A U-tube manometer is the simplest pressure gauge: a liquid in a bent tube rises on one side in proportion to the pressure difference. Mercury manometers gave us the “mmHg” still used for blood pressure.
How to use the Manometer Calculator
- Manometer liquid
- Liquid density
- Height difference h
- Atmospheric pressure
- Gravitational acceleration g
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the manometer liquid, enter the height difference (negative for vacuum) and the atmospheric pressure.
Tips
- Use absolute pressure in gas formulas and gauge pressure for depths and gauges.
- Density and viscosity depend strongly on temperature.
- The Reynolds number tells you whether the flow is laminar or turbulent.
- Flow rate = area × velocity (continuity).
Common mistakes to avoid
- Using diameter as radius in pipe formulas.
- Mixing dynamic and kinematic viscosity.
- Ignoring minor losses in short pipes with many fittings.
Inputs explained
| Field | What to enter |
|---|---|
| Manometer liquid | Choose one: Mercury, Water, Red gauge oil (SG 0.827), Ethanol … |
| Liquid density | Units: kg/m³, g/cm³, g/mL, lb/ft³, lb/in³ |
| Height difference h | Units: mm, cm, m, in, µm |
| Atmospheric pressure | Units: kPa, Pa, bar, psi, atm, MPa |
| Gravitational acceleration g | Units: m/s², ft/s², g, cm/s² |
Example values
How the main result changes when Liquid density changes and everything else stays at the example values above:
| Liquid density (kg/m³) | Gauge pressure (kPa) |
|---|---|
| 500 | 15.9268 |
| 750 | 15.9268 |
| 1,000 | 15.9268 |
| 1,250 | 15.9268 |
| 1,500 | 15.9268 |
| 2,000 | 15.9268 |
Formula
p_gauge = ρ·g·h, p_abs = p_atm + ρ·g·hWorked example
Inputs — Manometer liquid: Mercury; Height difference h: 120 mm; Atmospheric pressure: 101.325 kPa; Gravitational acceleration g: 9.80665 m/s².
Results — Gauge pressure = 15.92678 kPa; Absolute pressure = 117.25178 kPa.
Key terms
- Gauge pressure
- Pressure relative to atmospheric.
Frequently asked questions
Why use mercury?
Its high density keeps the column short.
What does a negative height mean?
The pressure is below atmospheric (vacuum).
What is 1 mmHg?
133.32 Pa.
Are manometers still used?
Yes – for calibration and low-pressure HVAC measurements.
What formula does the Manometer Calculator use?
p_gauge = ρ·g·h, p_abs = p_atm + ρ·g·h.