PSI supports two unit systems: SI (metric) and US customary. The selected unit system applies globally across all input forms — geometry, loads, pressures, temperatures, and report output all use the same unit system simultaneously.
Switch the unit system before entering any values. Changing the unit system after filling in form fields does not automatically convert existing values, which can lead to incorrect calculations.
SI vs US Units
The SI unit system uses metric units throughout PSI:| Quantity | Unit | Symbol |
|---|
| Length (diameter, height, thickness) | Millimeters | mm |
| Pressure | Megapascals | MPa |
| Temperature | Degrees Celsius | °C |
| Wind speed | Meters per second or kilopascals | m/s or kPa |
The US customary unit system uses imperial units throughout PSI:| Quantity | Unit | Symbol |
|---|
| Length (diameter, height, thickness) | Inches | in |
| Pressure | Pounds per square inch | psi |
| Temperature | Degrees Fahrenheit | °F |
| Wind speed | Miles per hour or pounds per square foot | mph or psf |
Unit Conversion Reference
Use this table when converting values between unit systems:
| Quantity | SI | US Customary | Conversion factor |
|---|
| Length | 1 mm | 0.03937 in | 1 in = 25.4 mm |
| Pressure | 1 MPa | 145.038 psi | 1 psi = 0.006895 MPa |
| Temperature | °C | °F | °F = (°C × 9/5) + 32 |
| Temperature | 0 °C | 32 °F | — |
| Temperature | 100 °C | 212 °F | — |
How Unit Labels Work in PSI
PSI displays the active unit label directly on each input field so you always know which unit is expected. For example:
- When SI is active: Diámetro exterior (mm), Espesor de pared (mm)
- When US is active: Diámetro exterior (in), Espesor de pared (in)
The same labeling applies to pressure and temperature fields in the design parameters form. In the Summary Report, values are printed with their unit appended — for example, 1.5 MPa or 250 psi — based on the unit system that was active when the report was generated.
The unit system selection is stored at the session level. If you reload the page, verify that the correct unit system is still selected before resuming data entry.
