Pool Calculator

What Is the Pool Calculator?

The Pool Calculator works out how much water a swimming pool holds. You choose the shape, enter the dimensions and the depth at each end, and it returns the volume in gallons, liters and cubic feet. It also shows the surface area and the weight of the water.

Pool volume is the number behind almost every maintenance decision. Chlorine, pH adjusters, algaecide and salt are all dosed per 10,000 gallons. Heaters are sized by the volume they must warm. Pumps and filters are selected so that the whole volume turns over within a set number of hours. A guess that is off by a few thousand gallons leads to water that is persistently over-treated or under-treated.

How the Calculator Works

Volume is surface area multiplied by average depth. The calculator first finds the surface area for the shape you choose. A rectangle is length times width. A circle is pi times the radius squared. An oval is treated as an ellipse, which is pi divided by four, times length, times width.

Most pools slope from a shallow end to a deep end. The calculator averages the two depths. For a bottom that slopes evenly, this average is accurate. For a flat-bottomed pool, enter the same depth twice.

The surface area times the average depth gives cubic feet. One cubic foot of water is 7.48 gallons, so the calculator multiplies by that figure. Liters follow at 3.785 per gallon.

The Pool Volume Formula

Average depth = (Shallow depth + Deep depth) ÷ 2

Rectangular: Gallons = Length × Width × Average depth × 7.48

Round: Gallons = 3.1416 × Radius² × Average depth × 7.48

Oval: Gallons = 0.7854 × Length × Width × Average depth × 7.48

Shortcut multipliers

Many pool guides round the constants. For a round pool, diameter × diameter × depth × 5.9 gives gallons. For an oval, length × width × depth × 5.9. For a rectangle, length × width × depth × 7.5. These give results within about one percent.

Step-by-Step Explanation

1. Measure the length and width of the water surface in feet. For a round pool, measure the diameter.
2. Measure the water depth at the shallow end and at the deep end.
3. Add the two depths and divide by 2 for the average depth.
4. Calculate the surface area for the pool’s shape.
5. Multiply the surface area by the average depth for cubic feet.
6. Multiply by 7.48 to convert to gallons.

Worked Example

A rectangular in-ground pool is 30 feet long and 15 feet wide. It is 3.5 feet deep at the shallow end and 8 feet at the deep end.

• Average depth: (3.5 + 8) ÷ 2 = 5.75 feet.
• Surface area: 30 × 15 = 450 square feet.
• Cubic feet: 450 × 5.75 = 2,587.5.
• Gallons: 2,587.5 × 7.48 = 19,355.

So the pool holds about 19,400 gallons, or 73,300 liters. The water weighs roughly 81 tons.

For a round above-ground pool 24 feet across with 4 feet of water: the radius is 12 feet, the area is 3.1416 × 144 = 452 square feet, the volume is 1,810 cubic feet, and it holds about 13,500 gallons.

How to Use the Calculator

1. Choose the pool shape.
2. Enter the length, or the diameter for a round pool.
3. Enter the width, unless the pool is round.
4. Enter the depth of water at the shallow end and the deep end.
5. Press Calculate.
6. Use the gallon figure for chemical dosing and equipment sizing.

For L-shaped or freeform pools, divide the surface into simple shapes, calculate each one, and add the volumes. The Square Footage Calculator helps with the surface area of each section. For generic volume conversions, see the Cubic Yard Calculator. If you are building a pool deck or surround, the Paver Calculator and Concrete Calculator estimate the materials, and the Fence Calculator covers the safety fence that most areas require.

Practical Tips

• Measure water depth, not wall height. The water line usually sits a few inches below the top of the wall.
• Write the volume somewhere permanent, such as inside the equipment shed. You will use it every time you add chemicals.
• Dose by the label rate per 10,000 gallons and scale to your volume.
• Add chemicals in stages and retest. It is easier to add more than to correct an overdose.
• A pump should turn over the full volume in about 8 hours. Divide the gallons by 480 minutes for the minimum flow rate in gallons per minute.
• Keep the water level at the middle of the skimmer opening for proper circulation.
• Check local rules before draining a pool. Many areas restrict where chlorinated water may be discharged.

Common Mistakes

Using the deep-end depth as the depth. This overstates volume significantly. Use the average.

Measuring the outside of the pool. Use the inside dimensions at the water surface.

Ignoring the shape. Treating an oval as a rectangle overstates the volume by about 27 percent.

Forgetting steps, benches and tanning ledges. These displace water. Large ledges can reduce the volume by several hundred gallons.

Using the wrong constant. There are 7.48 gallons in a cubic foot, not 7 and not 8.

Assuming the builder’s figure is exact. Builders often quote a nominal volume. Measuring gives a better number.

Pools With a Hopper Bottom

Some pools have a shallow section, a slope, and then a deep diving hopper. For these, split the pool into sections. Calculate the shallow section with its own depth, the sloped section with the average of its two depths, and the deep section with its depth. Add the three volumes. The simple average of shallow and deep works best when the slope runs evenly from one end to the other.

Freeform and Kidney Pools

For a kidney-shaped pool, measure the length and the two widest points across each lobe. Add the two widths, multiply by the length and by 0.45 to approximate the surface area. Then multiply by the average depth and by 7.48. For other freeform shapes, breaking the outline into a rectangle and half circles is usually accurate enough.

Why Volume Matters for Equipment

A heater’s output in BTU determines how quickly it raises the temperature. Raising one gallon of water by one degree Fahrenheit takes 8.34 BTU, so a 19,400 gallon pool needs about 162,000 BTU for each degree, before heat losses. Filters are rated by flow, and the flow you need follows from the volume and the turnover time. Salt chlorine generators are sized by the maximum gallons they can treat.

Filling Time and Cost

A garden hose delivers roughly 9 to 17 gallons per minute depending on pressure and hose size. At 10 gallons per minute, a 19,400 gallon pool takes about 32 hours to fill. Multiply your water utility’s price per thousand gallons by the volume to estimate the cost, and ask whether a sewer charge credit is available for pool fills.

Hot Tubs and Small Pools

The same formulas apply at any scale. A round hot tub 7 feet across filled to 3 feet holds about 860 gallons before seats are taken into account. Moulded seats and footwells displace a lot of water, so manufacturers’ stated capacities are usually 25 to 40 percent lower than the simple geometric volume. For spas, use the manufacturer’s figure when it is available. For stock tanks and plunge pools, which have simple shapes, the calculator’s result is accurate.

Evaporation and Top-Ups

An uncovered pool commonly loses about a quarter of an inch of water a day to evaporation in warm, dry weather. On a 450 square foot surface, a quarter inch is about 70 gallons. Over a week that is close to 500 gallons, and each top-up dilutes the chemistry slightly. A cover reduces evaporation by 90 percent or more and keeps heat in. If the level drops much faster than this, test for a leak by marking the level inside a bucket set on a step and comparing the pool and bucket after 24 hours.

Seasonal Dosing

At opening, closing and after heavy use or storms, pools are shocked with a larger dose of chlorine. The dose is given per 10,000 gallons, so a pool of 19,400 gallons needs just under twice the label quantity. Calculating that correctly is the difference between clear water in a day and a week of cloudy water.