How Many Gallons Is My Pool? — Volume Calculator
Every dose calculation — chlorine, acid, baking soda — depends on getting your volume right. Select your pool's shape and enter its dimensions.
Why pool volume matters
Every chemical dose is calculated per gallon. Add 20% too little chlorine because you underestimated your pool size and you'll be chasing algae all summer. Add 20% too much acid and you'll overshoot your pH target every time. Volume is the one number that makes every other calculation correct — or wrong.
How to measure accurately
For length and width, a tape measure along the water line is the most accurate method. For depth, use a telescoping pole or measuring rod. Measure at the shallow end, the deep end, and two or three spots in between. For a pool with a sloped bottom, average depth = (shallow depth + deep depth) ÷ 2, which is what the "Sloped" option above calculates.
For unusual shapes, the approach is always the same: break the pool into sections you can measure, calculate each section, and add them. An L-shaped pool is two rectangles. A kidney pool has two lobes — the (A + B) formula approximates the curved boundary.
Volume is always an estimate
Pool walls aren't perfectly vertical, floors aren't perfectly flat, and tape measures aren't perfectly level. A well-measured pool still has ±5–10% error. Dose chemicals conservatively — you can always add more, but you can't easily remove what's already in.
Formulas used
The constant 7.48 converts cubic feet to US gallons. The oval and round constant 5.9 is π/4 × 7.48 ≈ 5.87, rounded to 5.9. The kidney constant 0.45 accounts for the indented-lobe geometry and is consistent with the Pool & Hot Tub Alliance standard.
Common above-ground pool sizes in gallons
These figures use 4 ft as the water depth, which is the typical fill level for most above-ground round pools. For oval or irregularly shaped above-ground pools, use the calculator above with your actual measurements — the oval formula assumes a true ellipse and may not match your pool's shape.
| Pool size | Shape | Water depth | Gallons (approx.) | Litres (approx.) |
|---|---|---|---|---|
| 15 ft round | Round | 4 ft (1.2 m) | ~5,300 | ~20,100 |
| 18 ft round | Round | 4 ft (1.2 m) | ~7,600 | ~28,800 |
| 20 ft round | Round | 4 ft (1.2 m) | ~9,400 | ~35,600 |
| 21 ft round | Round | 4 ft (1.2 m) | ~10,400 | ~39,400 |
| 24 ft round | Round | 4 ft (1.2 m) | ~13,600 | ~51,500 |
| 27 ft round | Round | 4 ft (1.2 m) | ~17,200 | ~65,100 |
| 15×30 ft oval | Oval | 4 ft (1.2 m) | ~10,600 | ~40,100 |
| 18×33 ft oval | Oval | 4 ft (1.2 m) | ~14,100 | ~53,400 |
| 18×38 ft oval | Oval | 4 ft (1.2 m) | ~16,200 | ~61,300 |
Numbers rounded to the nearest 100. If your actual water depth differs from 4 ft, use the calculator above — volume scales directly with depth. Every additional 6 inches of water adds roughly 12–15% more volume. Oval figures assume a true ellipse; above-ground oval pools may hold slightly less due to their actual frame shape.
Common in-ground pool sizes in gallons
In-ground pools vary widely, but these are the most common backyard rectangle dimensions. Assumes the industry-standard 5 ft average depth (3.5 ft shallow, 6.5 ft deep).
| Pool size | Shape | Avg depth | Gallons (approx.) | Litres (approx.) |
|---|---|---|---|---|
| 12×24 ft | Rectangle | 5 ft (1.5 m) | ~10,800 | ~40,900 |
| 14×28 ft | Rectangle | 5 ft (1.5 m) | ~14,700 | ~55,600 |
| 15×30 ft | Rectangle | 5 ft (1.5 m) | ~16,800 | ~63,600 |
| 16×32 ft | Rectangle | 5 ft (1.5 m) | ~19,200 | ~72,700 |
| 16×36 ft | Rectangle | 5 ft (1.5 m) | ~21,600 | ~81,800 |
| 18×36 ft | Rectangle | 5 ft (1.5 m) | ~24,300 | ~92,000 |
| 20×40 ft | Rectangle | 5 ft (1.5 m) | ~30,000 | ~113,600 |
| 25×50 ft | Rectangle | 5 ft (1.5 m) | ~46,800 | ~177,200 |
Numbers rounded to the nearest 100. Depth has the largest effect on volume — a pool 1 ft deeper than average holds 15–20% more water. Use the calculator above with your actual measurements for an accurate dose calculation.
Worked example: rectangular pool
A standard backyard pool measures 16 ft × 32 ft with a sloped bottom — 3.5 ft at the shallow end, 6.5 ft at the deep end. Average depth = (3.5 + 6.5) ÷ 2 = 5 ft.
Volume = 16 × 32 × 5 × 7.48 = ~19,200 gallons. A 20% underestimate (15,360 gallons) would leave you chronically under-dosing chlorine; a 20% overestimate means you're overdosing every treatment. Measure carefully and round down slightly when in doubt.
Frequently asked questions
How do I find out how many gallons my pool holds?
Measure length, width, and depth, then use the formula for your pool's shape. The calculator above handles rectangle, oval, round, kidney, and L-shaped pools. For unusual shapes, break the pool into sections you can measure separately, calculate each section, and add them.
What if I don't know my pool's exact depth?
Use a telescoping pole or weighted string at the shallow end, deep end, and two points in between. For a sloped bottom, average depth = (shallow + deep) ÷ 2 — which is what the "Sloped" option above calculates. A single depth measurement taken at the midpoint is good enough if that's all you have.
How accurate is this pool volume calculator?
Within ±5–10% for careful measurements. Pool walls aren't perfectly vertical and floors aren't flat. That's accurate enough for chemical dosing — just dose toward the conservative end of your target range and retest before adding more rather than adding everything at once.
Should I include the spa in my pool volume?
Yes, if the spa shares water with the pool — most attached spas do. Use the "Include Spa" add-on above. If your spa is freestanding with a separate pump and water, calculate and treat it independently.
What pool volume do I use for chemical calculations?
Use this calculated gallons figure for every dose — chlorine, muriatic acid, baking soda, and calcium chloride all scale with water volume. An incorrect volume is the most common cause of consistent over- or under-dosing across every chemical you add.
Volume saved. Dosing automatic.
Enter your pool volume once in PoolChem Tracker and every recommendation — chlorine, acid, baking soda, calcium chloride — calculates the exact dose for your pool size automatically.
