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The pump's flow, which the valve sends backwards through the filter.
Read it from a flow meter if there is one; otherwise from the pump's curve at the system's head. The rate over the bed is what cleans it, so a variable-speed pump backwashing at a low speed may fall short of the rate the filter maker specifies — the page shows the rate the flow gives.
The filter's effective area, from its data plate or manual.
Hayward's Pro Series runs from 1.75 to 7.06 sq ft (0.163 to 0.660 m²). It is used only to show the rate the bed is washed at — flow divided by area — beside the Model Aquatic Health Code's 15 gallons per minute per square foot for public pools.
Until the sight glass runs clear — about 2 minutes, in Hayward's manual.
Longer than needed is water and chemical thrown away for nothing; shorter leaves dirt in the bed and brings the next backwash sooner. Watch the sight glass rather than the clock.
Half a minute to a minute, in Hayward's manual.
Rinse resettles the bed and sends the last dirty water to waste instead of back to the pool. It is part of the water lost, at the same flow.
As often as the gauge rises 8 to 10 psi (0.55 to 0.69 bar) above clean.
Backwash on the pressure gauge, not the calendar: a filter backwashed while it is still clean throws away water and chemicals for nothing, and the gauge is what Hayward's manual and the MAHC both key the backwash to.
Used for the share of the pool a month of backwashing replaces.
Every backwash is a small partial drain, refilled with fresh water, so it dilutes everything dissolved — which is also why a stabiliser level that creeps up on trichlor tablets creeps up more slowly in a pool that backwashes often.
Zero for a pool without a salt cell.
Salt only leaves with water, so each backwash takes its share and the cell's reading falls after a run of backwashes. The page shows how much, which is the salt to add back.
The stabiliser reading.
Stabiliser also leaves only with water; the page shows what each backwash takes.
Water per backwash
180 gallons
19.1 gallons per minute per square foot over the bed; public pools under the MAHC backwash at 15 or more.
- Backwash rate over the bed
- 19.11 gal/min per sq ft
- Share of the pool replaced each month
- 2.73 %
- Water to waste each month
- 360 gallons
- Salt thrown away each backwash
- 4.81 lb
- Stabiliser thrown away each backwash
- 0.08 lb
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Pool Filter Backwash Water Calculator: 180 gallons — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Backwash volume is the flow through the filter for the backwash and the rinse together; a residential sand filter backwashes with its own pump reversed through the valve, so the flow is the pump's
- Hayward Pro Series sand filter owner's manual (S270T): backwash when the pressure gauge rises 8 to 10 psi (0.55 to 0.69 bar) above clean; backwash until the sight glass is clear, approximately 2 minutes or less; rinse about 1/2 to 1 minute
- Hayward Pro Series performance data: effective filtration areas from 1.75 to 7.06 sq ft (0.163 to 0.660 m²), design flow based on 20 GPM per sq ft (814 LPM per m²), the maximum allowable NSF rating
- CDC Model Aquatic Health Code, 2023, 5.7.2.2.2: granular media filters shall be backwashed at a rate of at least 15 GPM per square foot (36.7 m/h) of filter bed surface area unless the manufacturer's NSF/ANSI 50 listing specifies otherwise; 5.7.2.2.3, backwashing should continue until the water leaving the filter is clear
Inputs used
- Pump flow during backwash
- 60 gal/min
- Filter bed area
- 3.14 sq ft
- Backwash time (minutes)
- 2
- Rinse time (minutes)
- 1
- Backwashes per month
- 2
- Pool volume
- 13210 gal
- Salt level (ppm)
- 3200
- Cyanuric acid (ppm)
- 50
Intermediate steps
- Backwash rate over the bed
- 19.11 gal/min per sq ft
- Share of the pool replaced each month
- 2.73 %
- Water to waste each month
- 360 gallons
- Salt thrown away each backwash
- 4.81 lb
- Stabiliser thrown away each backwash
- 0.08 lb
Confidence note: 19.1 gallons per minute per square foot over the bed; public pools under the MAHC backwash at 15 or more.
What this calculation does not cover
- The flow is what the pump delivers backwards through the valve, and a dirty filter, a long waste line or a pump running below high speed all deliver less. Where there is no flow meter the figure is an estimate from the pump's curve, and the water lost follows it exactly.
- The Model Aquatic Health Code sets public pools a minimum backwash rate over the filter bed, unless the filter's NSF/ANSI 50 listing says otherwise; the note beside the answer gives that minimum and the rate your flow reaches, in your units. A private pool is not bound by the figure, and the manufacturer's instructions govern the filter.
- Only sand and other granular filters backwash like this. A cartridge filter is hosed off by hand and sends no pool water to waste; a DE filter backwashes and then has fresh DE added, a separate order.
- Where the waste water may go is a local rule. For public pools the MAHC sends backwash water to a sanitary sewer or an onsite sewage system, not to storm drains or surface water without a permit, and DE backwash through a separation tank first.
- Each backwash dilutes everything dissolved in the pool by the same share — salt, stabiliser, calcium, alkalinity — and the fresh water refilling it brings its own chemistry. Retest after a run of backwashes rather than dosing from the numbers here alone.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-25 · v1.0.0
Regulatory standards & verification citations4
- Backwash volume is the flow through the filter for the backwash and the rinse together; a residential sand filter backwashes with its own pump reversed through the valve, so the flow is the pump's
- Hayward Pro Series sand filter owner's manual (S270T): backwash when the pressure gauge rises 8 to 10 psi (0.55 to 0.69 bar) above clean; backwash until the sight glass is clear, approximately 2 minutes or less; rinse about 1/2 to 1 minute
- Hayward Pro Series performance data: effective filtration areas from 1.75 to 7.06 sq ft (0.163 to 0.660 m²), design flow based on 20 GPM per sq ft (814 LPM per m²), the maximum allowable NSF rating
- CDC Model Aquatic Health Code, 2023, 5.7.2.2.2: granular media filters shall be backwashed at a rate of at least 15 GPM per square foot (36.7 m/h) of filter bed surface area unless the manufacturer's NSF/ANSI 50 listing specifies otherwise; 5.7.2.2.3, backwashing should continue until the water leaving the filter is clear
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