Divide 3 by 8 for the decimal, then multiply by 25.4 for millimetres. In 64ths it is exactly 24/64.

What is 5/16 as a decimal?

0.3125 in, or 7.938 mm. Note that 5/16 is exactly 20/64, so it sits on a 64th boundary.

How do I convert a decimal to a fraction?

Multiply by the denominator you want and round. To express 0.4375 in 64ths, multiply by 64 to get 28, giving 28/64, which reduces to 7/16. The 64ths column in the table does this conversion for every common fraction.

Why are fractions written in 64ths on drawings?

1/64 in is the practical resolution of a machinist's rule and the increment most fractional drill and stock sizes come in. Dimensions finer than 1/64 are normally given in decimals rather than fractions.

Is 2/8 the same as 1/4?

Yes — 0.2500 in either way. 2/8 reduces to 1/4 by dividing both terms by 2. Only reduced forms are listed in the table, but any equivalent fraction gives the same decimal.

Fraction to Decimal Chart

Every fraction from 1/64 through 1 inch, in simplest form, with its decimal inch equivalent and millimetre conversion.

Data verified 2026-09-29 · based on n/a — the conversions are exact arithmetic, not tabulated reference data

Quick Answer

Convert a fraction to a decimal by dividing the numerator by the denominator: 1/4 = 1 ÷ 4 = 0.2500. For millimetres, multiply by 25.4 — so 1/4 in is 6.350 mm. Only fractions in lowest terms appear here, because 2/8 and 1/4 are the same size.

Find a fraction or decimal

Fraction, Decimal and Millimetre Equivalents

Fraction^[1]	Decimal inch^[1]	Metric mm^[1]	In 64ths^[1]
1/64 #	0.0156	0.397	1/64
1/32 #	0.0312	0.794	2/64
3/64 #	0.0469	1.191	3/64
1/16 #	0.0625	1.587	4/64
5/64 #	0.0781	1.984	5/64
3/32 #	0.0938	2.381	6/64
7/64 #	0.1094	2.778	7/64
1/8 #	0.1250	3.175	8/64
9/64 #	0.1406	3.572	9/64
5/32 #	0.1562	3.969	10/64
11/64 #	0.1719	4.366	11/64
3/16 #	0.1875	4.762	12/64
13/64 #	0.2031	5.159	13/64
7/32 #	0.2188	5.556	14/64
15/64 #	0.2344	5.953	15/64
1/4 #	0.2500	6.350	16/64
17/64 #	0.2656	6.747	17/64
9/32 #	0.2812	7.144	18/64
19/64 #	0.2969	7.541	19/64
5/16 #	0.3125	7.938	20/64
21/64 #	0.3281	8.334	21/64
11/32 #	0.3438	8.731	22/64
23/64 #	0.3594	9.128	23/64
3/8 #	0.3750	9.525	24/64
25/64 #	0.3906	9.922	25/64
13/32 #	0.4062	10.319	26/64
27/64 #	0.4219	10.716	27/64
7/16 #	0.4375	11.112	28/64
29/64 #	0.4531	11.509	29/64
15/32 #	0.4688	11.906	30/64
31/64 #	0.4844	12.303	31/64
1/2 #	0.5000	12.700	32/64
33/64 #	0.5156	13.097	33/64
17/32 #	0.5312	13.494	34/64
35/64 #	0.5469	13.891	35/64
9/16 #	0.5625	14.287	36/64
37/64 #	0.5781	14.684	37/64
19/32 #	0.5938	15.081	38/64
39/64 #	0.6094	15.478	39/64
5/8 #	0.6250	15.875	40/64
41/64 #	0.6406	16.272	41/64
21/32 #	0.6562	16.669	42/64
43/64 #	0.6719	17.066	43/64
11/16 #	0.6875	17.462	44/64
45/64 #	0.7031	17.859	45/64
23/32 #	0.7188	18.256	46/64
47/64 #	0.7344	18.653	47/64
3/4 #	0.7500	19.050	48/64
49/64 #	0.7656	19.447	49/64
25/32 #	0.7812	19.844	50/64
51/64 #	0.7969	20.241	51/64
13/16 #	0.8125	20.637	52/64
53/64 #	0.8281	21.034	53/64
27/32 #	0.8438	21.431	54/64
55/64 #	0.8594	21.828	55/64
7/8 #	0.8750	22.225	56/64
57/64 #	0.8906	22.622	57/64
29/32 #	0.9062	23.019	58/64
59/64 #	0.9219	23.416	59/64
15/16 #	0.9375	23.812	60/64
61/64 #	0.9531	24.209	61/64
31/32 #	0.9688	24.606	62/64
63/64 #	0.9844	25.003	63/64
1 #	1.0000	25.400	64/64
2 #	2.0000	50.800	128/64
3 #	3.0000	76.200	192/64
4 #	4.0000	101.600	256/64

