How the static head calculator works
Enter the vertical height of liquid between two points, pick a liquid or type its specific gravity, and read the head in psi, kPa and bar. Add a gauge pressure at the top to get the pressure at the lower point, or start from a lower gauge and work up. The height is always the vertical distance through the liquid, from the liquid surface or the higher point down, never the length of the pipe or the shell.
Static head formula: p = ρ · g · h
Hydrostatic head is the pressure difference caused by a stationary liquid column: Δp = ρ × g × h. Here ρ is uniform liquid density in kg/m³, g is gravity in m/s², and h is vertical liquid height in meters. The result is in pascals (Pa). Source: NASA Glenn Research Center, Fluids Pressure and Depth.
This study model uses ρ = SG × 1,000 kg/m³ and standard gravity g = 9.80665 m/s². SG is specific gravity, a dimensionless density ratio. The nominal water reference is an approximation; actual water density varies with temperature. Sources: Spirax Sarco, Engineering Units and NIST, Guide to the SI, Footnotes.
- Height: h(m) = h(ft) × 0.3048.
- Head in kPa = SG × 9.80665 × h(m).
- Head in bar = head in Pa ÷ 100,000.
- Head in psi ≈ 0.433 × SG × h(ft).
- Pressure at lower point B = pressure at upper point A + Δp.
- Pressure at upper point A = pressure at lower point B − Δp.
The 0.433 shortcut is rounded. The calculator retains the SI calculation and uses 1 psi ≈ 6,894.757293 Pa, giving approximately 0.4335275 psi per foot at SG = 1. Unit definitions: NIST, Guide to the SI, Appendix B.8. Display values have six significant digits; exported numbers retain calculation precision.
Worked example: 20 ft of water and 100 psi at the top
These are fictional study inputs. SG = 1, so model density is 1,000 kg/m³. Height is 20 × 0.3048 = 6.096 m. Head = 1,000 × 9.80665 × 6.096 = 59,781.3384 Pa = 59.7813 kPa = 0.597813 bar ≈ 8.67055 psi.
With 100 psi gauge at A, pressure at B is 100 + 8.67055 = 108.671 psi gauge (rounded). The shortcut gives 0.433 × 1 × 20 = 8.66 psi of head; its small difference comes from rounding the coefficient.
For the reverse direction, if a gauge at the lower point reads 150 psi, the pressure at the top of the 20 ft column is 150 − 8.67055 = 141.329 psi gauge (rounded).
Where static head shows up in vessel work
A part low in a vessel carries the pressure at the top plus the liquid above it, so the pressure it allows at the top is its own MAWP minus that head (ASME VIII-1 UG-98). During a hydrotest the test pressure is set at the top, and the bottom of a tall vessel sees the test water's head on top of it (UG-99). And a gauge mounted low reads more than one at the top by the head between them. The MAWP and thickness calculator and the hydrotest pressure calculator pick up from here, and the API 510 formula sheet puts the steps side by side.
Common static head mistakes
- Using the vessel length. On a horizontal drum the liquid column is at most the inside diameter, however long the shell.
- Dropping specific gravity. 30 ft of glycerine at SG 1.26 is 16.3873 psi of head. The water figure, 13.0058 psi, misses about a fifth of it.
- Adding when you should subtract. Going down the column adds head and going up takes it away. Fix which point your known pressure is at before you touch the numbers.
- Mixing gauge and absolute pressure. Keep both points in gauge pressure, or both in absolute. Head added to an absolute reading and reported as gauge is off by about one atmosphere.
- Treating 0.433 as exact. It is rounded. On a 100 ft water column it comes out about 0.05 psi under the exact figure, so match the precision your answer key or design basis uses.
What this calculator does not do
Vessel and component pressure ratings come from the vessel's design data and nameplate under ASME Section VIII.This calculator does not compute or adjust them. It only converts a liquid column into a pressure difference between two elevations. Check any design question with the responsible inspector or engineer.
Liquid presets and service conditions
- Water (nominal): SG 1.00. Spirax Sarco, Engineering Units (density and specific gravity).
- Ethanol (reference): SG 0.79. Occupational Safety and Health Administration, Occupational Chemical Database: Ethyl Alcohol (Ethanol), physical properties.
- Glycerine (reference): SG 1.26. Occupational Safety and Health Administration, Occupational Chemical Database: Glycerine (Mist), physical properties.
These are approximate reference values, not guaranteed process-fluid properties. The OSHA pages do not state a reference temperature beside these SG values. Presets are not corrected for temperature, concentration or pressure. For actual service conditions, obtain the liquid density from your approved data and enter custom SG = density (kg/m³) ÷ 1,000.
Assumptions and limits
The model covers one continuous, stationary liquid with uniform density and constant standard gravity. It excludes friction and flow losses, pumps, acceleration, pressure transients, gas-column weight, stratified liquids and density changes with depth. It uses vertical liquid depth, not the vessel's overall length. In a partly filled vessel, the upper reference is the liquid surface; gas-space pressure is treated as uniform.
Pressure inputs and translated outputs are gauge pressures with a common atmospheric reference. Head itself is a pressure difference. The same addition and subtraction can be derived for a consistent absolute basis, but this interface does not accept absolute-pressure or vacuum inputs.
For a separate thickness-loss study calculation, use the remaining life calculator.
Static head pressure FAQ
How do you calculate static head pressure?
Multiply the liquid's density by gravity and by the vertical height of liquid: Δp = ρ × g × h. In US units that works out to about 0.433 psi per foot of water, times the specific gravity. Twenty feet of water gives 8.67 psi; the same 20 ft of a liquid with SG 0.85 gives about 7.37 psi.
How do you convert psi to feet of head?
Divide the pressure by 0.433 × SG. For water that is the same as multiplying psi by about 2.31, so 10 psi of water head is about 23.1 ft. For a liquid with SG 0.85, divide the water figure by 0.85: about 27.1 ft.
Does the shape or size of the vessel change the static head?
No. Only the vertical height of liquid and its density count. A narrow standpipe and a wide tank filled to the same level put the same pressure on their bottoms.
Is static head the same as total dynamic head?
No. Static head is the liquid column at rest. A pump's total dynamic head also adds friction and velocity losses in a flowing system, which this calculator leaves out.
Why does static head matter for API 510?
Static head is one of the vessel integrity calculations in the API 510 body of knowledge. It decides how much pressure a low part of the vessel really sees, which feeds part MAWP under ASME Section VIII Division 1 UG-98 and the pressure at the bottom during a hydrotest under UG-99.