Size your pool heater by pool volume, not guesswork.

Calculate your pool’s gallons first, then pick a heater that can raise the temperature by about 1°F per hour. That’s the rule of thumb.

Most people overthink this. If you have a 20,000-gallon pool, look for a heater that outputs roughly 100,000 BTU for gas or a heat pump sized to your climate. Gas heaters heat fast and are great for occasional use. Heat pumps are slower but cheaper to run if you keep the pool warm all season.

Don’t get too small—you’ll wait forever for the water to warm up and waste energy in the process. A little bigger than needed is fine; way too big just short-cycles and wears out faster.

One gotcha: heat pumps are rated differently than gas. Use the manufacturer’s guidelines for your region. And read the actual BTU rating, not the model number nonsense.

Future you doesn’t want to shiver through October because the heater can’t keep up.