Pressure and Head Conversion Chart

What it is

Pressure is stated in many units. US piping uses psi. Metric specs use bar, kPa or MPa. Pumps and water systems often use feet of head, the height of a water column that the pressure would support. NIST SP 811 gives the exact conversion factors used here.

How to use it

1. Find the unit you have in the left column.
2. Multiply by the factor to get the unit you want.
3. For head in feet of a fluid other than water, divide the water head by the fluid's specific gravity.

Conversion factors

Water head factors are based on water at about 39°F (4°C). Field practice often rounds to 2.31 ft per psi and 0.433 psi per foot.

Gauge and absolute pressure

• psig (gauge) reads zero at local atmospheric pressure.
• psia (absolute) reads zero at a perfect vacuum.
• psia = psig + atmospheric pressure (about 14.7 psi at sea level).
• Vacuum is often given in inches of mercury below atmosphere.

Key points

• Always state gauge or absolute on specs above a few psi and for any vacuum work.
• Atmospheric pressure drops with elevation. Use the local value for accurate absolute readings.
• Head to pressure: psi = feet of head × 0.4335 × specific gravity.
• 1 bar is close to, but not equal to, 1 atmosphere.
• Round only at the end of a calculation.

Common mistakes

• Mixing psig and psia in the same calculation.
• Using water head factors for oil, brine or glycol without the specific gravity.
• Reading kPa as bar, which is off by a factor of 100.
• Rounding 1 bar to 15 psi on test documents.

Governing standards: NIST SP 811 (guide for the use of the International System of Units).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.