Swimming Pool Volume & Chemical Dosing

Turning Pool Dimensions Into Gallons and a Chlorine Dose

A pool’s volume is its length times width times average depth (or pi times radius squared times average depth for a round pool), converted to US gallons at 7.48 gallons per cubic foot. Use Pool Volume mode to find your pool’s total gallons, then switch to Chemical Dosing mode to work out how much liquid chlorine (sodium hypochlorite) it takes to raise your Free Chlorine reading to a target level.

The Formula

Pool volume — a rectangular pool is a simple box, a round pool is a cylinder:

$$\vD{\text{Volume (gal)}} = \vA{\text{Length}} \times \vB{\text{Width}} \times \vC{\text{Avg. Depth}} \times 7.48$$

For a round pool, replace $\vA{\text{Length}} \times \vB{\text{Width}}$ with $\pi \times \left(\vA{\text{Diameter}} \div 2\right)^2$.

Chlorine dosing — scaled from a commonly-cited pool-industry rule of thumb for 10%-strength liquid chlorine (about 13 fl oz per 10,000 gallons to raise Free Chlorine by 1 ppm):

$$\vD{\text{Fl. Oz. Needed}} = 13 \times \frac{\vA{\text{Gallons}}}{10,000} \times \left(\vC{\text{Target ppm}} - \vB{\text{Current ppm}}\right)$$

A 12.5%-strength solution needs proportionally less volume for the same ppm increase.

Worked Example

A 20 ft × 10 ft rectangular pool with an average depth of 5 ft:

1. Volume: $\vA{20} \times \vB{10} \times \vC{5} = 1,000$ cubic feet, or $1,000 \times 7.48 \approx \vD{7,480}$ gallons.
2. To raise Free Chlorine from 1 ppm to 3 ppm with 10% liquid chlorine: $13 \times \left(7,480 \div 10,000\right) \times 2 \approx \vD{19.4}$ fl oz — about 0.15 gallons.

This calculator is deliberately scoped to raising Free Chlorine with liquid chlorine at 10% or 12.5% strength — the two most common retail/pool-store concentrations — rather than covering other chemicals or sanitizer types, since reliable published dosing rates for those weren’t available.

Key Factors to Consider

• Pool water chemistry involves more balanced factors than Free Chlorine alone. pH, total alkalinity, and calcium hardness all interact with each other and with chlorine’s own effectiveness — a pool with correct chlorine but badly unbalanced pH can still have water-quality or equipment issues, so chlorine dosing is one piece of a bigger balancing act, not the whole picture.
• Irregularly-shaped pools (kidney-shaped, freeform) need their volume estimated differently than this calculator’s rectangular or round formulas. For an irregular shape, many pool owners approximate the pool as a combination of simpler shapes (like a rectangle plus a half-circle) and sum their volumes, or use the average of several width measurements across the pool’s length.
• Chlorine demand changes with sunlight, bather load, and temperature, so a single dose doesn’t stay effective indefinitely. UV light from sunlight breaks down unstabilized chlorine relatively quickly, heavy pool use consumes chlorine faster, and warmer water accelerates both chemical reactions and organic growth — regular retesting, not a one-time dose, is how chlorine levels are actually maintained.
• Always add liquid chemicals to water, never water to concentrated chemicals, and follow the product’s own safety instructions. Liquid chlorine is a hazardous chemical that can cause harm if handled incorrectly — this calculator estimates a dosing quantity, not a safety guide, so always follow the product label’s own handling and storage instructions.

Common Mistakes

• Guessing pool volume instead of measuring average depth carefully. Many pools slope from a shallow end to a deep end — using the deepest point instead of a true average depth overstates the volume and leads to overdosing chemicals.
• Adding the full calculated dose all at once for a large chlorine gap. A very large jump in Free Chlorine is usually better split across a couple of doses with retesting in between, since factors like sunlight and circulation affect how quickly and evenly a large dose actually distributes.
• Assuming a 10% and 12.5% liquid chlorine bottle need the same dose for the same result. The stronger 12.5% solution needs proportionally less volume to raise Free Chlorine by the same amount — always confirm which concentration you’re using before dosing.

Useful to Know

• Building a poured concrete deck or pad around the pool? Concrete Calculator estimates the concrete needed for a slab project.
• Need to convert your pool’s gallon figure into a different volume unit? Volume Calculator calculates and converts volume for common shapes and units.