Pool Salt Calculator: How Much Salt Do I Add?
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Pool & Spa Calculator
Pool Volume Calculator: How Many Gallons Is My Pool?
Measure length, width and depth to get your pool’s volume in gallons and liters, then send it straight to the salt calculator.
Pool size
Shape
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ft
Depth
ft
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Your pool holds about
22,000 gallons
83,400 liters · 2,944 cubic feet
Surface 512 sq ft × average depth 5.75 ft.
Saved on this device. Open the pool salt calculator and your size is already filled in.
How we calculated this
Gallons = surface area × average depth × 7.4805
One US gallon is exactly 231 cubic inches, so a cubic foot (1,728 cubic inches) holds 7.4805 gallons; one gallon is 3.785 liters. A sloped floor is averaged as (shallow + deep) ÷ 2, which is exact for an even slope and slightly high for a steep “hopper” deep end. Methodology
Tip. Measure the water, not the pool wall: use the depth of the water itself and the inside length and width at the waterline.
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Key takeaways
Checked Oct 4, 2026- 1Gallons = surface area (sq ft) × average depth (ft) × 7.4805.
- 2Every chemical dose is per gallon, so a 10% volume error is a 10% dosing error.
- 3Measure the water depth, not the wall height; for a sloped floor, average the shallow and deep ends.
- 4For a straight-sided oval, the ellipse formula comes out about 12% low (15 × 30 ft example).
Pool volume is surface area times average depth times 7.4805, because one cubic foot holds 7.4805 US gallons. A 16 × 32 ft in-ground pool that slopes from 3.5 to 8 ft holds about 22,000 gallons; a 24 ft round above-ground pool with 4 ft of water holds about 13,500.
How to measure your pool
- Length and width at the waterline, inside the pool wall. For a round pool, measure straight across through the center.
- Depth of the water, not the wall. Above-ground pools are rarely filled to the top of the wall.
- Both ends of a sloped floor. Measure the shallow end and the deepest point; the calculator averages them.
The formulas behind the calculator
One US gallon is defined as exactly 231 cubic inches, and a cubic foot is 1,728 cubic inches, so a cubic foot holds 1,728 ÷ 231 = 7.4805 gallons.1 Many calculators round that to 7.5, which adds 0.3%. Every shape uses the same last step: area × average depth × 7.4805.
4 quarts = 1 gallon (gal) = 231 cubic inches
| Shape | Surface area | Gallons |
|---|---|---|
| Rectangle | length × width | area × average depth × 7.4805 |
| Round | π × (diameter ÷ 2)² | area × depth × 7.4805 |
| Oval with straight sides | (length − width) × width + π × (width ÷ 2)² | area × depth × 7.4805 |
| Kidney, L-shape, freeform | add up rectangles and circles | sum of sections × average depth × 7.4805 |
For a floor with an even slope, the average depth is (shallow + deep) ÷ 2. That is exact for a straight slope and slightly high for a pool with a steep “hopper” deep end, so treat the result as an upper bound in that case.
Worked examples
Worked example
16 × 32 ft in-ground pool, 3.5 to 8 ft deep
- Scenario
- A classic rectangle with a sloped floor.
- Math
- Area 32 × 16 = 512 sq ft. Average depth (3.5 + 8) ÷ 2 = 5.75 ft. 512 × 5.75 × 7.4805 = 22,023 gallons.
- Answer
- About 22,000 gallons (83,400 liters).
Worked example
24 ft round above-ground pool, 4 ft of water
- Scenario
- Water is 4 ft deep inside a taller wall.
- Math
- Area π × 12² = 452.4 sq ft. 452.4 × 4 × 7.4805 = 13,536 gallons.
- Answer
- About 13,500 gallons (51,200 liters).
Worked example
15 × 30 ft oval above-ground pool, 4 ft of water
- Scenario
- Straight sides with half-round ends.
- Math
- Area (30 − 15) × 15 + π × 7.5² = 225 + 176.7 = 401.7 sq ft. 401.7 × 4 × 7.4805 = 12,020 gallons.
- Answer
- About 12,000 gallons (45,500 liters).
The oval-pool mistake
Many calculators, and Google’s own AI summary for this search, treat an oval pool as a true ellipse: length × width × 0.785, or length × width × depth × 5.9 with the gallon factor folded in. That 5.9 is the round-pool factor: Hayward’s chart uses it for diameter × diameter.2 Most oval pools sold for backyards have straight sides and half-round ends, which hold noticeably more water than an ellipse of the same length and width. For the 15 × 30 ft pool above, the ellipse formula gives about 10,575 gallons, roughly 1,445 gallons or 12% too low. Every chemical dose based on that number would be 12% short.
