About SwimSeasonCheck

Why this exists

Ask when an outdoor pool is warm enough to swim in and you usually get "late May to September", which is true of almost nowhere in particular. Ask whether a heater is worth it and you get a sales page. SwimSeasonCheck answers both questions for 3,206 US cities with ten thousand or more people, using one published model, so any two cities can be compared directly.

What goes into the number

Air temperatures are the NOAA NCEI 1991-2020 U.S. Climate Normals (monthly mean and high) at the weather station nearest each city; the farthest any city sits from its station is under 35 miles. Sunshine is NASA's Prediction of Worldwide Energy Resources project's 2001-2025 monthly all-sky solar irradiance for each city's grid cell. Electricity prices are the EIA Electric Power Monthly residential average by state, and natural gas is EIA's residential price by state. City locations and populations come from the Census Bureau's 2025 Gazetteer and 2024 population estimates.

Water temperature is not measured; it is estimated from those inputs. The model counts a month as swimmable when the estimated water reaches 78°F, the low end of the U.S. Department of Energy's comfortable range, and prices each heater option on a typical 450-square-foot pool, including its purchase cost spread over its working life.

What the data shows

Across the 3,206 cities, 1,581 never get an unheated pool to 78°F in a normal year, and only 176 get six months or more; even the warmest manage 8, so no city in the data swims unheated all year. Most of the country sits between, where the honest answer depends on which heater, and what your state charges for the fuel it burns.

Known limits

Climate normals describe a typical year, not this one. A pool in full shade, behind a windbreak, or with a cover left on overnight will run cooler or warmer than the estimate. Energy prices are state averages, not your utility's tariff, and heater prices vary by installer. Treat each page as a starting point for a quote, not a quote.

Corrections

If a city's figures look wrong, say so through the contact page. Confirmed errors are fixed in the data or the model, which corrects every page that depends on it at once.

Who runs this

SwimSeasonCheck is independently run. It is not affiliated with any pool builder, heater manufacturer, retailer, utility or government agency, and nobody pays to change a verdict.