Read the flow at the head your return plumbing creates, using the curve for your exact pump and setting. The zero-head figure on the box is just one end of the curve. Our calculator can work out the flow between published points once you enter your vertical lift and a separate estimate for friction. It can’t work out your plumbing’s resistance from the lift alone.
Read your pump’s flow at your head height →
Check the units and the setting
Find the maker’s graph or table for your exact pump version. Note head in metres and flow in litres per hour before you enter any points. Feet, US gallons and litres are different units, so don’t copy a number and just change its label. Keep a note of where the figures came from and which setting they apply to.
A controllable pump may have several curves, one per setting. Points from different settings don’t make one curve. If you only have the full-speed curve, don’t multiply its flow by your controller percentage and treat it as a published lower-speed curve. Find proper data, or accept that the flow at that setting is uncertain.
Separate lift from plumbing resistance
Measure the height from the water surface the pump draws from to where the water comes out. That vertical lift is only part of the total head. EcoTech Marine’s guidance also points to friction in your plumbing, from pipe width, length and fittings, as part of head pressure.
Enter a friction estimate you can back up in the calculator. Entering zero looks at lift alone; it doesn’t mean your elbows, valves, pipe and outlets add no resistance. The tool can’t turn a list of pipe lengths and elbows into a friction figure, so don’t make up a rule of so many metres per elbow.
Working out flow between two points
Here’s a made-up curve, just to show the maths (it isn’t a real pump): 2,400 L/hour at 1 m and 1,600 L/hour at 2 m. Say your lift is 1 m and your extra friction head is 0.5 m. Your total head is 1.5 m, exactly halfway between the two points.
| Step | Calculation | Result |
|---|---|---|
| Total head | 1.0 + 0.5 | 1.5 m |
| How far between the points | (1.5 − 1) ÷ (2 − 1) | 0.5 |
| Change in flow between the points | 1,600 − 2,400 | −800 L/hour |
| Your estimated flow | 2,400 + 0.5 × (−800) | 2,000 L/hour |
What the calculator will and won’t accept
Enter between two and thirty points as head,flow pairs, one per line. Head must go up and flow must stay the same or go down. The result uses the two points either side of your head. If you’ve entered a point at exactly your head, it uses that.
Your total head has to fall within the points you entered; the calculator won’t guess beyond them. Maximum flow and maximum head alone don’t tell you the shape of the curve in between. Use the maker’s published points rather than building a curve from two marketing figures.
If you’re reading values off a graph by eye, there’s still some judgement involved. More decimal places can’t fix a blurry graph or an uncertain friction figure. Call the result an estimate, and keep the original graph to hand.
Check the uncertainty before you buy
With the made-up curve, a total head of 1.25 m gives 2,200 L/hour and 1.75 m gives 1,800 L/hour. That range shows how much the answer depends on the head you enter. It doesn’t cover every quirk of a real installation or guarantee the flow you’ll get.
Compare the estimate with the flow you need and what your system can handle. Return flow through your sump is a different job from circulation in the tank: don’t use a wavemaker’s flow figure as if it were return flow through the overflow. Our separate sizing guide helps you set a target.
Once it’s installed, measure the real flow if you can do it safely, and note the setting. Branches and outlets split the flow, and changes to plumbing or a dirty pump can change it too. Check your assumptions before deciding an unexpected result means the pump is faulty.
Common questions
Can I use the maximum flow and maximum head as the whole curve?
No. Those two end points don’t tell you the shape of the curve between them. Enter the maker’s points for the same pump and setting.
Does the calculator work out friction from pipe length?
No. Enter a friction estimate you’ve worked out separately. Entering zero shows lift only, with no friction.
What if my total head is outside the curve I entered?
The tool won’t give a result. Add more of the maker’s points or find better data; it won’t guess beyond what you’ve entered.
Sources and limits
Last checked 12 October 2026. Our worked examples use made-up inputs to show the maths. They aren’t measured costs or care targets for your tank.
Keep going
Read your pump’s flow at your head height · Free Reef Companion · All calculators