20x40 Pool Gallons Calculator

Volume and dosing for a 20x40 ft pool

A 20 by 40 ft pool is a classic Olympic-adjacent lap-pool footprint for backyards and is one of the largest common residential sizes. At a typical 4.5 ft average depth, it holds about 27,000 gallons -- length × width × average depth × 7.5.

Because this is a larger pool, salt and chemical amounts scale up accordingly: reaching a 3,200 ppm salt target from scratch takes about 720 lbs (eighteen 40 lb bags), a meaningful cost that's worth confirming with your chlorinator's actual target range before buying.

If your 20x40 pool has an unusually deep diving area or a shallower play area than the 4.5 ft default assumes, use the full Pool Volume Calculator with your specific shallow and deep depths.

Where these numbers come from

The formulas and default values in this calculator are based on:

• standard rectangular pool volume formula (7.5 multiplier)
• salt dosing at 1 lb per 120,000 gallons per 1 ppm change
• liquid chlorine dosing at approximately 10 fl oz of 12.5% chlorine per 10,000 gallons per 1 ppm free chlorine increase

These are planning estimates. Test your actual water, follow product labels, and check your equipment manufacturer's guidance before you buy or dose chemicals.