Pool chemistry depends on knowing how many gallons of water you're treating. Get the volume wrong by 20% — easy to do — and you're under-chlorinating, wasting shock, or burning swimmers' eyes. The good news: calculating it correctly takes ten minutes and a tape measure.
The basic formula
For a rectangular pool with consistent depth:
Cubic feet = Length × Width × Depth Gallons = Cubic feet × 7.48
The 7.48 conversion factor is fixed: there are 7.48 US gallons in one cubic foot. Memorize it.
For metric:
Cubic meters = L × W × D Liters = Cubic meters × 1000
The average-depth correction
Few pools have a single depth. To calculate volume correctly, use the average depth:
Average depth = (Shallow depth + Deep depth) ÷ 2
For a pool with a 3-foot shallow end and 7-foot deep end: (3 + 7) ÷ 2 = 5 feet.
For a 30 × 15 foot pool with 5-foot average depth: 30 × 15 × 5 × 7.48 = 16,830 US gallons.
When the slope isn't linear
The average-depth formula assumes the floor slopes uniformly. Most pools have a "hopper" design: shallow end is flat, then drops to deep end. For these:
- Calculate the shallow section as a rectangle (L × W × shallow depth).
- Calculate the deep section as a rectangle (L × W × deep depth).
- For the slope section, use the average depth.
- Sum all three.
Typically gives 5–10% less volume than the naive average-depth calculation.
Oval and kidney-shaped pools
For oval pools, multiply by 0.85 (empirical correction for curved sides):
Oval gallons = L × W × Average depth × 7.48 × 0.85
For a 30 × 15 × 5 foot oval: 30 × 15 × 5 × 7.48 × 0.85 = 14,305 gallons.
For kidney shapes, use the same 0.85 correction with the longest and widest measurements. For freeform, either measure surface area separately or accept approximation.
Common pool sizes
| Pool type | L × W × avg depth | Volume |
|---|---|---|
| Small backyard | 20 × 10 × 4 ft | 5,985 US gal |
| Standard residential | 30 × 15 × 5 ft | 16,830 US gal |
| Large residential | 40 × 20 × 5 ft | 29,920 US gal |
| Lap pool | 50 × 8 × 4 ft | 11,968 US gal |
| Above-ground 18 ft round | 18 ft × 4 ft | 7,650 US gal |
| Above-ground 24 ft round | 24 ft × 4 ft | 13,594 US gal |
For round pools, use π × radius² × depth × 7.48.
Chemical dosing — why volume matters
Standard targets:
- Free chlorine: 1–3 ppm
- pH: 7.2–7.6
- Total alkalinity: 80–120 ppm
- Calcium hardness: 200–400 ppm
- Cyanuric acid: 30–50 ppm
Quick reference for calcium hypochlorite (~65% available chlorine):
| Target adjustment | Per 10,000 gal |
|---|---|
| Raise free chlorine 1 ppm | ~2 oz |
| Shock dose (10 ppm) | ~20 oz |
| Raise pH 0.2 (soda ash) | ~3 oz |
| Raise alkalinity 10 ppm (baking soda) | ~1.5 lb |
If you think your pool is 16,830 gallons but it's actually 15,500, you've been overdosing by 8% on every chemical for years. That's $50–100 per season in wasted product, plus chronically high chlorine that irritates skin and bleaches swimsuits.
The numbers above are rules of thumb. Specific products vary in concentration. Always test water before dosing, and test again 4–6 hours after to verify.
The "fill it from a hose" method
If you have a flow meter on your hose, you can measure pool volume by filling from empty. Reset to zero, fill to normal water line, read the meter. More accurate than any calculation.
Water replacement and turnover
Pool turnover time is how long your pump takes to circulate the entire pool volume. Standard target is 8–12 hours per turnover.
Turnover time = Pool volume ÷ Pump flow rate
For 16,830 gallons with a 40 gpm pump: 16,830 ÷ 40 = 421 minutes ÷ 60 = 7 hours per turnover. Run the pump 8 hours daily and you're getting just over one full turnover per day.
The takeaway
Pool volume is the single most important number for pool maintenance. The figure your pool store has on file is almost certainly inherited from a previous owner. Spend ten minutes measuring. Calculate. Write the number on the inside of your pump room door. Use it for every dose, every shock, every chemistry adjustment for the next ten years.