Understanding Siphon Flow
A siphon allows liquid to flow from a higher liquid surface to a lower outlet through a tube. Once the tube is filled and primed, gravity drives the flow because the outlet is lower than the source liquid level.
The siphon flow rate depends mainly on the vertical height difference, tube diameter, and hydraulic losses inside the tube. This calculator gives a practical estimate using Torricelli's velocity formula with a discharge coefficient.
Siphon Flow Rate Formula
The ideal velocity from the height difference is estimated with Torricelli's equation:
A practical adjusted velocity can be estimated using a discharge coefficient:
The tube area is:
The siphon flow rate is:
Choosing a Discharge Coefficient
The discharge coefficient accounts for losses and real-world effects. A value of 1.0 means ideal lossless flow, which is usually too optimistic.
| Condition | Suggested Cd |
|---|---|
| Ideal theoretical estimate | 1.00 |
| Short, smooth tube with few losses | 0.80 – 0.95 |
| Typical practical siphon | 0.60 – 0.85 |
| Long tube, bends, rough hose, fittings | 0.40 – 0.70 |
| Air leaks, poor priming, restrictions | May be much lower |
How to Use the Calculator
- Enter the internal tube diameter in meters.
- Enter the vertical height difference between the source liquid level and the outlet.
- Enter a discharge coefficient. Use 0.80 as a reasonable starting estimate.
- Optionally enter a volume in liters to estimate transfer time.
- Click calculate to get velocity, flow rate, and step-by-step results.
Example Calculation
Suppose a siphon uses:
- Tube diameter: D = 0.025 m
- Height difference: h = 1.0 m
- Discharge coefficient: Cd = 0.80
The ideal velocity is:
The adjusted velocity is:
The internal area is:
The estimated flow rate is:
Important Limitations
- The siphon must be fully primed.
- The outlet must be lower than the source liquid level.
- Tube length, bends, fittings, entrance losses, and outlet losses reduce real flow.
- Very small tubes can be strongly affected by viscosity and surface tension.
- Air leaks or bubbles can stop the siphon.
- This simplified calculator does not perform a full Darcy-Weisbach pipe-loss calculation.
Frequently Asked Questions
Can a siphon move liquid uphill?
A siphon tube may rise above the liquid level temporarily, but the outlet must be lower than the source liquid level for sustained flow.
What happens if air enters the siphon?
Air can break the continuous liquid column and stop the siphon. A siphon must remain filled with liquid to keep working.
Does tube diameter affect siphon flow rate?
Yes. Flow area increases with the square of diameter, so diameter has a very strong effect on flow rate.
Does tube length affect siphon flow?
Yes. Longer tubes usually reduce the real flow rate because of friction losses. This simplified calculator accounts for losses only through the discharge coefficient.
What height difference should I use?
Use the vertical distance between the source liquid surface and the outlet. This is the driving head for the siphon.
Why is the real siphon flow lower than the ideal value?
Real siphons lose energy through friction, bends, fittings, entrance losses, outlet losses, and imperfect priming. That is why the discharge coefficient is included.