Use this free pool water calculator for rectangular or round pools and ponds. Enter the actual water depth to estimate US gallons, then an optional measured hose flow to estimate fill time. It does not calculate chemical doses.
How it works
Work out volume in cubic feet, then multiply by 7.48052 for US gallons. NIST lists 7.480519 liquid gallons per cubic foot; our calculator uses that conversion rounded to five decimal places.1
Worked example
A 12 x 8 foot rectangle at 3 feet average depth is 288 cubic feet, or about 2,154 gallons. At 10 gallons per minute that is about 215 minutes, or 3.6 hours.
Tips
- Use an area-weighted average water depth. Averaging shallow and deep ends is suitable for a simple, evenly sloped bottom, not every stepped pool or pond.
- Check local water rules and any fill restrictions.
- Ponds and pools may need permits or fencing. Check local rules.
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What you’ll need
A plain checklist. Needs vary by project, so follow the instructions on your materials.
- Garden hose or pump – to move water.
- Water meter or a bucket timer, optional – to check your actual flow.
- Water test kit – for pools, to check the water after filling.
- Local rules – check any permit or water use limits first.
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Estimates only. Confirm quantities and product details with your supplier before you order.
Pool volume formulas
- Rectangle: length x width x average water depth x 7.48052 = US gallons.
- Round: π x (diameter / 2)² x average water depth x 7.48052 = US gallons.
- Fill time in minutes = gallons to add / measured gallons per minute.
Measure dimensions in feet in US mode. The metric toggle accepts meters and liters per minute and converts them for the same calculation. Diameter runs across the full round pool, not from the center to the edge. Width is ignored in Round mode. NIST defines a US liquid gallon as 231 cubic inches; an Imperial gallon is a different unit.1
What average depth should I enter?
Measure from the bottom to the intended waterline. The utility guide at mcrw.org says to use waterline depth for a level floor and average the shallow and deep depths for a sloped floor. That simple average fits a bottom that slopes evenly over its area. For a large flat shallow section followed by a short steep deep section, or a pond with shelves, use an area-weighted depth or calculate sections separately. Do not average two extreme readings and assume they represent the entire basin.2
For example, a 20 x 10-foot rectangle split into two equal-area flat sections, 3 and 5 feet deep, has average depth (3 + 5) / 2 = 4 feet. Its volume is 200 x 4 x 7.48052 = 5,984 gallons, rounded. If three quarters of the area is 3 feet deep and one quarter is 5 feet deep, the weighted depth is 3 x 0.75 + 5 x 0.25 = 3.5 feet, and the volume is 5,236 gallons. Equal-depth averaging would overestimate that second shape.
Free pool gallons chart
These are our geometric calculations rounded to whole US gallons at the stated water depths, not manufacturer capacities. Actual water depth can be lower than advertised wall height. Steps and shelves are not deducted.
| Shape | Size | Water depth | US gallons |
|---|---|---|---|
| Rectangle | 12 x 24 ft | 4 ft | 8,618 |
| Rectangle | 16 x 32 ft | 4 ft | 15,320 |
| Rectangle | 20 x 40 ft | 5 ft | 29,922 |
| Round | 15 ft diameter | 4 ft | 5,288 |
| Round | 18 ft diameter | 4 ft | 7,614 |
| Round | 24 ft diameter | 4 ft | 13,536 |
The utility sheet uses the rounded shortcuts 7.5 for rectangles and 5.9 for round or oval shapes. Our rectangle and circle use geometry and the more precise US gallon conversion, so small differences are expected. This calculator has no oval mode: an ellipse and a rounded rectangle with straight sides do not have the same area. Use the pool manufacturer’s capacity or a shape-specific calculation for an oval.2
A checked round-pool example
An 18-foot-diameter pool with 4 feet of water has radius 9 feet. Volume = π x 9² x 4 = 1,017.876 cubic feet. Multiply by 7.48052 for about 7,614 US gallons. At a measured 10 gallons per minute, filling from empty would take 761.4 minutes, or 12.7 hours. The utility’s rounded 5.9 shortcut gives 7,646 gallons, about 32 gallons higher.2
How long does it take to fill a pool?
Use the volume you are adding, not always the full pool capacity. A half-full basin needs only the remaining volume. To measure hose flow, time filling a bucket with a known volume using the same hose setup. For example, 5 US gallons in 30 seconds is 10 gallons per minute. Five gallons in 60 seconds is 5 gallons per minute. Check again if household water use or pressure changes. Do not treat a hose rating as the actual flow at your tap.
| Gallons to add | 5 gal/min | 10 gal/min | 15 gal/min |
|---|---|---|---|
| 1,000 | 3.3 hours | 1.7 hours | 1.1 hours |
| 5,000 | 16.7 hours | 8.3 hours | 5.6 hours |
| 10,000 | 33.3 hours | 16.7 hours | 11.1 hours |
| 15,000 | 50.0 hours | 25.0 hours | 16.7 hours |
Frequently asked questions
Does this calculate pond volume accurately?
Only as well as the shape and depth approximate the pond. A rectangular or round footprint with representative average depth works as an estimate. Shelves, irregular edges and rocks displace water. Split an irregular pond into simple sections or use a measured fill volume when available.
Does this estimate my water bill?
No. Water rates, billing units, sewer charges and pool-fill allowances are local. Ask your utility about its current rules. The linked utility PDF has example prices, but we do not use those as current or national costs.
Can I use this result to dose pool chemicals?
It is not a dosing calculator. Confirm water volume with your pool manufacturer or measured fill, then follow the product label and appropriate water testing. A wrong volume estimate can lead to a wrong dose.
Sources
- NIST Handbook 44 (2025), Appendix C (US liquid volume definitions and conversion table; read October 6, 2026)
- mcrw.org: Swimming Pool Water Volume Calculation (utility-hosted measuring guidance and rounded formula multipliers; example prices not used; read October 6, 2026)