This calculator converts pressure into equivalent fluid head, converts head back into pressure, or determines the fluid’s specific gravity. Choose the variable to calculate and enter the other two. Pressure and head are two ways of expressing the same energy per unit weight, with specific gravity accounting for fluid density.
The formulas
In US units, H (ft) = 2.31 · P (psi) / SG. In metric units, H (m) = 10.2 · P (bar) / SG. Rearranging gives P = H · SG / K and SG = K · P / H, where K is 2.31 ft/psi or 10.2 m/bar.
Why specific gravity matters
A pressure gauge measures force per area, while head expresses energy as the height of a fluid column. At the same pressure, a lighter fluid produces a taller column and a denser fluid produces a shorter one. For example, 100 psi corresponds to 231 ft of water at SG = 1.0, but about 289 ft of a light hydrocarbon at SG = 0.8.
Keep the pressure basis consistent
The relationship works with gauge pressure, absolute pressure, or a pressure difference, but the pressure and head must describe the same basis. Gauge pressure produces gauge head; absolute pressure produces absolute head. In pump calculations, pressure rise normally means discharge pressure minus suction pressure using readings on the same basis.
Solve in any direction
- Head (H) — enter pressure and fluid specific gravity.
- Pressure (P) — enter fluid head and specific gravity.
- Specific gravity (SG) — enter corresponding pressure and head values.
Worked example
A pump raises the pressure of a liquid with SG = 0.85 by 100 psi. The equivalent head is H = 2.31 × 100 ÷ 0.85 = 271.8 ft. Entering 271.8 ft and SG = 0.85 in the reverse calculation returns approximately 100 psi.
Head versus pressure
A centrifugal pump develops essentially the same head at a given operating point regardless of fluid density, but the pressure rise changes with specific gravity. That is why pump curves are normally plotted in head rather than pressure. Use the result with the Total Dynamic Head Calculator to build a complete system requirement.