For cooling, you need 20 BTU per square foot as a baseline. For heating, you need 30–60 BTU per square foot depending on climate zone. These are starting points only — your actual number can vary by 40–60% based on insulation quality, ceiling height, window area, and eight other factors that this guide covers in detail.
The BTU-per-square-foot metric is the quickest way to estimate HVAC sizing, and it's the first number every contractor calculates. But using the wrong multiplier for your specific situation leads to oversized or undersized equipment that costs hundreds extra per year. Here's how to find your exact number.
Cooling BTU Per Square Foot: Complete Breakdown
The Baseline: 20 BTU/sq ft
ENERGY STAR and the DOE use 20 BTU per square foot as the standard cooling baseline. This assumes a home in DOE Climate Zone 4 (mixed climate) with average insulation, 8-foot ceilings, standard window area, and 2 occupants.
BTU Per Square Foot by Climate Zone (Cooling)
| DOE Climate Zone | States/Regions | BTU/sq ft for Cooling | Example: 1,500 sq ft Home |
|---|---|---|---|
| Zone 1 (Hot-Humid) | South FL, Hawaii, US territories | 25–30 | 37,500–45,000 BTU |
| Zone 2 (Hot) | Houston, Phoenix, New Orleans, Tampa | 22–28 | 33,000–42,000 BTU |
| Zone 3 (Warm) | Atlanta, Dallas, Las Vegas, Charlotte | 20–25 | 30,000–37,500 BTU |
| Zone 4 (Mixed) | Nashville, St. Louis, DC, Albuquerque | 18–22 | 27,000–33,000 BTU |
| Zone 5 (Cool) | Chicago, Denver, Boston, Pittsburgh | 16–20 | 24,000–30,000 BTU |
| Zone 6 (Cold) | Minneapolis, Milwaukee, Burlington | 14–18 | 21,000–27,000 BTU |
| Zone 7 (Very Cold) | Duluth, Fargo, Fairbanks | 12–16 | 18,000–24,000 BTU |
The range within each zone accounts for insulation quality. Use the lower end for well-insulated, newer homes and the upper end for older, poorly insulated homes.
BTU Per Square Foot by Insulation Level (Cooling)
Insulation is the second-largest factor after climate zone.
| Insulation Quality | BTU/sq ft Adjustment | Effective BTU/sq ft (Zone 4 baseline) |
|---|---|---|
| Poor (no wall insulation, single pane) | +30% | 26 |
| Below average | +15% | 23 |
| Average (code minimum) | 0% | 20 |
| Good (above code) | −10% | 18 |
| Excellent (high-performance) | −20% | 16 |
| Passive house | −40% | 12 |
A passive house in Climate Zone 5 might need only 8–10 BTU per square foot for cooling — less than half the standard baseline. Meanwhile, an uninsulated 1960s ranch in Zone 2 might need 32–35 BTU per square foot.
Heating BTU Per Square Foot: Complete Breakdown
Heating requirements are more variable than cooling because the temperature difference between indoors and outdoors is much larger in winter (60–90°F gap) than in summer (15–30°F gap).
BTU Per Square Foot by Climate Zone (Heating)
| DOE Climate Zone | States/Regions | BTU/sq ft for Heating | Example: 2,000 sq ft Home |
|---|---|---|---|
| Zone 1 (Hot-Humid) | South FL, Hawaii | 10–15 | 20,000–30,000 BTU |
| Zone 2 (Hot) | Houston, Phoenix, New Orleans | 15–25 | 30,000–50,000 BTU |
| Zone 3 (Warm) | Atlanta, Dallas, Las Vegas | 25–35 | 50,000–70,000 BTU |
| Zone 4 (Mixed) | Nashville, St. Louis, DC | 30–45 | 60,000–90,000 BTU |
| Zone 5 (Cool) | Chicago, Denver, Boston | 40–55 | 80,000–110,000 BTU |
| Zone 6 (Cold) | Minneapolis, Milwaukee | 50–65 | 100,000–130,000 BTU |
| Zone 7 (Very Cold) | Duluth, Fargo, Fairbanks | 60–80 | 120,000–160,000 BTU |
Heating vs. Cooling: Which Drives Your System Size?
In most of the US, heating load drives system size because the winter temperature differential is larger. Here's where each dominates:
| Scenario | Dominant Load | Why |
|---|---|---|
| Zones 1–2 (hot climates) | Cooling | Mild winters, brutal summers |
| Zone 3 (warm) | Cooling (usually) | Close to balanced |
| Zone 4 (mixed) | Varies by home | True mixed climate |
| Zones 5–7 (cool to very cold) | Heating | Severe winters dominate |
| High solar gain (large west windows) | Cooling (even in cold zones) | Solar heat adds significantly to cooling load |
| Well-insulated new build, any zone | May be balanced | Low loads both ways |
For heat pump sizing, this matters especially — the same unit provides both heating and cooling, so you size for the larger of the two loads.
