TOOL SIZE CONVERSION TOOL
mm to Inch Tool Size Converter
One inch is exactly 25.4 mm. Enter a tool dimension and get the decimal, the nearest fraction, and the error at the resolution you choose.
ONE OPERATION EACH WAY
How to Convert mm to Inches (and Back)
The conversion is one operation in each direction.
mm to Inches:
Divide by 25.4
To go from mm to inches, divide by 25.4. One operation, exact to the decimal.
Inches to mm:
Multiply by 25.4
To go from inches to mm, multiply by 25.4. Same factor, opposite operation.
Worked
Example
Take a 3/8″ dimension. Multiply by 25.4 and you get 9.525 mm exactly.
The Reverse
Returns the Start
Reverse it: 9.525 mm divided by 25.4 returns 0.375″, which is 3/8″. Nothing is lost either way.
A Definition,
Not a Measurement
Fixed by the international yard and pound agreement of 1959. NIST publishes it in Special Publication 811. BIPM defines the meter.
TOOL SIZE CONVERTER
mm to Inch Tool Size Converter
Enter a dimension. Get the exact decimal, the nearest fraction at the resolution you pick, and the error between them, with its direction.
Decimal Inch
0.472441
in
Exact at 1 inch = 25.4 mm. Nothing is rounded in the conversion itself.
Nearest Fraction
15/32 in
Error
0.003691 in
Undersize
mm ÷ 25.4 = inch
12 mm ÷ 25.4 = 0.472441 in
A near match is temporary, not permanent. Use a fraction that lands closest without going undersize for low-torque or already-damaged fasteners. Never on a torque you intend to last.
FRACTION LADDER
The mm to Inch Conversion Chart: Fractions at 1/64 Steps
EXACT FACTOR
1 in = 25.4 mm
Every value below is exact rather than rounded, which is why some carry six decimal places. 1/4" is 6.35 mm, not 6.3 and not 6.0, and the difference isn't cosmetic: a 0.1 mm error on a tap drill is enough to force a custom tool. Convert from the exact factor, round only the final answer, and never round an intermediate step.
Ladder Start
1/64"
0.396875 mm
Ladder End
1"
25.4 mm
Step
1/64"
0.015625 in
Rows
65
Exact values
| Fraction | Decimal Inch | Exact mm |
|---|---|---|
| 1/64" | 0.015625 | 0.396875 |
| 1/32" | 0.03125 | 0.79375 |
| 3/64" | 0.046875 | 1.190625 |
| 1/16" | 0.0625 | 1.5875 |
| 5/64" | 0.078125 | 1.984375 |
| 3/32" | 0.09375 | 2.38125 |
| 7/64" | 0.109375 | 2.778125 |
| 1/8" | 0.125 | 3.175 |
| 9/64" | 0.140625 | 3.571875 |
| 5/32" | 0.15625 | 3.96875 |
| 11/64" | 0.171875 | 4.365625 |
| 3/16" | 0.1875 | 4.7625 |
| 13/64" | 0.203125 | 5.159375 |
| 7/32" | 0.21875 | 5.55625 |
| 15/64" | 0.234375 | 5.953125 |
| 1/4" | 0.25 | 6.35 |
| 17/64" | 0.265625 | 6.746875 |
| 9/32" | 0.28125 | 7.14375 |
| 19/64" | 0.296875 | 7.540625 |
| 5/16" | 0.3125 | 7.9375 |
| 21/64" | 0.328125 | 8.334375 |
| 11/32" | 0.34375 | 8.73125 |
| 23/64" | 0.359375 | 9.128125 |
| 3/8" | 0.375 | 9.525 |
| 25/64" | 0.390625 | 9.921875 |
| 13/32" | 0.40625 | 10.31875 |
| 27/64" | 0.421875 | 10.715625 |
| 7/16" | 0.4375 | 11.1125 |
| 29/64" | 0.453125 | 11.509375 |
| 15/32" | 0.46875 | 11.90625 |
| 31/64" | 0.484375 | 12.303125 |
| 1/2" | 0.5 | 12.7 |
| 33/64" | 0.515625 | 13.096875 |
| 17/32" | 0.53125 | 13.49375 |
| 35/64" | 0.546875 | 13.890625 |
| 9/16" | 0.5625 | 14.2875 |
| 37/64" | 0.578125 | 14.684375 |
| 19/32" | 0.59375 | 15.08125 |
| 39/64" | 0.609375 | 15.478125 |
| 5/8" | 0.625 | 15.875 |
| 41/64" | 0.640625 | 16.271875 |
| 21/32" | 0.65625 | 16.66875 |
| 43/64" | 0.671875 | 17.065625 |
| 11/16" | 0.6875 | 17.4625 |
| 45/64" | 0.703125 | 17.859375 |
| 23/32" | 0.71875 | 18.25625 |
| 47/64" | 0.734375 | 18.653125 |
| 3/4" | 0.75 | 19.05 |
| 49/64" | 0.765625 | 19.446875 |
| 25/32" | 0.78125 | 19.84375 |
| 51/64" | 0.796875 | 20.240625 |
| 13/16" | 0.8125 | 20.6375 |
| 53/64" | 0.828125 | 21.034375 |
| 27/32" | 0.84375 | 21.43125 |
| 55/64" | 0.859375 | 21.828125 |
| 7/8" | 0.875 | 22.225 |
| 57/64" | 0.890625 | 22.621875 |
| 29/32" | 0.90625 | 23.01875 |
| 59/64" | 0.921875 | 23.415625 |
| 15/16" | 0.9375 | 23.8125 |
| 61/64" | 0.953125 | 24.209375 |
| 31/32" | 0.96875 | 24.60625 |
| 63/64" | 0.984375 | 25.003125 |
| 1" | 1 | 25.4 |
!
