Pool Filter Backwash Water Calculator
Calculate gallons per backwash from filter area, backwash rate and run time, with per-season totals for drain permits and sewer estimates. No signup.
How this is calculated
Three multiplications. The filter area sets the flow; the flow and run time set the volume; the volume and a season count set the annual discharge.
backwash_flow_gpm = backwash_rate_gpm_per_sqft x filter_area_sqft
gallons_per_bw = backwash_flow_gpm x run_time_minutes
liters_per_bw = gallons_per_bw x 3.785411784
seasonal_gallons = gallons_per_bw x backwashes_per_seasonWorked example. A 4.91 sq ft sand filter backwashed at 15 GPM per square foot for 3 minutes: flow is 73.65 GPM, volume is 220.95 gallons (shown as 221 on the result line, about 836 liters). Run that once a week for a 30-week season and the annual discharge is roughly 6,629 gallons.
15 GPM per square foot (36.7 m/h) is the minimum backwash rate for granular media filter systems published in the 2024 Model Aquatic Health Code (5th edition, a model code for public aquatic facilities), section 5.7.2.2.2, unless the filter manufacturer prohibits that rate or an alternate rate is approved per NSF/ANSI 50. It is the calculator's default starting point. The rate field is editable: your filter manual may specify a different rate, and that figure governs.
Sources: CDC Model Aquatic Health Code 2024, 5th edition, 5.7.2.2.2 (at least 15 GPM per sq ft for granular media, unless the manufacturer prohibits or NSF/ANSI 50 approves an alternate rate), 5.7.2.2.3 (backwash until water is clear), 4.11.6.1.1 (wastewater permits); LibreTexts Water 131: Advanced Water Mathematics (Alvord), section 6.2 Filtration Rates (backwash volume = flow rate x time, worked exercises).
Backwash water is the one pool expense that never appears on a chemical order. It leaves the system every time you flip the multiport, and the only record of it is the number this page computes: gallons per backwash from three measurements the filter already gives you, plus a seasonal total for the drain permit or sewer estimate that asks how much water you discharge in a year.
The formula, and where it comes from
The arithmetic is the same one state water-operator licensing exams drill: backwash flow (GPM) = backwash rate (GPM per square foot) x filter area (square feet), then gallons = flow x run time in minutes. The backwash rate is a starting-point input, not a hidden constant, because it comes from your filter manual and from the code your pool operates under. The 2024 Model Aquatic Health Code (5th edition, a model code for public aquatic facilities), section 5.7.2.2.2, states that granular media filter systems shall be backwashed at at least 15 GPM per square foot (36.7 m/h) of filter bed surface area, unless the filter manufacturer prohibits that rate or an alternate rate is approved per the NSF/ANSI 50 listing. That exception is why the rate field is editable. The MAHC figure is the calculator’s default.
The LibreTexts water-treatment mathematics course (Water 131, Alvord) covers the same arithmetic in its filtration-rates section: filtration and backwash rates are calculated as flow rate in GPM divided by the filter bed surface area in square feet, and the chapter includes backwash-volume exercises. No constant is hidden and no figure is asserted without a named source.
Backwash rate is not backwash volume
The rate, in GPM per square foot, is a property of the filter and the code it runs under. The volume, in gallons, is the rate multiplied by the area multiplied by the time. Confusing the two is what makes a permit estimate wrong by more than tenfold: a 4.91 sq ft sand filter at 15 GPM per square foot moves 73.7 GPM during the backwash, and three minutes of that is 221 gallons, not 15 gallons. The calculator separates the two and prints each step.
Why the seasonal total matters
A single backwash on a residential filter moves a couple of hundred gallons. Every backwash also removes that volume from the pool itself; the makeup water that refills it dilutes the remaining sanitizer and shifts water balance, so plan a rebalance test after each refill. Run one backwash a week for a 30-week season and the annual discharge is north of 6,000 gallons; a commercial filter with ten times the area can discharge ten times that. Municipalities that meter sewer use, or that require a discharge permit for pool and spa water, want that total. The calculator multiplies gallons per backwash by the number of backwashes per season you enter, so the figure on the permit application comes from the same formula as the one on the sight glass.
Where this sits alongside the other filter pages
Sizing the filter itself, so the area on the nameplate matches the flow the pool needs, is a different question answered by the pool filter size calculator. Dosing the DE that goes back into a DE filter after the backwash is the DE filter powder calculator. The flow and turnover requirement that sets the filter’s job in the first place lives on the commercial pool turnover rate and bather load calculator. And pricing the teardown clean that includes the backwash is the pool filter cleaning pricing calculator.
Everything runs in your browser: no account, no email gate, nothing stored or sent. This is a planning aid, not a compliance determination. Your filter manual and your local code are the authority on every figure here.
Frequently asked questions
How many gallons of water does a pool backwash use?
What is the correct backwash rate for a sand filter?
How long should you backwash a pool filter?
Do I need a drain permit for backwash water?
Sources
- 2024 Model Aquatic Health Code, 5th Edition (Code Language), a model code for public aquatic facilities: 5.7.2.2.2 granular media filter systems shall be backwashed at at least 15 GPM per square foot (36.7 m/h) unless prohibited by the filter manufacturer or approved at an alternate rate per NSF/ANSI 50; 5.7.2.2.3 backwashing continued until water leaving the filter is clear; 4.11.6.1.1 wastewater not directed to storm water or surface waters without permits from the AHJ or EPA. U.S. Centers for Disease Control and Prevention. Retrieved .
- Water 131: Advanced Water Mathematics (Alvord), section 6.2 Filtration Rates: filtration and backwash rates calculated as flow rate (GPM) divided by filter bed surface area (sq ft), with backwash-volume exercises. LibreTexts (College of the Canyons, Alvord). Retrieved .