Swimming Pool Circulation Calculator: Flow, Filter and Pipe Sizes

The circulation flow, filter area, pipe sizes and backwash flow for a swimming pool from its volume and turnover time.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

A pool circulation system takes water from the skimmers and the main drain, pumps it through the filter and the heater, treats it, and returns it through inlets. The turnover time is how long it takes to pass the whole pool volume through the filter, so the circulation flow is the volume divided by the turnover time. A shorter turnover needs a bigger pump, filter and pipes.

The filter area follows from the filter rate, which is the water speed through the media in metres per hour. The pipe sizes follow from the flow and a chosen water speed: suction pipes are kept slower than return pipes to protect the pump and to keep the suction at the drain low. The backwash flow is the filter area times the backwash rate.

Turnover times, filter rates and pipe velocities are set by the health regulator, the swimming pool hydraulic design standard (AS 3979 in Australia) and the filter maker, and they differ for public, commercial and domestic pools. The calculator takes them as inputs so you can use the ones that apply.

Formulas used

  • Q = V / t
  • A_f = Q / v_f
  • D = \sqrt{\frac{4Q}{\pi v}}
  • Q_b = A_f v_b

Worked example

With the default numbers in the calculator above:

  1. Circulation flow: 50 m³/h
  2. Turnovers each day: 6
  3. Filter area needed: 1.667 m²
  4. Diameter of one round filter of that area: 1.457 m
  5. Suction pipe: smallest inside diameter: 108.6 mm
  6. Return pipe: smallest inside diameter: 84.1 mm
  7. Backwash flow: 58.33 m³/h
  8. Water used by one backwash: 2.917 m³

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What turnover time should I use?

It depends on the type of pool and the rules that apply: public and commercial pools are turned over faster than domestic pools. Use the value from your regulator or standard.

Why are suction pipes bigger than return pipes?

Slower water in the suction pipe keeps the pressure loss and the suction at the pump low, which reduces the risk of cavitation and of a dangerous suction at the drain.

Does this size the pump?

It gives the flow. The pump head also depends on the filter, heater, pipes and fittings, so use the pipe pressure drop and pump curve calculators.

Related calculators

Further reading: White, Fluid Mechanics; Munson, Young and Okiishi, Fundamentals of Fluid Mechanics; Crane Technical Paper 410. The swimming pool hydraulic design standard that applies to your pool.

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