Only fractions in lowest terms are listed — 2/8, 4/16 and 1/4 are all the same size. The 64ths column shows the equivalent in 64ths of an inch, which is the granularity most fractional drill and stock sizes use.

The Conversion

decimal inch = numerator ÷ denominator
millimetres = decimal inch × 25.4
64ths = decimal inch × 64

All three columns are exact arithmetic from the fraction — there is nothing to look up and no rounding other than the display precision.

Reducing Fractions to Lowest Terms

A fraction is in lowest terms when the numerator and denominator share no factor other than 1. Divide both by their greatest common divisor:

8/32 → divide both by 8 → 1/4
6/16 → divide both by 2 → 3/8
10/64 → divide both by 2 → 5/32

This matters when reading a drawing: a dimension written 12/32 and one written 3/8 are the same size, and mixing reduced and unreduced forms is a common source of measurement error. The 64ths column here lets you compare any fraction against the 1/64 in increments most stock and drill sizes are sold in.

Frequently Asked Questions

What is 1/8 as a decimal?: 0.1250 in, or 3.175 mm. Divide 1 by 8 to get the decimal, then multiply by 25.4 for millimetres.
What is 3/8 as a decimal?: 0.3750 in, or 9.525 mm. Divide 3 by 8 for the decimal, then multiply by 25.4 for millimetres. In 64ths it is exactly 24/64.
What is 5/16 as a decimal?: 0.3125 in, or 7.938 mm. Note that 5/16 is exactly 20/64, so it sits on a 64th boundary.
How do I convert a decimal to a fraction?: Multiply by the denominator you want and round. To express 0.4375 in 64ths, multiply by 64 to get 28, giving 28/64, which reduces to 7/16. The 64ths column in the table does this conversion for every common fraction.
Why are fractions written in 64ths on drawings?: 1/64 in is the practical resolution of a machinist's rule and the increment most fractional drill and stock sizes come in. Dimensions finer than 1/64 are normally given in decimals rather than fractions.
Is 2/8 the same as 1/4?: Yes — 0.2500 in either way. 2/8 reduces to 1/4 by dividing both terms by 2. Only reduced forms are listed in the table, but any equivalent fraction gives the same decimal.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	Arithmetic conversion	derived	decimal = numerator ÷ denominator; millimetres = decimal × 25.4; 64ths = decimal × 64. All exact.

Data Sources

Standard	Revision	What it covers on this page
Arithmetic — no standard is involved	n/a — the conversions are exact arithmetic, not tabulated reference data	every value on this page

Cross-checked against:

Values recomputed from the fraction on every build and verified against the published decimal column
Millimetre column verified at build time against the inch column

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
All values	Exact arithmetic from the fraction — see the conversion above.

Decimal values are shown to four places. Any fraction can be evaluated exactly; the display precision is the only rounding.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Fraction to Decimal Chart." https://refmanual.com/charts/fraction-to-decimal/ (accessed 2026-09-29). Data from Arithmetic — no standard is involved, n/a — the conversions are exact arithmetic, not tabulated reference data.

BibTeX

@misc{fraction-to-decimal,
  title        = {Fraction to Decimal Chart},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/fraction-to-decimal/}},
  note         = {Accessed: 2026-09-29},
  source       = {Arithmetic — no standard is involved, n/a — the conversions are exact arithmetic, not tabulated reference data}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Fraction to Decimal Chart
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/fraction-to-decimal/
PY  - 2026
DA  - 2026-09-29
N1  - Source: Arithmetic — no standard is involved, n/a — the conversions are exact arithmetic, not tabulated reference data
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.