How many gallons is my pool?
Every pool-water calculation starts from one number: 7.48 — the gallons in a cubic foot. Multiply your pool's surface area by its average depth to get cubic feet, then by 7.48 to get gallons:
Gallons = length × width × average depth × 7.48 × shape factor
The shape factor corrects for pools that don't fill the rectangle around them. A rectangle uses 1.0. A round or oval pool only covers about 78.5% of its bounding rectangle (that's π ÷ 4), and an irregular kidney or freeform shape is typically estimated at about 75%. This is the same math behind the pool-industry shortcuts you may have seen — "length × width × depth × 7.5" for rectangles and "diameter × diameter × depth × 5.9" for round pools are just this formula with the constants pre-multiplied.
Pool gallons by size
Typical volumes at common sizes, using a 5 ft average depth for inground pools and a 4 ft water depth for above-ground rounds:
| Pool | Size | Approx. volume |
|---|---|---|
| Small inground (rectangle) | 12 × 24 ft | ~10,800 gal |
| Mid-size inground | 14 × 28 ft | ~14,700 gal |
| Standard inground | 16 × 32 ft | ~19,100 gal |
| Large inground | 18 × 36 ft | ~24,200 gal |
| Above-ground round | 15 ft dia. | ~5,300 gal |
| Above-ground round | 18 ft dia. | ~7,600 gal |
| Above-ground round | 24 ft dia. | ~13,500 gal |
| Above-ground oval | 15 × 30 ft | ~10,600 gal |
Volumes rounded; your pool's real average depth is what moves these numbers most.
Average depth — the number people get wrong
Most inground pools slope from a shallow end to a deep end, so use the average: add the two depths and divide by two. A classic 3.5 ft shallow and 6.5 ft deep end average 5 ft. Two things trip people up:
- Measure the water, not the wall. An above-ground pool with 52″ walls is usually filled to about 46–48″ — call it 4 ft of water, not 4.3.
- Diving pools aren't linear. If your deep end is a small diving well rather than half the pool, the true average sits closer to the shallow depth — nudge the deep-end number down a little.
From gallons to chemicals
Volume is the reason this number matters: every chemical dose is per gallon. The calculator applies the two most common starting rules for a stabilized-chlorine routine:
- Chlorine tablets: about one 3″ trichlor tab per 5,000 gallons per week, kept in a floating dispenser or inline feeder — so a 20,000-gallon pool starts at 4 tabs.
- Shock: about 1 lb of calcium-hypochlorite shock per 10,000 gallons, weekly in season and after storms, parties, or algae scares.
These are starting points, not gospel. Sun, heat, rain, and swimmers all consume chlorine, and product strengths differ — test the water once or twice a week and let the strips, not the calendar, set the dose. If your free chlorine keeps vanishing in summer sun, check your stabilizer (cyanuric acid) level before adding more tabs.
What do pool chemicals cost per month?
At typical prices — $1.50–$3 per tab bought in a bucket and $4–$8 per pound of shock — a 20,000-gallon pool runs roughly $50–$100 per month in season for the chlorine routine alone. Balancers (pH, alkalinity, stabilizer) add a few dollars more. The calculator's cost box multiplies your weekly tabs and shock by 4.33 weeks so you can budget the season; buying tabs in 25–50 lb buckets rather than small pails is the single biggest saving.
Tips for an accurate number
- Measure the water line, not the coping. Length and width at the waterline are what hold water.
- Round pools: one measurement. Measure the diameter and enter it as both length and width — the shape factor does the rest.
- Freeform pools: box it. Measure the longest length and widest width and use the kidney/freeform factor; for a very irregular shape, split it into two rectangles and add the results.
- Write the gallons down. Tape it inside the equipment-pad door — every future dose, heater sizing, and service quote starts from it.