How to Calculate Your Pool Volume

Knowing your pool's volume in gallons is the starting point for almost every chemical dosing decision you'll make. Get it wrong and you're either underdosing and chasing water problems, or overdosing and wasting money. The formula you use depends on your pool's shape, so find yours in the sections below.

What You'll Need Before You Start

A tape measure is all you need for most pools. If your pool was professionally installed, the original specs may already have the dimensions, which saves you a step.

One thing worth getting right before you run any numbers: your average depth. Most residential pools slope from a shallow end to a deep end, which means there's no single depth figure to plug in. To calculate average depth, measure the depth at the shallowest point and the depth at the deepest point, add them together, and divide by two.

Average depth = (shallow end depth + deep end depth) / 2

For example, if your shallow end is 3.5 feet and your deep end is 8 feet: (3.5 + 8) / 2 = 5.75 feet average depth.

If your pool has an unusually long shallow or deep end, a straight two-point average will be less accurate. In that case, treat the pool as two separate sections: calculate the volume of the shallow section and the deep section individually, then add them together. The same approach applies to pools with a sudden drop-off or hopper bottom rather than a gradual slope.

For example, if your deep end takes up about two thirds of the pool and your shallow end takes up one third: (8 x 0.67) + (3.5 x 0.33) = 6.52 feet.

Always measure actual water depth, not wall height. Your pool likely isn't filled all the way to the top, so using the wall measurement will inflate your number.

Rectangular Pools

Rectangular pools are the most straightforward to calculate.

Formula: Length (ft) x Width (ft) x Average Depth (ft) x 7.5 = Volume in gallons

The 7.5 figure converts cubic feet to gallons. One cubic foot of water holds 7.5 gallons.

Example: A pool that is 30 feet long, 15 feet wide, with a shallow end of 3.5 feet and a deep end of 7 feet.

Average depth: (3.5 + 7) / 2 = 5.25 feet

30 x 15 x 5.25 x 7.5 = 17,719 gallons

Circular Pools

For circular pools, you need the radius, which is half the diameter. Measure straight across the widest point of the pool to get the diameter, then divide by two.

Formula: 3.14 x Radius x Radius x Average Depth x 7.5 = Volume in gallons

Radius is multiplied by itself, which is the same as squaring it. If your radius is 12 feet, that part of the calculation is 12 x 12 = 144.

The most common mistake here is using the diameter instead of the radius. If you plug in the full diameter, your volume will come out roughly four times too large.

Example: A circular pool with a diameter of 24 feet and a consistent depth of 4.5 feet.

Radius: 24 / 2 = 12 feet

3.14 x 12 x 12 x 4.5 x 7.5 = 15,268 gallons

Oval Pools

Oval pools are measured at their longest length and widest width. The formula uses a multiplier of 0.785 to account for the oval shape rather than a full rectangle.

Formula: Length x Width x 0.785 x Average Depth x 7.5 = Volume in gallons

The 0.785 figure accounts for the oval shape (it is 3.14 divided by 4).

Example: An oval pool 30 feet long and 16 feet wide, with a shallow end of 3 feet and a deep end of 6 feet.

Average depth: (3 + 6) / 2 = 4.5 feet

30 x 16 x 0.785 x 4.5 x 7.5 = 12,726 gallons

Irregular and Kidney-Shaped Pools

Irregular pools are the ones most calculation guides skip, which is frustrating because they're common. There are two practical approaches depending on your pool's shape.

Method 1: Break it into two rectangles

If your pool has an L-shape or can be split into two rough rectangles, measure each section separately and add the volumes together.

Section A volume + Section B volume = Total volume

Example: Section A is 20 feet x 14 feet. Section B is 12 feet x 8 feet. Average depth across both is 5 feet.

Section A: 20 x 14 x 5 x 7.5 = 10,500 gallons Section B: 12 x 8 x 5 x 7.5 = 3,600 gallons Total: 14,100 gallons

In most cases, the average depth will be the same across both sections since the floor slope usually runs the full length of the pool. If your pool has a noticeably different depth in one section, measure each separately and use the relevant depth for each calculation.

Method 2: Kidney or freeform pools

For kidney-shaped or freeform pools without clean geometric sections, measure the longest length, the widest point, and your average depth, then apply a multiplier of 0.585 to approximate the shape.

Formula: Length x Width x Average Depth x 0.585 x 7.5 = Volume in gallons

Example: A kidney pool 32 feet long and 16 feet wide, with an average depth of 5 feet.

32 x 16 x 5 x 0.585 x 7.5 = 11,232 gallons

For chemical dosing purposes, an estimate within 10% of your actual volume is accurate enough. You do not need a precise figure down to the last gallon.

Above Ground Pools

Above ground pools use the same formulas as their in-ground equivalents. The main difference is that most above ground pools have a flat, consistent depth rather than a slope, so average depth is less of a concern.

Round above ground pools: 3.14 x Radius x Radius x Depth x 7.5

Example: An 18-foot diameter pool, 4 feet deep. Radius: 9 feet 3.14 x 9 x 9 x 4 x 7.5 = 7,630 gallons

Rectangular above ground pools: Length x Width x Depth x 7.5

Example: A 12 x 24-foot pool, 4 feet deep. 12 x 24 x 4 x 7.5 = 8,640 gallons

Oval above ground pools: Length x Width x 0.785 x Depth x 7.5

Example: A 15 x 30-foot pool, 4 feet deep. 15 x 30 x 0.785 x 4 x 7.5 = 10,598 gallons

Now You Have Your Volume: What Next?

Your gallon figure is what every product label is referencing when it says "per 10,000 gallons." Once you have it, dosing becomes straightforward: take your volume, divide by 10,000, and multiply by the recommended dose.

The most common mistake I see at this stage is people rounding their pool size down rather than up. If your pool comes out at 14,500 gallons and you dose for 10,000, you are leaving 4,500 gallons undertreated. That can leave chlorine levels lower than intended, making it easier for algae and cloudy water to develop. When in doubt, round up to the nearest 5,000 gallons.

If you would rather skip the manual calculation, Pool Day's pool volume calculator does the math for you. Enter your dimensions and shape and it gives you an instant volume figure ready to use for dosing.