Venturi Flow Calculator
Venturi throat speed and flow from diameters, pressure drop, and Cd.
What it does
Venturi meters read flow from a squeeze: the throat runs faster than the inlet, pressure drops with the speed gain, and diameters plus that drop fix the volume rate. Power plants and labs trust the same Bernoulli trade.
The tool above takes both diameters with pressure drop, density, and Cd. It returns throat velocity with volume flow and inlet velocity.
The default 0.1 to 0.05 m at 5,000 Pa runs 3.27 m/s in the throat for 6.41 L/s. The imperial mirror of 3.94/1.97 in at 0.725 psi reads 10.71 ft/s and 101.8 GPM.
Equations follow NASA Glenn Bernoulli and venturi pages with UT Austin fluids notes behind them. All three agree on the area ratio form used here.
How to use
- Pick metric or imperial units.
- Enter inlet and throat diameters.
- Enter the pressure drop.
- Enter density and Cd.
- Read throat velocity first.
- Read volume flow beside it.
Formula
- D1, D2
- Inlet and throat diameters.
- dP
- Inlet minus throat pressure.
- rho
- Fluid density, water 1000.
- Cd
- Discharge coefficient, 1 ideal.
Imperial converts first (in x 0.0254, psi x 6894.76), then displays ft/s and GPM. Density stays SI in both modes. Cd defaults to 1 ideal; billing meters use measured 0.95 to 0.98.
Worked example
Example: 2 to 1 venturi
Metric defaults: 0.1/0.05 m, 5,000 Pa water, Cd 1.
- Pipe: 0.1 m inlet, 0.05 m throat.
- Areas: A1 0.007854, A2 0.0019635 sq m.
- Ratio squared: 0.25^2 = 0.0625, 1 - = 0.9375.
- Head: 2 x 5000 / (1000 x 0.9375) = 10.667.
- Throat: root = 3.27 m/s.
- Flow: 0.0019635 x 3.266 = 0.00641 m3/s = 6.41 L/s.
- Inlet: 0.00641 / 0.007854 = 0.82 m/s.
- Tool shows 3.27 m/s, 6.41 L/s, 0.82 m/s.
Reference table
| Pipes (m) | dP (Pa) | Flow |
|---|---|---|
| 0.1 / 0.05 | 5,000 | 6.41 L/s |
| 0.2 / 0.1 | 20,000 | 51.3 L/s |
| 0.1 / 0.05, Cd 0.97 | 5,000 | 6.22 L/s |
Limits
- Ideal flow; viscosity needs Cd.
- Incompressible fluids only.
- No cavitation at throat.
- Straight runs assumed.
Common mistakes
- Using outlet taps for drop.
- Skipping the Cd correction.
- Mixing psi with SI density.
- Sizing throats near inlets.
- Ignoring elevation terms.
Try it yourself
Quick checks. Answers sit tucked under each question.
Defaults with Cd 0.97?
Same areas: 6.41 x 0.97 = 6.22 L/s.
0.2/0.1 m at 20,000 Pa?
v2 6.53 m/s, Q 0.0513 m3/s = 51.3 L/s.
Imperial defaults?
10.71 ft/s, 101.8 GPM from rounded inputs.
Questions
What is the venturi formula?
Continuity A1v1 = A2v2 with Bernoulli: v2 = root(2 x dP / (rho x (1 - (A2/A1)^2))).
Do real meters match ideal?
Multiply by Cd, about 0.95 to 0.98 for real meters.
What assumptions hold?
Horizontal, steady, incompressible, inviscid flow without cavitation at the throat.
How is volume flow derived?
Yes: Q = A2 x v2 gives m3/s; x1000 for L/s, x3600 for m3/h, x15850 for GPM.
Can throat nearly equal inlet?
No. Near-equal areas blow up the denominator; keep throats well under inlets.
What density applies?
Water 1,000 kg/m3; enter any fluid density for oils or gases within incompressible limits.