For cooling, plan on about 20 BTU per square foot. For heating, it's higher and depends on your climate — from ~32 BTU/sq-ft in the warm south up to ~55 BTU/sq-ft in the cold north. Those two numbers are the fastest way to ballpark an AC or furnace. Below is the full chart for both, plus the reason the per-square-foot rule is only a starting point — and a calculator that adjusts it for your ceiling, insulation, sun, and occupancy.
Cooling: BTU per square foot
The ~20 BTU/sq-ft baseline (ENERGY STAR) assumes an 8-ft ceiling and average insulation. Climate shifts it:
| Climate | BTU per sq ft (cooling) | 500 sq ft | 1,000 sq ft |
|---|---|---|---|
| Hot (e.g. Phoenix, Chennai) | ~25 | 12,500 BTU | 25,000 BTU |
| Warm (e.g. Atlanta) | ~22 | 11,000 BTU | 22,000 BTU |
| Temperate (e.g. NYC) | ~20 | 10,000 BTU | 20,000 BTU |
| Cool (e.g. Seattle) | ~18 | 9,000 BTU | 18,000 BTU |
| Cold (e.g. Minneapolis) | ~17 | 8,500 BTU | 17,000 BTU |
Divide the BTU by 12,000 to get tons — 24,000 BTU is a 2-ton AC.
Heating: BTU per square foot by climate zone
Heating is far more climate-sensitive than cooling, because the colder it gets outside, the more heat leaks out. Furnace sizing uses these published per-sq-ft factors:
| Climate zone | BTU per sq ft (heating) | 1,500 sq ft | 2,500 sq ft |
|---|---|---|---|
| Zone 1 — hot | ~32 | 48,000 BTU | 80,000 BTU |
| Zone 2 — warm | ~37 | 55,500 BTU | 92,500 BTU |
| Zone 3 — temperate | ~42 | 63,000 BTU | 105,000 BTU |
| Zone 4 — cool | ~47 | 70,500 BTU | 117,500 BTU |
| Zone 5 — cold | ~55 | 82,500 BTU | 137,500 BTU |
This is why the same 1,500 sq ft home needs a ~63,000 BTU furnace in a temperate climate but over 80,000 BTU up north.
What a climate zone means here
"Climate zone" is shorthand for how hard your equipment has to work across the year. Hotter regions demand more cooling capacity per square foot; colder regions demand far more heating capacity. Most of the U.S. falls in the temperate-to-cool band; the deep south and desert southwest run hot, and the upper Midwest and Northeast run cold. Pick the row that matches your summers (for cooling) and winters (for heating).
Worked examples
- A 1,500 sq ft temperate home: ~30,000 BTU (2.5 tons) of cooling and a ~63,000 BTU furnace. See the full breakdown →
- An 8,000 BTU window unit: cools about 400 sq ft in a temperate climate.
- A 2,500 sq ft home: ~50,000 BTU cooling and a ~105,000 BTU furnace. Details →
Why per-square-foot is only a starting point
A single BTU/sq-ft number treats every room the same. In reality, five things move it a lot:
- Ceiling height — over 8 ft scales the load up (a 10-ft ceiling adds ~25%).
- Insulation — poor adds ~15%, excellent trims ~10%.
- Sun — a very sunny room needs ~10% more; a shaded one ~10% less.
- Occupants — +600 BTU per person beyond two.
- Kitchen — +4,000 BTU for appliance heat.
Stack a hot climate, a sunroom's worth of glass, and poor insulation and you're 40% above the chart. That's why the chart gets you close, but the BTU calculator gets you right — it applies every one of these to your actual room.
Frequently asked questions
How many BTU per square foot do I need? About 20 BTU/sq-ft for cooling in a temperate climate (25 in a hot one, 17 in a cold one), and 32–55 BTU/sq-ft for heating depending on how cold your winters are.
Why is heating BTU per square foot higher than cooling? Heat loss scales with how big the indoor–outdoor temperature gap is, and winter gaps are usually larger than summer ones — so a furnace has to overcome more.
Is 20 BTU per square foot accurate? It's a solid starting point for an average room. Ceiling height, insulation, sun, climate, and occupancy can each move it 10–25%, so confirm with a calculator or a Manual J load calculation before buying.
The bottom line
Use ~20 BTU/sq-ft for cooling and the 32–55 BTU/sq-ft heating table as your first pass, then adjust for your room's real conditions. The free BTU calculator does the adjusting and gives you the exact number, the tonnage, and the unit to buy.
Estimates for planning only. Confirm a purchase or install with a Manual J load calculation and a licensed HVAC professional.