Quick Answer: How Do I Convert Engineering Units?
An engineering unit converter turns one measurement into its equivalent in another system, for example millimeters to inches or PSI to MPa. Machinists and engineers use it to read prints, compare material data sheets and set up CNC jobs in mixed metric and imperial units. The core conversions are fixed ratios: 1 inch is exactly 25.4 mm, 1 MPa is 145.038 PSI and 1 Ra µm is 39.37 Ra µin.
This converter uses exact ratio factors plus ASTM E140 and ISO N-grade lookup tables to compute: length (mm, inches, thou and microns with nearest fractional inch), surface finish (Ra, Rz, RMS and N-grade), hardness (HRC, HB and HV), and pressure, temperature, weight and torque. Inputs are any single field per tab, every other field updates as you type.
Common machinist conversions: 1 in = 25.4 mm | 1 thou = 0.0254 mm | 1 MPa = 145 PSI | 1 ksi = 6.895 MPa | 1 Ra µm = 39.37 Ra µin | 68°F = 20°C | 1 lb = 0.4536 kg | 1 ft·lb = 1.356 N·m. Standard CNC finish: 63–125 µin Ra (1.6–3.2 µm).
Common Conversions Every Machinist Knows
You will use some conversions so often that you should know them by heart. Here are the ones that come up every day in a machine shop.
Length
- 1 inch = 25.4 mm (exact)
- 1 thou (mil) = 0.001 inch = 0.0254 mm
- 1 micron = 0.001 mm = 0.00004 inch
- 1 foot = 304.8 mm
Pressure and Stress
- 1 MPa = 145 PSI (roughly)
- 1 ksi = 1,000 PSI = 6.895 MPa
- 1 bar = 14.5 PSI (roughly)
Temperature
- Room temp: 68°F = 20°C
- Water boils: 212°F = 100°C
- Quick formula: °C = (°F − 32) × 5/9
Surface Finish
- 1 µm Ra = 39.37 µin Ra
- Standard CNC finish: 63–125 µin Ra (1.6–3.2 µm)
- Fine finish: 16–32 µin Ra (0.4–0.8 µm)
Tip: Keep a small reference card at your workstation. Even the best machinists double-check their conversions. Getting it right the first time saves scrap and rework.
Hardness
- Mild steel: ~80–90 HRB (too soft for HRC)
- Heat-treated 4140: ~28–32 HRC
- Tool steel (hardened): ~58–62 HRC
- HRC 60 is very hard. Most cutting tools are above HRC 60.
Weight and Torque
- 1 lb = 0.4536 kg
- 1 kg = 2.205 lbs
- 1 ft-lb = 1.356 N·m
- 1 N·m = 8.851 in-lbs