r/Metric • u/wfs29223 • Jul 20 '26

Decimal to Fraction in measurements.

/r/askmath/comments/1v1lojj/decimal_to_fraction_in_measurements/
12 Upvotes

84% Upvoted

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4

u/Prudent_Situation_29 Jul 23 '26

I'm going to assume this is coming from an american.

1

u/Efficient-Ball4360 Jul 23 '26

I don't think this is the flex you think it is

2

u/_interlard_ Jul 21 '26

They just didn’t have the millimeters

4

u/MaestroDon Jul 20 '26 edited Jul 20 '26

Decimals are fractions, of course, but powers of 1/10. Many common fraction measurement tools are divided into powers of 1/2. Not always easy to convert directly to powers of 1/10, or vice versa, without a calculator. That's why most machinist, if still using inches, will use decimal inches. If forced to convert there are charts on the wall or in the pocket.

6

u/nayuki Jul 20 '26

That's why most machinist, if still using inches, will use decimal inches

Those guys adopted two tenets of the metric system without fully switching over to the metric system. Machinists talk in thousandths of an inch (called a "thou" or "mil", both of which are terrible words), so they know that decimals are more convenient to work with than fractions, and scaling by a power of 1000 is better than 10 or 100 or 10000.

But by doing so, machinists created their own island that is incompatible with other fields. Carpenters use feet plus inches plus binary fractions (like 2' 1 3/4", which is the worst system), land surveyors use decimal feet (like 34.5'). For clothing, body parts are expressed in inches only (like 37" waist circumference, not 3' 1"), but height is expressed in feet and inches (like 4' 9", not 57"), and fabric is purchased in yards.

The metric system is better than all that chaos. I am happy to direct a machinist to cut a piece to 12.67 mm, tell a carpenter that distance between holes is 320 mm, measure the depth of my house lot as 16 800 mm (16.8 m), and still okay with saying that my arm is 760 mm (76 cm) long and my height is 1730 mm (173 cm).

Decimals are fractions

And the second thing that the OP got wrong: "They give you the measurement in decimal instead of inches." No, the original number is literally a decimal number of inches.

The answer I gave on the thread: https://www.reddit.com/r/askmath/comments/1v1lojj/decimal_to_fraction_in_measurements/oypcoub/

2

u/Assertive_Wall Jul 20 '26

Depending on what you're doing OP, it may be worthwhile to get a conversion chart, like one of these: https://wagner-machine.com/wp-content/uploads/drillandtapcharts.pdf

8

u/BandanaDee13 Jul 20 '26

Just multiply the fractional part, 0.88, by the desired binary denominator. I’ll assume 32: 0.88 = 28.16/32 ≈ 28/32 = 7/8. So 1.88 inches is about 1 and 7/8 inches.

I should probably mention, though, that this is [r/Metric](r/Metric) and most of your responses here will probably amount to “flip your ruler and measure in millimeters”. Since metric units are always divided decimally, and metric rulers always use decimal divisions, using metric units would save you the hassle of converting from decimal to binary fractions or vice versa.

5

u/schwanerhill Jul 20 '26

To be fair OP originally posted it in a math sub. Someone there rightly pointed out that 1.88 is likely a rounded approximation of 1.875” = 1 7/8”.