Pool equipment makers size it differently. The volume chart in Hayward’s AquaRite salt-system manual gives oval pools as length × width × average depth × 6.7,2 which works out to 0.893 of the rectangle instead of the ellipse’s 0.785. That is the same answer our straight-sided formula gives for a pool twice as long as it is wide.
| Pool | Ellipse (0.785) | Hayward chart (6.7) | Exact shape | Ellipse is low by |
|---|---|---|---|---|
| 12 × 24 ft | 6,768 | 7,718 | 7,693 | 12% |
| 15 × 30 ft | 10,575 | 12,060 | 12,020 | 12% |
| 16 × 32 ft | 12,032 | 13,722 | 13,676 | 12% |
| 18 × 33 ft | 13,959 | 15,919 | 15,693 | 11% |
| 16 × 40 ft | 15,040 | 17,152 | 17,506 | 14% |
Tip. Check your pool’s sides: if the long sides run straight for a stretch before curving, choose “Oval” here. Only a pool that curves continuously all the way around is a true ellipse.
Gallons in common pool sizes
The sizes people ask about most, worked through with the formulas above. Rectangles are shown at an even 4 ft and at a 5.5 ft average (for example, a floor sloping from 3.5 to 7.5 ft); ovals at 4 ft of water, typical of an above-ground pool.
| Size | Rectangle, 4 ft | Rectangle, 5.5 ft average | Straight-sided oval, 4 ft |
|---|---|---|---|
| 12 × 24 ft | 8,618 | 11,849 | 7,693 |
| 15 × 30 ft | 13,465 | 18,514 | 12,020 |
| 16 × 32 ft | 15,320 | 21,065 | 13,676 |
| 20 × 40 ft | 23,938 | 32,914 | 21,369 |
Your own depths will differ, which is why the calculator asks for them. A 6-inch difference in average depth on a 20 × 40 ft pool is about 3,000 gallons.
Why the number matters
Salt, chlorine, shock and balancing chemicals are all dosed per gallon, so an error in volume becomes the same percentage error in every dose. Measure once, then press “Use in my pool tools”: the result is saved on your device and the pool salt calculator fills it in automatically. Our guides to ideal pool chemical levels and opening and closing a pool build on the same number, including when to close your pool for winter.
For the full method and its limits, see our methodology page.
Frequently asked questions
About 13,500 gallons with 4 ft of water: π × 12² × 4 × 7.4805. Use your actual water depth; with 4.5 ft of water it is about 15,200 gallons.
About 22,000 gallons if the floor slopes evenly from 3.5 to 8 ft deep (average 5.75 ft). A pool that is 4 ft deep throughout holds about 15,300 gallons.
Add the shallow-end and deep-end water depths and divide by 2. For an evenly sloped floor that is exact; with a steep hopper-shaped deep end, the true volume is a little lower.
Split it into simple pieces, such as two circles or a rectangle and a circle, measure each one and add the areas. Choose “Other” in the calculator to add up to three sections.
Gallons = surface area in square feet × average depth in feet × 7.4805. The factor comes from the definition of the US gallon as 231 cubic inches.
Multiply gallons by 3.785. A 20,000-gallon pool holds about 75,700 liters.
About 23,900 gallons with an even 4 ft of water, or about 32,900 gallons at a 5.5 ft average depth: 800 sq ft × average depth × 7.4805.
A rectangle with 4 ft of water holds about 13,500 gallons. A straight-sided oval of the same size holds about 12,000; the ellipse formula many calculators use would say about 10,600.
About 8,600 gallons for a rectangle with 4 ft of water, or about 7,700 gallons for a straight-sided oval above-ground pool with 4 ft of water.
Sources
- 1.NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement. National Institute of Standards and Technology, 2026. Used for: definition of the US gallon (231 cubic inches); 7.4805 gallons per cubic foot Back to text
- 2.AquaRite Electronic Chlorine Generator Owner’s Manual (092538 Rev D). Hayward Pool Products, 2019. Used for: manufacturer pool-volume chart (oval factor 6.7, round factor 5.9) Back to text
Behind this tool
How this page was made
- Built by the Pool Desk and reviewed by the Data & Methodology Desk.
- Data: unit definitions from NIST Handbook 44; the oval comparison uses the pool-volume chart in Hayward’s AquaRite manual. Every rule is on our methodology page.
- AI use: AI tools help us research, draft and check this page. The calculator runs our own code; every figure in the text is checked against the source cited for it.
- Spotted an error? Tell us. The form opens with this page’s address filled in.
Change log
- Oct 4, 2026Review: re-tested the calculator (20 × 40 ft rectangle sloping 3.5–7.5 ft; 24 ft round at 4.5 ft; 15 × 30 ft oval at 4 ft; a 10 × 5 m metric pool) against the formulas; re-checked the NIST definition and Hayward's volume chart (oval × 6.7, round × 5.9); noted that the × 5.9 oval shortcut in search summaries is the ellipse formula.
- Oct 3, 2026Added the oval-formula comparison (ellipse vs Hayward chart vs exact shape), a common pool sizes table and NIST quote; calculator saves average depth for the salt tool’s rain estimate.
- Oct 3, 2026Published with rectangle, round, straight-sided oval and multi-section shapes.
Maintained by Pool Desk · Reviewed by Data & Methodology Desk · Last reviewed Oct 4, 2026 · Corrections
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