Layout and shop math
Metric conversions for machinists
1 in = 25.4 mm exactly, so 0.001 in is 25.4 µm and a tenth (0.0001 in) is 2.54 µm; 400 SFM is 121.92 m/min (x 0.3048, also defined exactly).
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
Converted
mm
0.375 in x 25.4 = 9.525 mm
Full value 9.525 mm. Shop value rounded to 2 places. Starting point for setup; round tolerances toward the tighter side.
| Unit | Full value | Shop value | Multiply by, with your number |
|---|---|---|---|
| mm | 9.525 mm | 9.53 mm | 0.375 in x 25.4 = 9.525 mm |
| µm | 9,525 µm | 9,525.00 µm | 0.375 in x 25,400 = 9,525 µm |
| thou | 375 thou | 375.00 thou | 0.375 in x 1,000 = 375 thou |
| tenths | 3,750 tenths | 3,750.0 tenths | 0.375 in x 10,000 = 3,750 tenths |
Source: NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B; NIST Handbook 44, Appendix C (General Tables of Units of Measurement); NIST Special Publication 1038. 25.4 mm per inch and 0.3048 m per foot are defined exactly. Reference temperature: ISO 1:2022.
Conversions are unit arithmetic. Round only the final number, and round tolerances toward the tighter side.
How it works
Converted value = value x factor; the reverse divides by the same factor. Temperature uses °C = (°F - 32) / 1.8 and °F = °C x 1.8 + 32. A fraction is turned into a decimal first: 3/8 in = 0.375 in, then x 25.4. SFM (surface feet per minute) is cutting speed; IPM (inches per minute), IPR (inches per revolution) and IPT (feed per tooth) are feeds. Thou is 0.001 in, a tenth is 0.0001 in, µm is a micrometer (micron), µin a microinch, Ra the arithmetic average roughness, lbf pound-force, N newton, and hp is mechanical horsepower (550 ft·lbf/s).
| Quantity | From | To | Multiply by | Status | Source |
|---|---|---|---|---|---|
| Length | in | mm | 25.4 | Exactly, by definition | NIST SP 811, App. B; NIST HB 44, App. C |
| Length | thou (0.001 in) | µm | 25.4 | Exactly, derived from 25.4 | Derived |
| Length | tenths (0.0001 in) | µm | 2.54 | Exactly, derived from 25.4 | Derived |
| Length | mm | in | 1 / 25.4 = 0.03937008 | Inverse of a defined factor | Derived |
| Cutting speed | SFM | m/min | 0.3048 | Exactly, by definition | NIST SP 811, App. B; NIST HB 44, App. C |
| Cutting speed | m/min | SFM | 1 / 0.3048 = 3.280840 | Inverse of a defined factor | Derived |
| Feed rate | IPM | mm/min | 25.4 | Exactly, derived from 25.4 | Derived |
| Feed per rev | IPR | mm/rev | 25.4 | Exactly, derived from 25.4 | Derived |
| Feed per tooth | IPT | mm/tooth | 25.4 | Exactly, derived from 25.4 | Derived |
| Metal removal rate | in³/min | cm³/min | 16.387064 | Exactly (25.4^3 / 1000) | NIST HB 44, App. C; NIST SP 811, App. B |
| Power | hp (550 ft·lbf/s) | kW | 0.7456999 (shown 0.7457) | Rounded | NIST SP 811, App. B; NIST SP 1038 |
| Force | lbf | N | 4.448222 | Rounded | NIST SP 811, App. B; NIST SP 1038 |
| Torque | ft-lb (lbf·ft) | N·m | 1.355818 | Rounded | NIST SP 811, App. B; NIST SP 1038 |
| Surface finish | µin Ra | µm Ra | 0.0254 | Exactly, derived from 25.4 | NIST SP 811, App. B; NIST SP 1038 |
| Temperature | °F | °C | °C = (°F - 32) / 1.8; °F = °C x 1.8 + 32 | Exactly, by formula | NIST SP 811, App. B; NIST HB 44, App. C |
Electric horsepower is 0.746 kW; this page uses mechanical horsepower (0.7457 kW), as the metal removal rate calculator does. The difference is about 0.3 W per horsepower, 0.04%.
