Find the right pool
Address and parcel checks keep the measurement tied to your property.
Gallons guide
How many gallons is a 20 x 40 pool?
A 20 x 40 rectangular pool with a 5 ft average depth holds about 29,900 gallons. Change the average depth and the number moves fast.
800 sq ft footprintDepth drives volumeCalculator feeder
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Pool volume formula
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From guide to measurement
Apply this to your actual pool.
Gallons guides are useful shortcuts. PoolSavr measures the real shape and carries that number into the estimate path.
Measure my poolMeasure the shape
The app traces footprint, perimeter, surface area, and gallons.
Price the project
Scope and finish choices turn the measured pool into an estimate path.
Field guide
What to look for.
The formula
Length times width times average depth times 7.48.
The footprint is 800 sq ft. Multiply that by 5 ft average depth, then by 7.48 gallons per cubic foot, and the pool lands around 29,900 gallons.
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Pool volume formula
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Depth sensitivity
Average depth is the variable that changes everything.
| Average depth | Approx. gallons | What changed |
|---|---|---|
| 4.5 ft | 26,900 | Shallower play-pool style average. |
| 5 ft | 29,900 | Baseline for this large backyard pool. |
| 5.5 ft | 32,900 | A classic shallow-to-deep profile. |
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Why it matters
Chemicals, heaters, pumps, and startup all use gallons.
Chemical labels are written per 10,000 gallons. Guess your volume wrong and every dose is wrong by the same percentage. Surface life is a chemistry story, so gallons are not trivia.
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Startup water testing
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FAQs
Quick answers.
Is 29,900 gallons a big pool?
Yes, that is on the larger side for many backyard pools.
Does the spa add to my volume?
If it shares water with the pool, yes — dose the combined system. Attached spas typically add several hundred to a couple thousand gallons.
How do I know my exact volume?
Use real shape times real depths. The calculator handles formulas, and the app traces the actual pool shape from satellite imagery.
National averages can’t price your pool. They don’t know its size.
Enter your address and we’ll measure your actual pool from satellite imagery — surface area, perimeter, gallons — and price your project the way a premium contractor would. Free, about two minutes, and the measurement is yours either way.
Measure my pool