Where our numbers come from
Every SizeSums result comes from public U.S. government data and the formulas described here. Each calculator page also shows its own formula, assumptions and a worked example.
Data sources
| Data | Source | Updated |
|---|---|---|
| Climate zone by county | U.S. Department of Energy, Building Energy Codes Program (IECC climate zones) | Per code edition |
| Design temperatures (1% cooling, 99% heating) | Computed by SizeSums from NOAA Integrated Surface Database hourly airport observations, 2015-2024 | Computed once, years shown |
| County population centers | U.S. Census Bureau, 2020 centers of population | Per census |
| ZIP code to county | U.S. Census Bureau, ZCTA to county relationship file | Per census |
| Equipment sizing guidance | ENERGY STAR; U.S. Department of Energy | Checked yearly |
Climate: zone and design temperatures
- Your ZIP code is matched to the county that covers most of it. If a ZIP crosses county lines, pick your state or type your own design temperatures.
- The climate zone is the county's IECC zone (1 hottest to 8 coldest), with A, B or C for moist, dry or marine.
- The cooling design temperature is the outdoor temperature exceeded in only 1% of the hours of a year (about 88 hours). The heating design temperature is the one the air stays above in 99% of hours. Equipment is sized for these, not for the single hottest or coldest day.
- We compute both from ten years (2015-2024) of hourly readings at the airport weather station nearest each county's center of population, skipping stations with gaps. Contractors use ACCA Manual J tables built from an older, longer record. Recent winters have been milder, so our heating figure can run a few degrees warmer than theirs; cooling figures usually agree within a degree or two. If you have your contractor's numbers, type them in.
- The inlet water temperature used for tankless water heaters is estimated as the local mean annual air temperature plus 2°F, which is close to the temperature of shallow groundwater. Water from lakes and rivers runs colder in winter.
Estimates, not a load calculation
Our calculators use the size of your home, insulation, windows and climate. A Manual J load calculation also looks at each wall, window and duct run in your actual house. For central air, furnaces and heat pumps, use our result to check a quote, and let a contractor's Manual J decide the final size.