Heating Load by Climate Zone
Heating BTU-per-square-foot rules of thumb by IECC climate zone, from negligible in tropical Zone 1 to about 60 BTU/sqft in subarctic Zone 8, with representative cities for each zone. These are estimates for a first pass — a Manual J calculation is authoritative, and the BTU figures are a general rule of thumb only approximately aligned to the IECC zones.
Heating BTU/sqft by IECC zone
| IECC zone | Climate | Representative cities | Heating BTU/sqft |
|---|---|---|---|
| 1 | Very hot | Miami, Honolulu | Negligible heating load |
| 2 | Hot | Houston, Phoenix, Tampa | 30–35 |
| 3 | Warm | Atlanta, Los Angeles, Dallas | 35–45 |
| 4 | Mixed | Baltimore, Washington DC, Seattle | ~45 |
| 5 | Cool | Chicago, Denver, Boston | ~50 |
| 6 | Cold | Minneapolis, Milwaukee, Helena | ~55 |
| 7 | Very cold | Duluth, Fargo, Anchorage | ~60 |
| 8 | Subarctic | Fairbanks | ~60+ |
Two things this table does, and one it doesn’t
The zone-to-city column is solid: the IECC climate zones are a real, well-defined map, and knowing your zone tells you your design conditions. The BTU/sqft column is a general heating rule of thumb — useful for a ballpark and a sanity check, running from about 30 BTU/sqft in warm continental zones to about 60 in the coldest. What the table cannot do is size a furnace or heat pump: insulation, air-tightness, window area, and ceiling height swing the real load far more than the zone alone, and a tight new home in Zone 6 can need less heat per square foot than a leaky old one in Zone 4.
Pair this with the cooling load rules of thumb for the summer side. For anything past a first estimate, a full load calculation is the tool — the rule of thumb is there to catch a sizing that is obviously wrong, not to produce the final number.
Common questions
How many BTU per square foot to heat a house?
It depends on the climate. Published rules of thumb run roughly 30–35 BTU/sqft in hot zones, ~45 in mixed climates, and up to ~60 BTU/sqft in the coldest zones — with tropical climates needing essentially no heating. But these are estimates; a well-insulated home lands near the low end and a drafty older one near the high end, and a Manual J calculation is the authoritative answer.
What are the IECC climate zones?
The IECC divides the US into climate zones numbered 1 (hottest — Miami, Honolulu) through 8 (subarctic — Fairbanks), with letters for moisture (A moist, B dry, C marine). Zone 4 is the mixed middle (Baltimore, DC), Zone 5 is cool (Chicago, Denver), and Zone 6 is cold (Minneapolis). The zone sets your design temperatures and drives the heating load.
Are these heating numbers tied to the IECC zones exactly?
Not exactly. The IECC zone-to-city mapping is reliable, but the published heating-BTU/sqft "zone" tables actually use a generic warm-to-cold region map rather than IECC numbering. They run in the same direction, so we map them approximately to the IECC zones — but treat the BTU numbers as a general rule of thumb, not a literal IECC citation.