Room-Specific BTU Per Square Foot
Different rooms within the same home have different BTU requirements:
| Room Type | Cooling BTU/sq ft | Heating BTU/sq ft | Why Different |
|---|---|---|---|
| Bedroom | 18–22 | 30–40 | Lower occupancy, smaller windows |
| Living room | 20–25 | 30–45 | More occupants, electronics |
| Kitchen | 25–35 | 25–35 | Cooking heat reduces heating need, increases cooling need |
| Sunroom/conservatory | 30–45 | 40–60 | Extreme glass area |
| Basement (finished) | 10–15 | 20–30 | Ground-coupled, naturally cool |
| Attic/bonus room | 28–40 | 45–65 | Roof exposure |
| Garage (insulated) | 25–35 | 35–55 | Minimal insulation, door losses |
| Home office | 22–28 | 30–40 | Computer equipment adds heat |
| Bathroom | 20–25 | 35–45 | Humidity concerns, comfort expectations |
Zoning tip: If you're using mini splits or a zoned system, size each zone based on its room-specific BTU per square foot — not the whole-home average. A sunroom might need 2–3× the BTU/sq ft of a basement room directly below it.
Commercial and Non-Residential BTU Per Square Foot
For comparison, commercial HVAC sizing metrics differ significantly from residential:
| Building Type | Cooling BTU/sq ft | Heating BTU/sq ft | Notes |
|---|---|---|---|
| Office (standard) | 25–35 | 25–35 | High occupancy, equipment loads |
| Office (server room) | 100–300 | 0–10 | Equipment heat dominates |
| Retail store | 30–45 | 25–35 | Open doors, high foot traffic |
| Restaurant | 40–60 | 30–40 | Kitchen heat, high occupancy |
| Church/assembly | 30–50 | 25–40 | Variable occupancy spikes |
| Hospital | 40–60 | 35–50 | 24/7 operation, strict codes |
| Warehouse | 5–15 | 10–25 | Minimal conditioning |
| Data center | 200–500 | 0 | Cooling only, massive heat loads |
Factors That Change Your BTU Per Square Foot Number
Here's a consolidated reference of all factors and their impact:
| Factor | Cooling Impact | Heating Impact |
|---|---|---|
| Climate zone | ±30% | ±60% |
| Insulation quality | ±30% | ±40% |
| Ceiling height (9–12 ft) | +12% to +50% | +12% to +50% |
| Window area and orientation | ±20% | ±20% |
| Air sealing | ±15% | ±20% |
| Ductwork condition | ±20% | ±20% |
| Floor level (upper vs. lower) | ±25% | ±15% |
| Occupancy | +5–10% | +5–10% (body heat helps in winter) |
| Shade/landscaping | ±15% | ±5% (less impact in winter) |
| Humidity level | ±15% | ±5% |
| Appliances/equipment | +5–15% | +5% (equipment heat helps in winter) |
When multiple factors compound, the total adjustment can be dramatic. A worst-case home (hot climate, poor insulation, upper floor, unshaded, leaky ducts) might need 40+ BTU/sq ft for cooling. A best-case home (mild climate, excellent insulation, shaded, tight envelope) might need only 10–12 BTU/sq ft.
How to Use BTU Per Square Foot for Quick Sizing
Method 1: Simple Multiply
The fastest approach — multiply your square footage by your climate zone's midpoint BTU/sq ft value.
2,000 sq ft × 20 BTU/sq ft = 40,000 BTU cooling capacity needed
This gets you within 20% of the right answer for an average home. Good enough for budget planning, not precise enough for equipment selection.
Method 2: Adjusted Multiply
Start with the climate zone value, then apply the top 3 adjustment factors (insulation, ceiling height, windows).
2,000 sq ft × 22 BTU/sq ft (Zone 3) × 1.15 (below-avg insulation) × 1.12 (9-ft ceilings) × 1.10 (large windows) = 62,340 BTU
This gets you within 10% for most homes and is suitable for selecting equipment size ranges.
Method 3: Full Calculation
Use all 11 factors from our BTU calculator guide. This gets you within 5–10% of a professional Manual J calculation and is suitable for final equipment selection.
Quick comparison: Same 2,000 sq ft home in Atlanta (Zone 3), 9-ft ceilings, below-average insulation, large windows
| Method | Cooling BTU Result | Accuracy vs Manual J |
|---|---|---|
| Simple (20 BTU/sq ft) | 40,000 BTU | ±20% |
| Adjusted (zone + 3 factors) | 62,340 BTU | ±10% |
| Full calculation (all 11 factors) | 58,000 BTU | ±5–10% |
| Professional Manual J | 55,200 BTU | Reference standard |
The simple method undersized by 27%. The adjusted method was within 13%. The full calculation was within 5%. All three methods took less than 5 minutes — the Manual J took 2–4 hours and cost $300–$500.
