01
The purpose
What does this calculator estimate?
How much water filter backwashing sends to waste over a year — and what it costs you.
Water Loss & Leaks · E4
Backwash Water Loss & Cost Calculator
How much water filter backwashing sends to waste over a year — and what it costs you.
Measure the inputsBackwash on pressure (8–10 PSI over clean), not on a fixed weekly schedule.
Read the full resultCartridge filters skip backwashing entirely — a big water saver in drought areas.
Check the boundaryCapture backwash water for landscape irrigation where local rules allow.
Useful forWater Loss & LeaksPlanningComparisonVerification
Live calculation
Backwash loss inputs
✓ Checked May 2026
Enter backwash rate, durations, and frequency.
Estimated rate
—
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 Backwash Water Loss & Cost Calculator result with context.
How much water filter backwashing sends to waste over a year — and what it costs you.
02
The tool
What information does it use?
- Backwash flow rate: GPM during backwash.
- Backwash duration: Minutes per backwash.
- Rinse duration: Minutes of rinse after.
- Backwashes per month: enter a measured or verified value
- Water rate: $ per gallon.
03
The method
How to use this calculator
- Backwash flow rateGPM during backwash.
- Backwash durationMinutes per backwash.
- Rinse durationMinutes of rinse after.
- Backwashes per monthEnter the value you can verify.
- Water rate$ per gallon.
04
The relationship
The formula in plain English
Annual loss = Rate × (Backwash min + Rinse min) × backwashes/month × 12
Only sand and DE filters backwash. Cartridge filters are hosed off and lose almost no water.05
Worked through
A worked example: 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 ≈ $7.20
| Per backwash | 2/month | 4/month | Weekly |
|---|---|---|---|
| 40 gal | 960 gal/yr | 1,920 gal/yr | 2,080 gal/yr |
| 60 gal | 1,440 gal/yr | 2,880 gal/yr | 3,120 gal/yr |
| 88 gal | 2,112 gal/yr | 4,224 gal/yr | 4,576 gal/yr |
| 120 gal | 2,880 gal/yr | 5,760 gal/yr | 6,240 gal/yr |
06
Why it matters
How the result supports the next decision
Backwash on pressure (8–10 PSI over clean), not on a fixed weekly schedule.
Cartridge filters skip backwashing entirely — a big water saver in drought areas.
07
Good inputs
Improve the estimate before relying on it
- Backwash on pressure (8–10 PSI over clean), not on a fixed weekly schedule.
- Cartridge filters skip backwashing entirely — a big water saver in drought areas.
- Capture backwash water for landscape irrigation where local rules allow.
- Shorter, pressure-triggered backwashes cut annual water loss substantially.
- A slightly dirty sand bed actually filters finer — don't over-backwash.
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 waste in a year?
It depends on filter size and how often you backwash. A typical sand filter using around 60 gallons per cycle, backwashed twice a month, loses roughly 1,400 gallons a year. Larger filters or more frequent backwashing push that higher — which is why backwashing only when pressure demands it matters.
Which filters waste water on backwashing?
Sand and diatomaceous earth filters both backwash, sending water to waste each cycle. Cartridge filters don't — they're cleaned by removing the element and rinsing it with a hose, using only a few gallons. In water-restricted regions, that difference is a key reason to choose cartridge.
How can I reduce backwash water loss?
Backwash based on the pressure gauge — when it rises about 8 to 10 PSI over the clean reading — rather than on a calendar schedule. Keep backwash and rinse cycles only as long as needed to run clear. Where permitted, redirect backwash water to irrigate lawns and landscaping.
Can I reuse backwash water?
Often yes, for non-edible landscape irrigation where local regulations allow, since it's mostly pool water with filtered debris. DE backwash may need to be captured rather than discharged to storm drains. Always check your municipality's rules before redirecting it.
Does over-backwashing hurt filtration?
With sand filters, yes — a freshly cleaned bed actually filters slightly coarser than one with a little accumulated dirt, which helps trap fine particles. Backwashing more often than the pressure requires wastes water and can leave the water marginally less clear, so let pressure be your guide.
How many gallons does backwashing waste?
Around 60 gallons per cycle for a 24-inch sand filter; roughly 1,400 gallons a year at twice monthly.
Do cartridge filters backwash?
No — they're hosed off by hand, saving the water a backwash would consume.
When should I backwash?
When the filter pressure rises 8–10 PSI above its clean reading — by pressure, not schedule.
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.