Pool & Spring

Pool Volume Calculator

Knowing your pool's exact gallons matters more than people think. Chemical dosing math depends on it (chlorine, alkalinity, pH adjustments all scale with volume). Heater sizing depends on it. Liner replacement and chemical purchases scale by it. Initial fill cost depends on it (~$60-200 for a 20K-gallon pool). This calculator handles three pool shapes — rectangular, round, oval — using the standard shape factor (7.5 for rectangles; 5.9 for round/oval; 6.7 for kidney) that converts cubic feet to gallons. Pick shape, enter dimensions, get gallons, liters, and cubic feet.

Common pool sizes (gallons)

Above-ground round 24-ft diameter × 4.5 ft deep: ~12,200 gallons
Above-ground round 27-ft diameter × 4.5 ft deep: ~15,500 gallons
In-ground 14x28 rectangular × 5 ft avg depth: ~14,700 gallons
In-ground 16x32 rectangular × 5 ft avg depth: ~19,200 gallons (the residential standard)
In-ground 20x40 rectangular × 5.5 ft avg depth: ~33,000 gallons (large)
Kidney 17x34, 5 ft avg depth: ~19,300 gallons
Lap pool 8x40 × 5 ft: ~12,000 gallons
Spa / hot tub: 300-500 gallons typical
Inflatable / kiddie 10-ft diameter × 2.5 ft: ~1,150 gallons

The math behind the factors

Volume in gallons = surface area (sq ft) × average depth (ft) × multiplier.

The multiplier converts cubic feet to gallons: 1 cubic foot = 7.48 gallons (rounded to 7.5). Different pool shapes use different effective multipliers because their actual surface area isn't always equal to the bounding rectangle:

Rectangle / square: 7.5 (full coverage in the bounding box)
Round / oval: 5.9 (~79% of the bounding box area; π/4)
Kidney: 6.7 (~89% of bounding box)
Free-form: use 7.0 as default, or compute area more carefully

Worked example: 16x32 rectangular pool with 5 ft average depth. Surface area = 512 sq ft. Volume = 512 × 5 × 7.5 = 19,200 gallons. About 73,000 liters or 2,560 cubic feet.

How to use this calculator

Pick shape: rectangle, round, or oval.
Dimensions: length, width (rectangle) or diameter (round/oval).
Average depth: (shallow end + deep end) / 2. For uniform depth (above-ground), just the depth.
Output: gallons, liters, cubic feet.
For irregular pools: break into geometric shapes (rectangle + half-circle for kidney), calculate each, sum volumes.

Common scenarios

Building a 16x32 in-ground pool, average 5 ft depth. ~19,200 gallons. Initial fill cost: $60-200 from city water (varies by rate). Heater sizing: 200,000-400,000 BTU for fast heat. Annual chemical cost: $300-600.

Above-ground 24-ft round pool, 4.5 ft deep. ~12,200 gallons. Smaller chemistry math but still significant volume. Fill from garden hose: 5-10 hours.

Spa repair, 350 gallons, replacing water annually. Drain, clean filter, refill, add startup chemicals. ~$15-30 in startup chemical cost. Heater might handle full reheat in 6-8 hours.

FAQ

Why different multipliers for different shapes? +

The 7.5 gallons per cubic foot conversion is fixed (1 cu ft = 7.48 gal). But the surface area calculation depends on shape: a 24-ft round pool isn't a 24x24 square in terms of water surface — it's smaller (πr² vs r²). Combining the conversion with the shape factor gives you a single multiplier per shape that works directly on the bounding-box dimensions.

What if my pool has different depths in different ends? +

Use average depth: (shallow + deep) / 2. A pool with 3-ft shallow end and 7-ft deep end has 5 ft average. This is accurate for chemistry dosing (chemicals diffuse). For heating, the math is the same.

How accurate does my volume need to be? +

For chemistry dosing: within 10-15% is fine — just round to nearest thousand gallons. For initial filling: same accuracy. For heater sizing: same. For new construction cost estimates: probably more precision warranted, but volume contributes only modestly to total cost.

How long does it take to fill a pool? +

Garden hose (5-8 gpm): 25-40 hours for a 12K gallon pool, 40-60+ hours for 19K gallon. Use a water meter to track. For new pool fills, many homeowners hire a water truck delivery ($200-500 for 5K gallons) to speed it up significantly.

What's the cost of filling a pool? +

City water rates: $3-10 per 1,000 gallons. A 20,000 gallon pool: $60-200 to fill. Well water: free but check your well's recovery rate; filling a pool from a small well can take days. Some areas charge sewer fees on pool fills — ask your utility to remove that from your bill.

How much water do I lose to evaporation? +

Outdoor pool in warm, dry climate: 1/4 to 1/2 inch per day from the surface = roughly 100-300 gallons per day for a 16x32 pool. Pool covers cut evaporation 95%+. Annual makeup water: 5,000-15,000 gallons typical for uncovered pools.

Do I need to drain the pool ever? +

For chemistry reset (high CYA, calcium hardness): partial drain and refill. For winter closing (cold climates): partial drain below skimmer. For liner replacement: full drain. For major repairs: as needed. Generally, the less draining, the better — it's expensive water and wastes treated chemistry.

Why is my heater taking forever to warm the pool? +

Heaters are sized to maintain temp, not rapidly heat from cold. Standard 250-400K BTU heaters raise 20K gal pool 1-2°F per hour. From 60°F to 85°F = 25-50 hours of heater run time. Use a solar cover overnight to retain heat — saves enormous energy and time.