🏠 Home NPSH Calc Guide FAQ
Engineering Fluid Calculator

Head ⇄ Pressure
Converter

Convert between fluid column head and pressure for any process fluid — with temperature-corrected density, 11 output units, and a full fluid library.

31 Fluids 11 Pressure Units Temperature Correction Custom Density / SG Fluid Comparison Click-to-Copy SI + Imperial Free · No Login
Converter P = ρ·g·H
m

Enter the fluid column height in your chosen unit.

20°C
m/s²

Earth 9.81 · Moon 1.62 · Mars 3.72 · Jupiter 24.79

Custom Density optional override
kg/m³

Leave empty for auto temperature-corrected density. Use for slurries or unlisted fluids.

SG

Entering SG auto-fills density. SG = ρ / 1000 kg/m³.

RESULT
—
P = ρ·g·H
Select fluid · set value · press convert
► 10 m of water = 0.981 bar = 14.22 psi
► 10 m of mercury = 13.29 bar = 192.8 psi
► 1 bar = 10.20 m of water = 33.47 ft
► 1 psi = 0.703 m H²O = 2.307 ft H²O
► 760 mmHg = 1 atm = 10.33 m H²O
Engineering Reference

Physics, formulas and practical guidance for every engineer

1
The Fundamental Formula
The relationship between fluid head H and pressure P comes from hydrostatics:
P = ρ × g × H
H = P / (ρ × g)
P = pressure (Pa)  |  ρ = density (kg/m³)
g = gravity (m/s²)  |  H = head (m)
Head is fluid-specific. 10 m of mercury exerts 13.5× more pressure than 10 m of water.
 ℹ  Quick check: 1 m of water at 20°C ≈ 0.0979 bar ≈ 1.420 psi ≈ 9.79 kPa.
2
Why Pump Curves Use Head
Centrifugal pump curves are in metres of head — the same impeller at the same speed produces the same head for any fluid.

Example — 50 m head pump:
  • Water (998 kg/m³) → 4.89 bar
  • Diesel (840 kg/m³) → 4.12 bar
  • Sulfuric acid (1840 kg/m³) → 9.01 bar
 ⚠  Never size pipe or valves using water pressure if pumping a denser fluid like acid, brine, or mercury.
3
Temperature & Density
Most liquids expand when heated — density decreases.
ρ(T) ≈ ρ&sub2;&sub0; × [1 − β × (T − 20)]
β = thermal expansion coefficient (1/°C)
Water: β ≈ 2.1×10²/°C
TempWater ρ (kg/m³)m per bar
4°C1000.010.197
20°C998.210.215
60°C983.210.372
100°C958.410.639
For organic solvents β is 4–7× higher — always apply temperature correction.
4
Pressure Unit Conversions
1 bar = 100,000 Pa
1 psi = 6,894.757 Pa
1 atm = 101,325 Pa
1 mmHg = 133.322 Pa
1 bar ≈ 14.504 psi ≈ 0.987 atm ≈ 750.1 mmHg
UnitTypical Use
bar / mbarProcess plants, Europe
psi / psigUSA industry, piping
kPa / MPaSI standard, civil
atmChemistry, altitude
mmHg / inHgMedical, barometers
m H&sub2;O / ft H&sub2;OPumps, HVAC
5
Gauge vs Absolute Pressure
P_abs = P_gauge + P_atm
P_atm = 1.01325 bar = 14.696 psia
Absolute pressure is measured from perfect vacuum. Used for NPSH, vapour pressure, and gas law calculations.

Gauge pressure is measured above atmospheric. What industrial gauges display.
 ⚠  For NPSHa calculations always use absolute pressures. See our NPSH Calculator.
6
Specific Gravity & Custom Fluids
Specific gravity (SG) = fluid density / 1000 (dimensionless):
SG = ρ_fluid / 1000
P = H × SG × 1000 × g
For slurries: ρ_mix = ρ_liq ×(1−Cv) + ρ_sol ×Cv
Common SG: diesel 0.84 · seawater 1.025 · H&sub2;SO&sub4; 1.84 · mercury 13.55
 ℹ  Enter SG in the Custom Density panel for any slurry, blend, or mixture.
7
How to Read Pump Curves

A pump performance curve is a graph published by the pump manufacturer showing how a specific pump model behaves across its full operating range. Understanding it is essential for correct pump selection, system design, and troubleshooting. Every pump curve contains the same core elements.

