The 1.25 covers the heat the pool sheds while you're adding it — mostly evaporation off the surface. A cover cuts that margin dramatically, which is why the calculator shows the covered sizing too: sometimes a $150 cover downsizes a $3,500 heater to a $2,400 one.
A worked example
15,000 gallons up 15°F overnight: 15,000 × 8.34 × 15 ÷ 12 × 1.25 ≈ 195,000 BTU/hr — buy the 200k or 250k gas unit. A typical 120k BTU-equivalent heat pump would need ~20+ hours for the same job: fine for maintaining warmth all week, wrong for Friday-night spontaneity. Same pool, two different right answers depending on how you swim.
Before you buy
Price the operating cost difference with the heating cost calculator — purchase price is one-time, fuel is forever. And whichever you choose, the cover savings calculator shows the accessory that makes either heater half as expensive to run.
Frequently asked questions
What size pool heater do I need?
Size to how fast you want heat, not just pool size: gallons × 8.34 × degrees of rise ÷ hours available = BTU/hr needed, then add ~25% for surface losses while heating. A 15,000-gallon pool warmed 15°F overnight (12h) wants roughly a 200–250k BTU gas heater.
Is a bigger pool heater better?
For gas, generally yes within budget — a bigger unit runs shorter for the same heat, wears less, and gives weekend-warming flexibility. Heat pumps are different: they're marathon runners sized for maintenance heating, not sprints.
Gas heater or heat pump?
Gas: fast, works in any weather, higher operating cost — right for occasional/weekend heating and cold climates. Heat pump: 4–6× cheaper per degree, but slow and weak below ~50°F air — right for keeping a pool warm all season in a mild climate.
Can a heater be too big for the pool?
Practically, gas heaters top out at plumbing flow requirements — very large units need adequate GPM. The realistic constraint is gas line sizing: 400k BTU units often need a larger meter/line, which is quote territory, not calculator territory.