1. Fractions, Decimal Inches and Millimeter Dimensions
Usage, Conversion and Rounding Practices
Fractional inch dimensions (with 1/64 as the base unit) have permanent presence in all industries as standard
size designations for fasteners, wrenches, drills, plumbing fittings and many other parts and common items in
use around the world.
Arithmetically convenient decimal inch dimensions have replaced fractions on most mechanical engineering
drawings. To change a fraction into a decimal value, divide the numerator with the denominator to yield a
quotient. Example: 3 (numerator) divided by 64 (denominator) equals .046875 (quotient).
In the United States, decimal inch dimensions are expressed on engineering drawings to a precision of two or
three, sometimes four, decimal places and the quotients in the chart are rounded to three places. Note that
standard practice does not use a preceding zero to the left of the decimal point for inch values less than 1.
Zeroes are appended to inch values to present a uniform number of digit places to the right of the decimal,
such as .250 and .266, or .484375 and .500000.
In other countries, the metric system is used for all manner of product definition on engineering drawings,
with the millimeter (mm) the unit of measure for mechanical drawings. The metric system has its own series
of fasteners, tools, standard parts and common items, and most are not interchangeable with inch items.
Millimeter dimensions are expressed as whole numbers, and as decimal fractions to one or two places of
precision. International practice always precedes the decimal point with a zero for values less than 1. Metric
values are expressed only to the number of digit places that precision requires. Zeroes to the right of the
decimal point are not used.
The chart terminates with the integer 1. The 25.4mm metric value for one inch is an exact conversion factor.
Multiplying any decimal inch dimension by 25.4 yields the millimeter equivalent. Conversely, dividing a
millimeter dimension by 25.4 yields the decimal inch equivalent.
The 3-place inch values and millimeter equivalents in the chart (except those noted as *exact) have been
rounded in compliance with the engineering standard IEEE SI 10, American National Standard for Metric
Practice. The rounding method can be described as a set of five conditional rules:
1) Do not change the last digit if the discarded digit is less than 5.
Example: 1.1244… rounded to three places resolves to 1.124
2) Do not change the last digit if the discarded digit is 5 followed by a zero, and the last digit is even.
Example: 1.12450… rounded to three places resolves to 1.124
3) Increase the last digit by 1 if the discarded digit is 5 followed by a zero, and the last digit is odd.
Example: 1.12550… rounded to three places resolves to 1.126
4) Increase the last digit by 1 if the discarded digit is 5 followed by any value other than zero.
Example: 1.12451… rounded to three places resolves to 1.125
5) Increase the last digit by 1 if the discarded digit is greater than 5.
Example: 1.1246… rounded to three places resolves to 1.125
Cross-familiarity with inches and the metric system:
1 inch = 25.4mm exact (or approximately 25mm)
1 foot = 305mm or 30.5 centimeters or 3 decimeters, rounded.
1 millimeter [0.001 meter] = .039 inch rounded (approximately .040 inch).
1 centimeter [0.01 meter] = .394 inch rounded (approximately .400 inch).
1 decimeter [0.1 meter] = 3.937 inches rounded (approximately 3 and 15/16 inches).
1 meter = 39.370 inches rounded (approximately 39 and 3/8 inches).
GDT Brain, Inc. Copyright © 2015. All Rights Reserved. F. Tolmie 2015MY29
Inch Fraction
Decimal Inch Value
quotient (rounded)
Metric Value
*exact
33/64 .515625 .516 13.1mm
17/32 .531250 .531 13.49mm
35/64 .546875 .547 13.89mm
9/16 .562500 .563 14.29mm
37/64 .578125 .578 14.68mm
19/32 .593750 .594 15.08mm
39/64 .609375 .609 15.48mm
5/8 .625000 .625 15.88mm
41/64 .640625 .641 16.27mm
21/32 .656250 .656 16.67mm
43/64 .671875 .672 17.07mm
11/16 .687500 .688 17.46mm
45/64 .703125 .703 17.86mm
23/32 .718750 .719 18.26mm
47/64 .734375 .734 18.65mm
3/4 .750000 .750 *19.05mm
49/64 .765625 .766 19.45mm
25/32 .781250 .781 19.84mm
51/64 .796875 .797 20.24mm
13/16 .812500 .812 20.64mm
53/64 .828125 .828 21.03mm
27/32 .843750 .844 21.43mm
55/64 .859375 .859 21.83mm
7/8 .875000 .875 22.22mm
57/64 .890625 .891 22.62mm
29/32 .906250 .906 23.02mm
59/64 .921875 .922 23.42mm
15/16 .937500 .938 23.81mm
61/64 .953125 .953 24.21mm
31/32 .968750 .969 24.61mm
63/64 .984375 .984 25mm
1.000 *25.4mm
Inch Fraction
Decimal Inch Value
quotient (rounded)
Metric Value
*exact
1/64 .015625 .016 0.4mm
1/32 .031250 .031 0.79mm
3/64 .046875 .047 1.19mm
1/16 .062500 .062 1.59mm
5/64 .078125 .078 1.98mm
3/32 .093750 .094 2.38mm
7/64 .109375 .109 2.78mm
1/8 .125000 .125 3.18mm
9/64 .140625 .141 3.57mm
5/32 .156250 .156 3.97mm
11/64 .171875 .172 4.37mm
3/16 .187500 .188 4.76mm
13/64 .203125 .203 5.16mm
7/32 .218750 .219 5.56mm
15/64 .234375 .234 5.95mm
1/4 .250000 .250 *6.35mm
17/64 .265625 .266 6.75mm
9/32 .281250 .281 7.14mm
19/64 .296875 .297 7.54mm
5/16 .312500 .312 7.94mm
21/64 .328125 .328 8.33mm
11/32 .343750 .344 8.73mm
23/64 .359375 .359 9.13mm
3/8 .375000 .375 9.52mm
25/64 .390625 .391 9.92mm
13/32 .406250 .406 10.32mm
27/64 .421875 .422 10.72mm
7/16 .437500 .438 11.11mm
29/64 .453125 .453 11.51mm
15/32 .468750 .469 11.91mm
31/64 .484375 .484 12.3mm
1/2 .500000 .500 *12.7mm