Irregular Pool Area & Surface Calculator
Two ways to get your pool’s area. The Sections method is the most accurate for any real pool shape. The Kidney Quick Formula is faster but is an estimate, not exact.
Depth & Volume (Optional)
Kidney pools. Freeform pools. Lagoon shapes with a curve on every side. Most backyard pools are not rectangles. That’s the whole point — a curved pool looks better in a yard than a plain box does.
But curved pools are harder to measure. You need the surface area for a pool cover, a resurfacing job, or a new liner. You need the volume for chemicals. This tool gets you both, for any shape.
The Problem With “Magic Number” Formulas
Search for a kidney pool calculator and you’ll find a formula everywhere: 0.45 × (A+B) × L. It’s fast. It’s also just an estimate.
Here’s proof. Take a kidney pool with a 25 ft wide lobe, a 15 ft narrow lobe, and 50 ft length. Run it through 0.45 × (A+B) × L. You get 900 square feet.
Now run the exact same pool through a different formula you’ll find on other sites — 3/4 × L × average width. You get 750 square feet.
Same pool. Two “standard” formulas. A 150 square foot gap between them. That’s not a rounding error — it’s almost 17% apart.
Neither formula is wrong, exactly. They’re both rough shortcuts built for speed, not precision. If your pool cover costs $2 a square foot, a 150 square foot gap is $300 you didn’t plan for.
The Accurate Way: Break It Into Sections
Every real pool builder measures a curved pool the same way: break it into shapes you can measure exactly, then add them up. A kidney pool is really a rectangle in the middle with two rounded or tapered ends. A lagoon pool is a big rectangle with curved corners. A freeform pool might need four or five sections.
This is what the Sections tool above does. Add a rectangle for the straight middle. Add a circle for a rounded end. Add a tapered end for a lobe that narrows. The math for each piece is exact — no empirical multiplier, no guessing.
Worked Example
A pool has a straight middle section 30 ft by 16 ft, plus a rounded end with a 16 ft diameter.
Middle: 30\times16=480 sq ft.
Rounded end (half circle, radius 8): \frac{1}{2}\times\pi\times8^2=100.5 sq ft.
Total: 580.5 sq ft — exact, not a guess.
When the Quick Kidney Formula Is Fine
Not every job needs exact numbers. If you just want a rough sense of your pool’s size, or you’re doing early budget math before calling a contractor, the Kidney Quick Formula tab is fast and close enough. Just don’t use it for a final material order.
Getting Water Volume, Not Just Area
Surface area tells you how much cover or liner you need. Volume tells you how much water — and how much chemical — your pool holds.
Volume needs one more number: depth. Most pools aren’t one depth all the way across, so use the average:
\text{Average Depth}=\frac{\text{Shallow End}+\text{Deep End}}{2}
Multiply area by average depth for cubic feet. Multiply cubic feet by 7.48 for US gallons.
Getting this wrong matters more than people think. Overestimate your pool’s volume and you’ll underdose chlorine — the water looks fine but isn’t actually safe. Underestimate it and you’ll overdose, which stings eyes and dries out skin. Neither mistake is harmless.
What Pool Area Is Actually Used For
Pool covers. Custom covers for curved pools are cut to the pool’s exact shape and priced by the square foot. A wrong area means an expensive custom order that doesn’t fit.
Resurfacing. Plaster, pebble, and tile resurfacing jobs are quoted by square foot of surface. Contractors need your real number, not a rough guess.
Liners. Vinyl liners are cut to fit the pool’s exact shape and depth changes. Precise measurements avoid a liner that’s too small or wastes material.
Chemical dosing. Chlorine, pH balancers, and algaecide are all dosed by volume, in gallons or liters. Get the volume right and dosing takes care of itself.
Frequently Asked Questions
The most common quick formula is 0.45 × (A+B) × L, where A and B are the widths of the two lobes and L is the length. This is an estimate, not an exact calculation — different sources use slightly different multipliers, which can produce results up to 17% apart on the same pool.
Break the pool into sections — rectangles, circles, and tapered ends — and calculate each one exactly, then add them together. This is more accurate than any single multiplier formula, since it uses real geometry instead of an empirical shortcut.
Add the shallow end depth and the deep end depth, then divide by two. Use the actual water depth, not the wall height — most pools aren’t filled all the way to the top.
7.48 US gallons. Multiply your pool’s volume in cubic feet by 7.48 to get gallons.
Chemical doses are calculated per gallon of water. Overestimating volume leads to underdosing, so the water looks fine but isn’t properly sanitized. Underestimating leads to overdosing, which can irritate skin and eyes.
Bottom Line
A curved pool doesn’t have to be a guessing game. Use the quick kidney formula for a fast ballpark. Use the Sections tool when the number actually matters — a cover order, a resurfacing quote, a liner that has to fit right the first time.