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.
Run your whole job on the same numbers
These NORDIX tools are a taste of the full platform — bid pipeline, estimating, and job costing that carry your numbers from the first bid to the final invoice. Our team sets it up for your shop and walks you through your next real job.
Request access →