Common sizes, inch to mm
| Fraction (in) | Decimal (in) | Metric, full value (mm) | Metric, 2 places (mm) |
|---|---|---|---|
| 1/64 | 0.0156 (0.015625) | 0.396875 | 0.40 |
| 1/32 | 0.0313 (0.03125) | 0.79375 | 0.79 |
| 1/16 | 0.0625 | 1.5875 | 1.59 |
| 1/8 | 0.1250 | 3.175 | 3.18 |
| 3/16 | 0.1875 | 4.7625 | 4.76 |
| 1/4 | 0.2500 | 6.35 | 6.35 |
| 5/16 | 0.3125 | 7.9375 | 7.94 |
| 3/8 | 0.3750 | 9.525 | 9.53 |
| 7/16 | 0.4375 | 11.1125 | 11.11 |
| 1/2 | 0.5000 | 12.7 | 12.70 |
| 9/16 | 0.5625 | 14.2875 | 14.29 |
| 5/8 | 0.6250 | 15.875 | 15.88 |
| 11/16 | 0.6875 | 17.4625 | 17.46 |
| 3/4 | 0.7500 | 19.05 | 19.05 |
| 13/16 | 0.8125 | 20.6375 | 20.64 |
| 7/8 | 0.8750 | 22.225 | 22.23 |
| 15/16 | 0.9375 | 23.8125 | 23.81 |
| 1 | 1.0000 | 25.4 | 25.40 |
Source: Derived from 25.4 mm per inch (exactly, NIST Handbook 44)
| Inch (in) | Shop name | Metric (mm) | Micrometers (µm) |
|---|---|---|---|
| 0.0001 | one tenth | 0.00254 | 2.54 |
| 0.0005 | half a thou (five tenths) | 0.0127 | 12.70 |
| 0.001 | one thou | 0.0254 | 25.40 |
| 0.005 | five thou | 0.127 | 127.00 |
| 0.010 | ten thou | 0.254 | 254.00 |
Source: Derived from 25.4 mm per inch
Number, letter and every 1/64 in drill size, with mm, are on the drill size chart.
Worked example: an inch job sheet for a metric setup
400 SFM, 0.008 IPR, 15.0 IPM on the mill, 1.500 in³/min, a 5 hp spindle.
- 400 x 0.3048 = 121.92 m/min.
- 0.008 x 25.4 = 0.2032 mm/rev, shown 0.203 mm/rev.
- 15.0 x 25.4 = 381.00 mm/min.
- 1.500 x 16.387064 = 24.58 cm³/min (24.580596 cm³/min).
- 5 x 0.7456999 = 3.73 kW (3.7285 kW).
Result: 121.92 m/min, 0.203 mm/rev, 381.00 mm/min, 24.58 cm³/min, 3.73 kW.
Worked example: a metric drawing read in inch
A 12.70 mm bore, +/-0.02 mm, Ra 1.6 µm, dimensions at 20 °C.
- 12.70 / 25.4 = 0.5000 in.
- 0.02 / 25.4 = 0.000787 in, shown 0.0008 in (about 8 tenths).
- 1.6 / 0.0254 = 62.99 µin, shown 63 µin Ra.
- 20 °C x 1.8 + 32 = 68.0 °F.
- Reverse checks: 63 µin x 0.0254 = 1.60 µm Ra (1.6002 µm Ra); (68 - 32) / 1.8 = 20.0 °C.
Result: 0.5000 in bore, +/-0.000787 in (hold +/-0.0007 in on a tenths gauge), 63 µin Ra, at 68.0 °F.
Other checks
- 120 m/min / 0.3048 = 394 SFM (393.70 SFM).
- 500 lbf x 4.448222 = 2,224.1 N.
- 30 ft-lb x 1.355818 = 40.67 N·m; 100 N·m / 1.355818 = 73.76 ft-lb.
- 7.5 kW / 0.7456999 = 10.06 hp.
- 0.0005 in = 0.0127 mm = 12.70 µm.
Shop notes
- The inch was defined as exactly 25.4 mm in 1959, so inch and mm conversions carry no rounding error until you round the result.
- Round a converted tolerance inward, not to the nearest: +/-0.02 mm is +/-0.000787 in, so hold +/-0.0007 in if the gauge reads tenths.
- Dimensions on a drawing apply at 20 °C (68 °F) unless the drawing says otherwise (ISO 1:2022, geometrical product specifications, standard reference temperature).
- Horsepower on this page is mechanical (550 ft·lbf/s, 0.7457 kW). Electric horsepower is 0.746 kW; the difference is about 0.3 W per horsepower, 0.04%.
- For feeds from RPM and flutes, use the feed rate conversion calculator; this page converts units only.
FAQ
How many mm are in an inch?
25.4 mm exactly; the inch is defined that way, so 3/8 in is 9.525 mm with no rounding.
What is 0.001 in in microns?
25.4 µm. A tenth (0.0001 in) is 2.54 µm, and half a thou (0.0005 in) is 12.70 µm.
How do you convert SFM to m/min?
Multiply by 0.3048: 400 SFM is 121.92 m/min. Going back, divide by 0.3048: 120 m/min is 394 SFM.
What is 63 µin Ra in µm?
1.60 µm Ra: 63 µin x 0.0254. 1.6 µm going the other way is 63 µin Ra.
How do you convert horsepower to kW?
Multiply by 0.7457: a 5 hp spindle is 3.73 kW.
At what temperature are drawing dimensions measured?
20 °C (68 °F), the standard reference temperature in ISO 1, unless the drawing states another.
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