Measurement calculator
Metric and Imperial Length Converter
Convert millimeters, centimeters, meters, inches, feet, yards, fractions, and mixed feet and inches. The physical length stays put; only the name tag changes, which is more than I can say for half the hardware bins in a busy workshop.
Sage’s guided lesson
Unit conversion changes the description, not the object
A part that is 1 inch long is also exactly 25.4 millimeters long. Nothing stretches during the calculation. A correct conversion preserves the physical quantity while replacing one unit with an equivalent amount of another unit.
The labels change; the length does not
One measurement can be written many correct ways
The measurement 2 ft 7 3/8 in can also be written as 31.375 decimal inches, about 2.614583 decimal feet, about 0.871528 yard, 796.925 millimeters, 79.6925 centimeters, or 0.796925 meter.
Some forms are easier for different jobs. A tape measure favors mixed feet, inches, and fractions. A digital caliper may show decimal inches or millimeters. Architectural work may use decimal feet. Machine drawings often use millimeters or decimal inches. The useful form is the one that matches the drawing, tool, and tolerance.
The same length in several forms
2 ft 7 3/8 in = 24 in + 7.375 in
= 31.375 in
= 31.375 × 25.4 = 796.925 mm
= 0.796925 m
Write the unit beside every important number. A bare 25 can be 25 inches, 25 millimeters, or 25 reasons to remake the part.
Use a conversion factor equal to one
How to do the conversion math yourself
The international inch is exactly 25.4 millimeters. From that definition, the foot is exactly 304.8 millimeters, and the yard is exactly 914.4 millimeters. These are definitions, not experimental approximations.
Inches to millimeters
millimeters = inches × 25.4
3/4 in = 0.75 × 25.4 = 19.05 mm
Millimeters to inches
inches = millimeters ÷ 25.4
50 mm ÷ 25.4 = 1.9685039… in
Meters to feet
feet = meters ÷ 0.3048
2 m ÷ 0.3048 = 6.5616798… ft
Cancel the starting unit on paper when you use a conversion factor. For example, multiply inches by 25.4 millimeters per inch. The inch units cancel, leaving millimeters. It is a small habit that catches a remarkable number of upside down conversions.
I know this is off topic, but this reminds me of a joke I heard when teaching this to one of my interns, why did the inch feel important? It was part of a larger foot. Yeah, I do not see the connection either, but I think it is cute. Back to the math.
A few anchors make mental conversion easier
Useful workshop equivalents worth recognizing
| Starting measurement | Equivalent | How to use it |
|---|---|---|
| 1 in | 25.4 mm exactly | Primary inch to metric factor |
| 1/2 in | 12.7 mm exactly | Common hardware and stock reference |
| 3/4 in | 19.05 mm exactly | Useful when comparing nominal panels and hardware |
| 1 ft | 304.8 mm exactly | Architectural and layout conversion |
| 1 yd | 0.9144 m exactly | Fabric, landscaping, and longer stock |
| 10 mm | 0.3937008 in | A little less than 13/32 in |
| 25 mm | 0.984252 in | Close to, but not equal to, 1 in |
| 100 mm | 3.937008 in | A little less than 4 in |
| 1 m | 39.3700787 in | About 3 ft 3 3/8 in |
Approximate mental shortcuts are useful for checking scale. One inch is about 25 millimeters, and one meter is about 39.37 inches. Do not use the shortcut in place of the exact factor when a close fit matters.
Twenty five millimeters is not one inch. It is about 0.984 inch, short by 0.4 millimeter. That difference can matter in bearings, shafts, hardware, and anything else that expects an actual fit.
The popular name is not the whole story
U.S. customary and British Imperial are related, but not identical
People in the United States often call inch based measurements “Imperial.” The more precise name for the collection used in the United States is U.S. customary measurement. British Imperial and U.S. customary units share modern definitions for the inch, foot, and yard, so ordinary length conversions agree.