The ladder stops at 1" because the steps widen above it. Fractional sizes at 1-1/16" and up are still sold, but they are no longer spaced at 1/64", so one table cannot carry them honestly.
ROUNDING RULE
Which Way to Round, and Why It Is Not Symmetric
The working rule: prefer the nearest fraction that will still fit, and treat any near match as temporary. A near match is for removing a low-torque or already-damaged fastener. It's never for a torque application you intend to last.
THE CASE PEOPLE GET WRONG
13 mm vs 1/2"
1/2" in mm
12.7 mm
Gap
0.300 mm
Gap as % of fastener
2.36%
Effect
Fits, then rounds the hex
TWO PUBLISHED RULES, OPPOSITE ANSWERS
Machinist Guidance
Take the nearest size up
13
mm
VS
Wrench Reference
Metric and SAE are not interchangeable
1/2
in
0.300
mm apart — small enough to feel right,
large enough to round the fastener
Apply the first rule and you round up. Apply the second
to the same pair and you get the failure it warns about.
The two describe different jobs, and neither one says which. The condition below settles it.
01
Use the Near Match When
The load is low, the fastener is already damaged, or you're removing something you intend to replace.
02
Don't Use It When
You're torquing to a specification, or working on a fastener softer than the tool itself.
03
Need the Nearest Standard Size?
The metric to SAE socket conversion works one size at a time, and the wrench size chart covers both ladders side by side.
LABELS, NOT MEASUREMENTS
When a Tool Size Is Not a Dimension
Some tool sizes are labels. Convert a label and you get a confidently wrong number, because the input was never a measurement.
01
Pipe and Hose
Nominal bore, not diameter
02
Wrench Rating
Gripping capacity, not jaw opening
03
Thread Designation
Size label, not a length
04
Measure First
Convert the dimension, not the label
EXAMPLE 01
Pipe and Hose
Label
1/2" pipe
Label implies
12.7 mm
Actual OD (NPS Sch 40)
21.3 mm
Difference
≈ 68% wider
WHY THE NUMBER MOVES
The label is a nominal bore. Outside diameter is governed by the pipe standard and the wall schedule: change the schedule and the bore changes while the outside diameter stays put.
21.3
mm outside diameter for a pipe labelled 1/2 inch
EXAMPLE 02
Pipe Wrench
Published gripping capacity
76 mm
Published jaw opening
103 mm
Buyer converts jaw opening
4" expected
Actual error
27 mm
GRIPPING CAPACITY IS NOT JAW OPENING
A pipe wrench is rated by gripping capacity, which is not jaw opening. A buyer who converts the 103 mm figure and expects a 4 inch capacity is working from the wrong number by 27 mm.
27
mm of error from converting the wrong dimension
!
The procedure is the same in both cases.
Measure the part across the dimension that matters, then convert that. Outside diameter for a pipe or a tube. Across-flats distance for a hex. Bore for a hose or a fitting. Never convert the label.
If a measured dimension sits between the sizes the charts carry,
solve it with better instruments before it becomes a tool problem.
OUSHKE's measuring tools range covers calipers and gauges, and a caliper reading removes the guesswork a tape measure leaves behind.
FOUR SYSTEMS
Inch Tool Sizes That Are Not Fractions
CONVERTER
Assumes a fraction
A converter assumes an inch dimension is a fraction. Four systems run alongside each other in tooling, and only one of them is.
| System | Example | What It Actually Is |
|---|---|---|
| Fractional | 1/4", 3/8", 1/2" | The one every converter handles |
| Number | #7 = 0.2010" = 5.1054 mm | A series that runs backwards, with #1 the largest and #80 the smallest |
| Letter | A = 0.2340" through Z = 0.4130" | Also not a fraction, and not a length you can convert |
| Thread | 3/8" NPT, 1/4" BSP | A designation, where the actual diameter differs from the number |
!
Type 7 into a mm to inch converter and you get an answer for 7 mm, or for 7 inches. Nothing tells you the input was never a length. For a number or letter drill, look the size up rather than converting it.
PRACTICAL NOTE
Convert only measurements. Look up designations.
If the number in front of you is a size label rather than a measured dimension, a converter is the wrong tool for it. Look up the size in its own table.
Frequently Asked Questions (FAQs)
Find answers to common questions about our hand tools, their care, and usage. If you need additional information, our customer support team is ready to assist you.
How many mm are in an inch exactly?
Exactly 25.4 mm. The figure is a defined constant rather than a measurement, agreed internationally in 1959, so it carries no tolerance wherever the inch is used.
What is 13 mm in inches as a fraction?
13 mm is 0.511811". At 1/64 resolution the nearest fraction is 33/64", which is 13.096875 mm and 0.097 mm oversize. The nearest fraction most people reach for is 1/2", which is 0.300 mm undersize.
Is 13 mm the same as 1/2 inch?
No. They are 0.300 mm apart, or 2.36% of the fastener. That's close enough for the tool to feel right and far enough to round a hex under load.
Is a 1/2 inch pipe 12.7 mm?
No. Under NPS Schedule 40 a 1/2 inch pipe measures about 21.3 mm outside diameter. The 1/2 inch figure is a nominal bore, not a dimension you can convert.
Why does my measurement not match a standard tool size?
Because inch tooling is sold in steps and most metric dimensions fall between them. The converter gives you the nearest step and the error, so you can judge whether the fit is good enough for the job.