01
The purpose
What does this calculator estimate?
How much water — and money — each filter backwash sends down the drain, per cycle and per year.
Pumps & Filtration · C8
Pool Backwash Water Loss Calculator
How much water — and money — each filter backwash sends down the drain, per cycle and per year.
Measure the inputsOnly backwash when filter pressure rises 8–10 PSI above clean — not on a fixed schedule.
Read the full resultOver-backwashing wastes water and actually filters worse; some dirt helps a sand bed trap fines.
Check the boundaryCartridge filters don't backwash — you hose them off, saving hundreds of gallons a year.
Useful forPumps & FiltrationPlanningComparisonVerification
Live calculation
Backwash inputs
✓ Checked May 2026
Enter filter type, size, rates, and frequency.
Calculated result
—
Enter the required values to calculate.
Enter the known values. Let the engine handle the calculation.
✓ Formula checked↻ Live result○ No account needed
Planning estimate only. Confirm product labels, equipment specifications, qualified-professional advice and local requirements before acting.
The result, explained
Use the Pool Backwash Water Loss Calculator result with context.
How much water — and money — each filter backwash sends down the drain, per cycle and per year.
02
The tool
What information does it use?
- Filter type: Sand or DE (cartridge filters don't backwash).
- Filter size: Tank diameter.
- Backwash flow rate: GPM during backwash.
- Backwash duration: Minutes per backwash.
- Rinse duration: Minutes of rinse after backwash.
- Backwashes per month: enter a measured or verified value
- Water rate: $ per gallon.
03
The method
How to use this calculator
- Filter typeSand or DE (cartridge filters don't backwash).
- Filter sizeTank diameter.
- Backwash flow rateGPM during backwash.
- Backwash durationMinutes per backwash.
- Rinse durationMinutes of rinse after backwash.
- Backwashes per monthEnter the value you can verify.
- Water rate$ per gallon.
04
The relationship
The formula in plain English
Gallons per backwash = Rate × (Backwash min + Rinse min)
Annual loss = per-backwash × frequency × 12. Cartridge filters are rinsed by hand, not backwashed.05
Worked through
A worked example: 24" sand filter, 15 GPM, 3-min backwash + 1-min rinse, twice/month
- Per backwash = 15 × (3 + 1) = 60 gal
- Monthly = 60 × 2 = 120 gal
- Annual = 120 × 12 = 1,440 gal
| Filter size | Backwash rate | Per backwash | Annual (2/month) |
|---|---|---|---|
| 18 inch | 10 GPM | 40 gal | 960 gal |
| 24 inch | 15 GPM | 60 gal | 1,440 gal |
| 30 inch | 22 GPM | 88 gal | 2,112 gal |
| 36 inch | 30 GPM | 120 gal | 2,880 gal |
06
Why it matters
How the result supports the next decision
Only backwash when filter pressure rises 8–10 PSI above clean — not on a fixed schedule.
Over-backwashing wastes water and actually filters worse; some dirt helps a sand bed trap fines.
07
Good inputs
Improve the estimate before relying on it
- Only backwash when filter pressure rises 8–10 PSI above clean — not on a fixed schedule.
- Over-backwashing wastes water and actually filters worse; some dirt helps a sand bed trap fines.
- Cartridge filters don't backwash — you hose them off, saving hundreds of gallons a year.
- Capture or redirect backwash water for lawn irrigation where local rules allow.
- A push-pull (slide) valve wastes less rinse water than a multiport in some setups.
08
The boundary
How to interpret the result
Measurements, equipment specifications, site conditions and local requirements can change the practical result.
The engine reports invalid inputs and safety warnings beside the result. Resolve those messages before using the estimate.
09
Before acting
What should override this estimate?
- Measured site conditions and current test results
- Product labels and manufacturer specifications
- Qualified-professional advice and applicable local codes
10
Common questions
Frequently asked questions
How much water does backwashing a pool filter use?
It varies with filter size and how long you run the cycle. A typical 24-inch sand filter backwashed at 15 GPM for three minutes plus a one-minute rinse uses about 60 gallons per cycle. Over a season of regular backwashing that adds up to hundreds or thousands of gallons, which this calculator totals for you.
How often should I backwash my pool filter?
Backwash based on pressure, not the calendar. When the filter's pressure gauge reads about 8 to 10 PSI above its clean starting pressure, it's time. Backwashing more often than that wastes water and can reduce filtration, because a slightly dirty sand bed actually traps fine particles better.
Do cartridge filters need backwashing?
No. Cartridge filters are cleaned by removing the element and hosing it off, which uses far less water than backwashing a sand or DE filter. That water savings is one of the main reasons many owners in drought-prone areas choose cartridge systems.
Can I reuse pool backwash water?
Often, yes — backwash water can irrigate lawns and non-edible landscaping where local regulations permit, since it's mostly pool water with filtered debris. Check your municipality's rules first, especially regarding DE, which may require capture rather than discharge to storm drains.
Why does backwashing waste so much water?
Backwashing reverses flow through the filter at high rate to flush trapped dirt to waste, then a rinse cycle resettles the media — all of that water leaves the pool. Larger filters and longer cycles use more. Backwashing only when pressure demands it keeps the annual total down.
How many gallons does a backwash use?
Roughly 60 gallons for a 24-inch sand filter on a typical 3-minute backwash plus 1-minute rinse.
Should I backwash every week?
Only if pressure has risen 8–10 PSI over clean. Backwash by pressure, not on a weekly schedule.
Do cartridge filters save water?
Yes — they're hosed off by hand instead of backwashed, saving hundreds of gallons a year.
Primary sources
Use the result with current guidance.
These official references support safe planning context. Product labels, local requirements and qualified-professional advice take priority over a general calculator estimate.