The systems are not interchangeable in every category. A U.S. gallon and an Imperial gallon are different sizes, as are their pints and fluid ounces. That difference does not affect this length calculator, but it matters enormously when converting volume. A label that says “gallon” needs a country and context before it gets trusted with paint, fuel, or pie filling.
Modern exact length definitions
1 international inch = 25.4 mm exactly
1 international foot = 0.3048 m exactly
1 international yard = 0.9144 m exactly
Modern U.S. customary length units are not floating independently in space. Their legal values are tied exactly to the meter.
The foot began with bodies; the meter escaped into physics
From barleycorns and meridians to the speed of light
Many early units were connected to the human body, including feet, hands, fingers, arms, and paces. That made them convenient, but not universal. One person’s foot was not always another person’s foot, and local standards could vary between towns, trades, and rulers.
In medieval England, the inch was officially described using three barleycorns placed end to end. It was an attempt to turn an everyday object into a repeatable standard, although barley was never famous for precision manufacturing.
The metric system emerged during the French Revolution as an effort to create a rational, decimal, widely shared system. The original meter was intended to equal one ten millionth of the distance from the equator to the North Pole along a meridian. A platinum meter standard was deposited in 1799.
In 1959, several English speaking countries agreed on the international yard as exactly 0.9144 meter. That made the inch exactly 25.4 millimeters. Today the meter is defined by fixing the speed of light in vacuum, so one meter is the distance light travels in 1/299,792,458 of a second.
The old U.S. survey foot differed very slightly from the international foot. It was deprecated at the end of 2022, and the international foot became the standard for new U.S. work beginning in 2023. The tiny difference mattered over long mapping and surveying distances.
Exact factors do not rescue vague measurements
Preserve tolerance, and watch for nominal sizes
The factor 25.4 millimeters per inch is exact, but the starting measurement may not be. If a board was measured to the nearest 1/16 inch, converting it to 1.5875 millimeters does not make the board known to four decimal places. It only converts the stated increment exactly.
Nominal sizes require extra caution. A product called 3/4 inch plywood may measure less than 0.750 inch. A 20 millimeter bearing bore is intended to be metric, while a 3/4 inch bore is exactly 19.05 millimeters. Those are different parts even though they look close on a rough tape.
Sage’s conversion checklist
Identify the source unit before entering the number. Use the exact conversion factor during the calculation. Keep extra digits until the final step. Round to a precision the tool and tolerance can support. Confirm whether a labeled product size is nominal or actual. Convert the answer back once when the consequence of an error is expensive.A conversion can print eight decimal places while the tape mark was read only to the nearest sixteenth. The calculator knows arithmetic; it was not standing beside you when the measurement was taken.
The units agree now
One length, several languages, no international incident.
You now know the exact inch and metric relationships, why U.S. customary and Imperial are not perfect synonyms, how the meter moved from an Earth based idea to the speed of light, and why a nominal 25 millimeter part should not be mistaken for a one inch part.
That earns cheese fries. I will accept the order in ounces or grams, but I refuse to convert individual fries unless somebody is paying overtime.
Quick answers
Metric and imperial conversion questions
Is one inch exactly 25.4 millimeters?
Yes. The international inch is defined as exactly 25.4 millimeters.
How do I convert inches to millimeters?
Multiply inches by 25.4. For example, 0.75 × 25.4 = 19.05 mm.
How do I convert millimeters to inches?
Divide millimeters by 25.4. For example, 50 ÷ 25.4 ≈ 1.9685 in.
Are U.S. customary and British Imperial measurements the same?
Their modern inch, foot, and yard length definitions agree, but the systems are not identical overall. Their gallons, pints, and fluid ounces differ.
Why does an exact conversion still show a rounded result?
The conversion relationship can be exact while the displayed decimal is shortened for readability. The original measurement may also have limited precision.
What happened to the U.S. survey foot?
It was deprecated at the end of 2022. Beginning in 2023, the international foot, exactly 0.3048 meter, is the standard foot for new U.S. applications.