Orifice Size Calculator
Enter the flow you want, the pressure drop across the orifice, the liquid and the edge type. The calculator gives the orifice diameter in millimetres and inches and the jet velocity.
- Diameter from flow
- Any pressure drop
- Edge type
Pressure drop is the difference between pressure before and after the orifice.
An orifice limits flow by forcing liquid through a small hole. Flow depends on the hole area, the square root of the pressure drop and a discharge coefficient for the edge. A sharp-edged hole passing 10 L/min of water at 2 bar drop is about 4.2 mm across.
Orifice size chart
Water, sharp edge. Search for a flow.
| Flow (L/min) | 1 bar drop | 2 bar drop | 5 bar drop |
|---|---|---|---|
| 1 | 1.57 | 1.32 | 1.05 |
| 5 | 3.51 | 2.95 | 2.34 |
| 10 | 4.96 | 4.17 | 3.32 |
| 20 | 7.01 | 5.9 | 4.69 |
| 50 | 11.08 | 9.32 | 7.41 |
| 100 | 15.68 | 13.18 | 10.48 |
Diameters in mm. Doubling the pressure drop shrinks the hole by about 16%.
Flow and orifice
Water, 2 bar drop. Slide the flow.
The dashed lines show the jet narrowing after the plate, called the vena contracta.
How to size an orifice
Q = Cd × A × √(2ΔP/ρ). Step through it.
Flow in m³/s
L/min ÷ 60,000.
Small holes block easily. Fit a strainer upstream of any orifice under about 2 mm.
Sharp, chamfered or nozzle
Edge types. Compare them.
Square edge plate.
Bevelled inlet.
Smooth rounded inlet.
Sharp-edged plates are the easiest to make and the most predictable, which is why flow meters use them.
Before drilling
Orifice checks. Tick each item.
Tick each item as you go.
Drill undersize first and test. It is easy to open a hole up and impossible to make it smaller.
What you get
Orifice sizing. Each tile explains one result.
mm and inches.
m/s.
By edge.
Density.
L/min and gpm.
For gas flow, compressibility matters and this liquid equation is not accurate. Use a gas orifice method instead.
Frequently Asked Questions
No question matches that search.
How do I size an orifice?
Use Q = Cd × A × √(2ΔP/ρ) and solve for the area, then the diameter.
What is a discharge coefficient?
The ratio of real to ideal flow, about 0.61 for a sharp-edged hole.
What size orifice for 10 L/min at 2 bar?
About 4.2 mm with a sharp edge.
How does pressure change orifice flow?
Flow rises with the square root of pressure drop.
Does fluid density matter?
Yes, lighter liquids flow faster through the same hole.
What is the vena contracta?
The narrowest point of the jet just after the hole.
Can I use this for gas?
No, gas needs a compressible flow method.
The right hole for the flow
Enter flow and pressure drop to get the orifice size. Drill a little small, test and open it up to hit your target.
More size calculators
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