How do you convert psi to feet of head?

Use Head (ft) = 2.31 × Pressure (psi) ÷ specific gravity. For water (SG = 1), 100 psi equals 231 ft of head. The inverse is Pressure (psi) = Head (ft) × SG ÷ 2.31, so 150 ft of water equals 64.9 psi.

Why does specific gravity matter when converting head and pressure?

Pressure equals density times height (P = ρ·g·h), so a denser fluid makes more pressure per foot. Head scales as 1/SG: 100 psi is 231 ft of water but 271.8 ft of diesel (SG 0.85). A pump develops fixed head, so its pressure rise changes with the fluid's SG.

What is the 2.31 factor in the head-to-pressure formula?

2.31 is the feet of water that exert 1 psi at the base of the column at SG = 1, from 144 in²/ft² ÷ 62.4 lb/ft³ = 2.3094 ft per psi. Its reciprocal, 0.433, is the psi per foot of water. Dividing by SG adapts both to any fluid.

Head vs Pressure: The 2.31 ft/psi Factor & Specific Gravity

1. Head vs Pressure

Head and pressure are two ways of describing the same thing: the energy a fluid carries because of the height of its column. Head is the vertical height of a column of the fluid (measured in feet or metres); pressure is the force that column exerts on its base (measured in psi, bar or kPa). They are linked directly through fluid statics.

The static relationship (P = ρ·g·h): Head (ft) = 2.31 × P (psi) ÷ SG P (psi) = Head (ft) × SG ÷ 2.31 Where SG = specific gravity of the fluid (water = 1.0)

Why pumps are rated in head, not pressure

A centrifugal pump adds a fixed amount of head to whatever it is pumping, regardless of the fluid's density — the impeller throws the liquid the same height whether it is water or gasoline. The pressure rise it produces, however, depends on how heavy that liquid is. Rating pumps in feet of head therefore makes a single performance curve valid for every fluid; the engineer simply converts to pressure for the specific service using SG.

Quantity	Symbol	Common Units	Depends on density?
Head (column height)	H	ft, m	No — set by the pump
Pressure	P	psi, bar, kPa	Yes — rises with SG
Specific gravity	SG	dimensionless	It is the density ratio

Quick example: A pump developing 100 ft of head produces 43.3 psi with water (SG = 1.0) but only 36.8 psi with light oil (SG = 0.85). The head is constant; the pressure varies with density.

2. The 2.31 ft/psi Constant

The whole conversion hinges on one number: at SG = 1, a column of water 2.31 ft tall exerts exactly 1 psi at its base. That constant is not arbitrary — it falls straight out of the density of water and the definition of a pound per square inch.

Where 2.31 comes from

Derivation from 144 / 62.4: Water density ρ = 62.4 lb/ft³ 1 ft² of base = 144 in² Pressure from a 1-ft column of water: P = 62.4 lb/ft³ × 1 ft ÷ 144 in²/ft² P = 0.4333 psi per foot of water Invert to get feet per psi: 1 ÷ 0.4333 = 2.3094 ≈ 2.31 ft of water per psi

Two faces of the same constant: 0.433 psi/ft is how much pressure each foot of water adds; 2.31 ft/psi is how many feet of water make 1 psi. They are reciprocals (0.4333 × 2.3094 = 1.0).

Adapting the constant to any fluid

Those two numbers are for water (SG = 1). For any other liquid you divide head by SG (or multiply pressure by SG), because the column's weight scales directly with density:

Head (ft) = 2.31 × P (psi) ÷ SG P (psi) = Head (ft) × SG ÷ 2.31 Equivalent per-foot form: ΔP per foot = 0.433 × SG (psi/ft)

Quantity	Value at SG = 1 (water)	General fluid
Feet of column per psi	2.31 ft/psi	2.31 ÷ SG
psi per foot of column	0.433 psi/ft	0.433 × SG
Metres per bar (≈)	10.2 m/bar	10.2 ÷ SG

3. Role of Specific Gravity

Specific gravity is the lever that turns the water-based 2.31 constant into a true answer for any liquid. Because pressure at the bottom of a column equals density times height (P = ρ·g·h), a heavier fluid pushes harder for every foot of column. Two consequences follow directly:

For a fixed pressure, head scales as 1/SG. A denser fluid needs a shorter column to make the same psi, so the equivalent head is smaller.
For a fixed head, pressure scales as SG. The same column height of a denser fluid produces more psi.

