Accurate volume is the foundation everything else in this app builds on — chemical dosing, turnover targets, pump sizing, all of it. Miscalculating volume doesn't just throw off one number, it throws off every calculation downstream.
The 7.5 vs. 7.48052 Question
One cubic foot of water actually holds ≈7.48052 gallons. The trade rounds that to 7.5 for simplicity — what this app uses throughout. That 0.02 gal/ft³ rounding is trivial on a small pool, but it scales: on a 40,000-gallon pool, the rounding difference is ≈100 gallons — enough to matter for tight commercial chemical automation, though not for everyday residential dosing.
Shape Formulas — Where They Come From
| Shape | Formula | Source |
|---|
| Rectangle | L × W × Avg. Depth × 7.5 | Standard, undisputed |
| Round | R × R × 3.14 × Avg. Depth × 7.5 | Standard, undisputed |
| Oval | L × W × Avg. Depth × 6.7 | This app's original source deck |
| Kidney | (A + B) × L × 0.45 × Avg. Depth × 7.5 | Standard, undisputed |
You may run into a different Oval formula elsewhere: L × W × 0.785 × Avg. Depth × 7.5, a 5.89 multiplier instead of 6.7. That 0.785 number isn't a rough estimate — it's the exact area formula for a true mathematical ellipse (π/4 = 0.7854), so it looks authoritative. The problem is the shape it's built for: almost no real "oval" pool is a true ellipse. A true ellipse curves inward along its entire length, like a stretched circle. Most oval pools are actually closer to a stadium shape — flat, straight sides in the middle with rounded ends — which holds more water than a pure ellipse of the same length and width because it doesn't taper in through the middle section. Using the pure-ellipse 0.785 on a real stadium-shaped pool will consistently undercount the water.
This app uses 6.7, and that's the number to use. It sits between the pure-ellipse ratio (0.785) and a full rectangle (1.0) — right where a real stadium-shaped oval pool belongs — and it's sourced from the original training material this whole project is built on. Don't swap in 0.785/5.89 for an oval pool in this app; it's solving for the wrong shape and will undercount gallons, which risks over-concentrating chemicals when dosing off that number.
Sloped & Hopper-Bottom Pools
The Rect. w/Slope shape on this tab uses a 3-point average depth (Shallow + Mid-Point + Deep, ÷3) instead of the standard 2-point average — a better approximation than a simple shallow/deep average for a pool with an uneven, continuously-sloping floor. For a true hopper-bottom pool (a distinct flat deep well, not a continuous slope), the more rigorous method is to split the pool into separate zones — shallow flat section, sloped transition, and deep hopper box — calculate each zone's volume independently, and add them together before the final ×7.5. That level of precision isn't built into this calculator, but it's the right move for a pool with a genuine diving well.
Real-World Variables This App Can't See For You
Waterline, Not Deck Height
Measure depth from the pool floor to the skimmer/tile waterline — not to the top of the deck. Pools are never filled to the brim, and this mistake alone can throw off totals by hundreds or thousands of gallons.
Displacement — Steps, Shelves, Benches
Sun shelves, steps, and benches take up space the water can't occupy. Calculate their volume (L × W × H for a shelf; stacked blocks for steps) and subtract it from the total — otherwise you're overestimating.
Freeform Pools — the Grid Method
For a shape too irregular for any preset, measure the width at regular intervals (e.g., every 5 ft) along the pool's length, average those widths, then multiply by total length for an approximate surface area.
Attached Spas
Shared filtration with the pool (spillway spa)? Calculate both volumes and add them for total system dosing. Isolatable via valves for independent heating/therapy use? Keep the spa's volume separate so it can be dosed on its own.
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