Set the altimeter to 29.92 inHg
Read the altitude shown on the altimeter
Use the formula: Pressure Altitude = Field Elevation + (29.92 – Current Altimeter Setting) × 1000
If the altimeter setting is below 29.92, add the difference in inches of mercury multiplied by 1000
If the altimeter setting is above 29.92, subtract the difference in inches of mercury multiplied by 1000
Use the airport elevation as the field elevation if calculating from the ground
Verify the result against a flight computer or performance chart if needed