Same 100 psi, different fluids: Water (SG 1.00): H = 2.31 × 100 ÷ 1.00 = 231.0 ft Diesel (SG 0.85): H = 2.31 × 100 ÷ 0.85 = 271.8 ft Brine (SG 1.15): H = 2.31 × 100 ÷ 1.15 = 200.9 ft The lighter the fluid, the taller the column for the same pressure.

Fluid	Typical SG	Head for 100 psi	psi for 100 ft of head
Gasoline	0.74	312.2 ft	32.0 psi
Diesel / light crude	0.85	271.8 ft	36.8 psi
Water	1.00	231.0 ft	43.3 psi
Seawater	1.03	224.3 ft	44.6 psi
Heavy brine	1.20	192.5 ft	52.0 psi

Pump-curve takeaway: A centrifugal pump's curve is plotted in feet of head precisely so it works for every row above. The developed pressure in psi changes with the SG of whatever is being pumped — which is exactly why SG belongs in the conversion.

⚠ Watch the gauge vs absolute basis. The 2.31·psi/SG relation converts a pressure difference (or a gauge pressure) into a column height. When you need an absolute pressure (e.g. for NPSH or vapor-pressure work), add atmospheric pressure separately before converting.

4. Worked Examples

Example 1 — Pressure to head (water)

A gauge reads 100 psi on a water line. How many feet of head is that?

H = 2.31 × P ÷ SG H = 2.31 × 100 ÷ 1.00 H = 231.0 ft (= 70.4 m)

Example 2 — Same pressure, lighter fluid (gasoline vs water vs brine)

The same 100 psi acting on three different fluids produces three different column heights — the heart of why SG matters:

Gasoline (SG 0.74): H = 2.31 × 100 ÷ 0.74 = 312.2 ft Water (SG 1.00): H = 2.31 × 100 ÷ 1.00 = 231.0 ft Brine (SG 1.20): H = 2.31 × 100 ÷ 1.20 = 192.5 ft Same 100 psi → the lighter the fluid, the taller the column.

Example 3 — Head to pressure

A pump develops 150 ft of head on water. What discharge pressure is that?

P = H × SG ÷ 2.31 P = 150 × 1.00 ÷ 2.31 P = 64.9 psi On diesel (SG 0.85) the same 150 ft would be only: P = 150 × 0.85 ÷ 2.31 = 55.2 psi

Example 4 — Metric / non-psi input

A vendor quotes 10 bar on water. Convert to feet of head. First convert the unit on an SG-independent basis, then apply the formula:

10 bar × 14.5038 = 145.04 psi H = 2.31 × 145.04 ÷ 1.00 = 335.0 ft (= 102.1 m)

5. Unit Conversions

Head–pressure conversion happens in two steps: first put the input on a common basis (psi for pressure, feet for head) using density-independent unit factors, then apply the 2.31/SG formula. Mixing those two steps up — folding SG into a unit factor — is the most common conversion error.

Pressure unit factors

From	To psi	Note
1 bar	14.5038 psi	bar × 14.5038
1 kPa	0.145038 psi	kPa ÷ 6.89476
1 psi	6.89476 kPa	psi × 6.89476
1 psi	0.0689476 bar	psi ÷ 14.5038

Head unit factors

From	To	Note
1 m	3.28084 ft	m × 3.28084
1 ft	0.304800 m	ft ÷ 3.28084

Order of operations: (1) convert the input pressure to psi or the input head to feet using the factors above — these never involve SG; (2) apply Head = 2.31·psi/SG (or its inverse); (3) convert the answer to whatever output unit you want. The SG term appears once, in step 2 only.

Common Mistakes to Avoid

❌ Forgetting the SG term — using plain 2.31 for a non-water fluid.
❌ Folding SG into a unit factor instead of applying it once in the formula.
❌ Mixing gauge and absolute pressure when the application needs absolute.
❌ Confusing 0.433 (psi per foot) with 2.31 (feet per psi) — they are reciprocals.
❌ Assuming the pump's developed psi is fixed — it tracks the fluid SG even though head is fixed.

Key References

Fluid statics – P = ρ·g·h (hydrostatic pressure of a fluid column)
Water density – 62.4 lb/ft³ at 60°F, giving 0.433 psi/ft and 2.31 ft/psi
Cameron Hydraulic Data – head/pressure conversion tables
Crane TP-410 – Flow of Fluids Through Valves, Fittings, and Pipe