Flow Rate Q (m³/h) Head H (m) Efficiency η (%) / Power P (kW) 0 50 100 150 200 250 0 10 20 30 40 50 60 H-Q curve η curve P curve System Operating Point BEP NPSHr Shutoff head
H-Q Curve
Efficiency η
Power P
System Curve
NPSHr
Curve / Point What It Shows How to Use It
H-Q curve Head (m) the pump delivers at each flow rate. Always falls from left (high head, zero flow) to right (low head, max flow). For your required flow Q, read up to this curve to find the head the pump will produce. Convert to pressure using P = ρgH.
Shutoff head Maximum head at zero flow — where the H-Q curve meets the Y-axis. Pump is running but no fluid is moving. Never operate at shutoff for more than a few seconds. Heat builds rapidly in the casing and can damage the pump.
BEP Best Efficiency Point — the flow rate where the pump converts the most shaft power into useful hydraulic energy. Efficiency peaks here. Select a pump so normal operating flow is within ±10% of BEP. Operating far from BEP increases vibration, heat, and wear.
η curve Pump efficiency (%) at each flow rate. Bell-shaped, peaking at BEP. Typical centrifugal pumps peak at 70–90%. Use to calculate shaft power: P_shaft = (ρ × g × Q × H) / η. Higher efficiency = lower running cost.
P curve Shaft power (kW) drawn from the motor at each flow rate. For most centrifugal pumps, power rises with flow. Motor must be sized for the maximum power point on the curve (usually at max flow). Add 10–15% safety margin.
System curve The head your system demands at each flow rate: H_sys = static head + (friction losses × Q²). Parabola from the static head point. Draw your system curve on the pump curve. Where they intersect is the operating point — actual flow and head in service.
Operating point The intersection of the H-Q curve and the system curve. This is where the pump will actually run. Read off the flow (Q) and head (H) — use this calculator to convert that head to pressure for your fluid.
NPSHr curve Net Positive Suction Head required by the pump at each flow. Rises steeply at high flows. Pump will cavitate if NPSHa < NPSHr. Your available NPSHa (calculated from suction conditions) must always exceed NPSHr + 0.5 m safety margin. Use our NPSH Calculator.
 ⚠  Pump curves are for water at 20°C. When pumping a different fluid, head stays the same but pressure changes with density (P = ρgH). Motor power also changes — denser fluids draw more power at the same flow.
 ℹ  Multiple impeller speeds: Manufacturers often publish a family of curves at different speeds or impeller diameters on one chart. The Affinity Laws govern how head, flow and power scale: Q ∝ N, H ∝ N², P ∝ N³.

Frequently Asked Questions

Multiply head in metres by fluid density and gravity, then divide by 100,000: P (bar) = H (m) × ρ (kg/m³) × 9.81 / 100,000. For water at 20°C (ρ = 998.2 kg/m³): P = H × 0.09791. So 10 metres of water = 0.979 bar.
Pressure (Pa, bar, psi) is force per unit area. Head is the equivalent height of fluid that generates that pressure: H = P / (ρ × g). 1 bar = 10.2 m of water = 0.75 m of mercury. Pump engineers use head because pump curves in metres are valid for any fluid.
For water at 20°C (SG = 0.998): H (ft) = P (psi) × 2.308. So 1 psi = 2.31 ft of water. Exact formula: H (ft) = P (psi) × 6894.757 / (ρ × 9.81 × 0.3048). For non-water fluids, divide the water result by the SG of your fluid.
Because ΔP = ρ × g × H — pressure depends on fluid density. A pump producing 50 m of head gives 4.89 bar with water but 9.01 bar with sulfuric acid (1840 kg/m³). Pump curves use head precisely because head is independent of fluid density.
Higher temperature → lower density (most liquids) → less pressure per metre of head. For water, this is about 0.2% per 10°C. For organic solvents the thermal expansion is 4–7× higher — temperature correction is much more important. This calculator applies temperature-corrected density automatically using each fluid's expansion coefficient.
Copied!