BTU Per Square Foot for Specific Home Sizes
Here's a quick reference for the most commonly searched home sizes:
| Home Size | Cooling BTU (Zone 3–4 avg) | Cooling Tonnage | Heating BTU (Zone 5 avg) | Furnace Size |
|---|---|---|---|---|
| 500 sq ft | 10,000–12,000 | 1 ton | 20,000–30,000 | 40,000 input |
| 750 sq ft | 15,000–18,000 | 1.5 ton | 30,000–40,000 | 40,000–60,000 |
| 1,000 sq ft | 20,000–24,000 | 2 ton | 40,000–55,000 | 60,000–80,000 |
| 1,200 sq ft | 24,000–28,000 | 2–2.5 ton | 48,000–66,000 | 60,000–80,000 |
| 1,500 sq ft | 30,000–36,000 | 2.5–3 ton | 60,000–82,500 | 80,000–100,000 |
| 2,000 sq ft | 40,000–48,000 | 3.5–4 ton | 80,000–110,000 | 100,000–120,000 |
| 2,500 sq ft | 50,000–60,000 | 4–5 ton | 100,000–137,500 | 120,000–140,000 |
| 3,000 sq ft | 60,000–72,000 | 5–6 ton | 120,000–165,000 | 140,000–2 units |
| 3,500 sq ft | 70,000–84,000 | 6–7 ton | 140,000–192,500 | 2 units typical |
| 4,000 sq ft | 80,000–96,000 | 7–8 ton | 160,000–220,000 | 2 units typical |
Why ranges? The spread within each home size reflects the impact of insulation, climate zone, ceiling height, and other factors. A well-insulated 2,000 sq ft home might need the same cooling capacity as a poorly insulated 1,200 sq ft home. Your specific number depends on your home's characteristics.
How New Construction Standards Are Changing BTU Requirements
Building codes have dramatically reduced the BTU per square foot needed for new homes. The 2021 IECC (International Energy Conservation Code), now adopted in most states by 2026, requires:
| Code Requirement | Impact on BTU/sq ft |
|---|---|
| R-20 or R-13+5 wall insulation | −10–15% vs older homes |
| R-38 to R-60 attic insulation | −10–15% vs older homes |
| Air sealing (≤3 ACH50 blower door) | −15–20% vs older homes |
| Low-E double/triple-pane windows | −10–15% vs single pane |
| Insulated ductwork in conditioned space | −15–20% vs attic ducts |
A code-built 2026 home typically needs 30–40% less HVAC capacity than a comparable 1990s home. A 2,000 sq ft new-build in Zone 5 might only need 28,000–32,000 BTU for cooling (14–16 BTU/sq ft) compared to 40,000+ BTU for a 1990s home of the same size.
This is why "rule of thumb" sizing from older references tends to oversize modern homes. If your HVAC contractor quotes the same tonnage for your 2026 new-build as they'd quote for a 1990s home of the same size, get a second opinion or request a Manual J calculation.
Special Cases and Exceptions
Homes with Radiant Floor Heating
Radiant floor systems operate at lower BTU per square foot (15–30 BTU/sq ft in most climates) because they heat from the floor up, creating a more even temperature profile. The lower supply temperature (90–130°F vs 120–170°F for forced air) means less heat loss through ducts and more usable heat per BTU.
Homes with Ductless Mini Splits
Mini splits eliminate duct losses (15–25% in many homes), effectively reducing the required BTU per square foot by that same percentage. A home that needs 20 BTU/sq ft with a ducted central system might only need 15–17 BTU/sq ft with ductless mini splits.
Homes at High Altitude
At elevations above 4,000 feet, air density decreases, reducing the BTU output of gas furnaces and air conditioners by approximately 4% per 1,000 feet. A home at 6,000 feet (Denver) needs to derate equipment capacity by about 8%. In practice, this means you may need to select a slightly larger unit to compensate.
Homes with Significant Internal Heat Gains
If your home has a server room, commercial kitchen, grow operation, or other major internal heat source, your cooling BTU per square foot for those areas can jump to 50–300 BTU/sq ft. These zones should be calculated separately from the rest of the home and may need dedicated cooling equipment.
Key Takeaways
- Cooling baseline: 20 BTU/sq ft. Heating baseline: 30–60 BTU/sq ft. Your specific number depends on climate zone and insulation quality
- Climate zone has the biggest impact: a home in Phoenix needs nearly twice the cooling BTU/sq ft as the same home in Seattle
- New 2026 code-built homes need 30–40% less HVAC capacity than 1990s homes of the same size
- For quick sizing, multiply sq ft × BTU/sq ft for your zone. For accuracy, apply the adjustment factors
- Heating typically drives system size in Zones 4–7; cooling drives in Zones 1–3
- Room-specific sizing varies dramatically: a sunroom needs 2–3× the BTU/sq ft of a basement
Frequently Asked Questions
Related Articles
5,000 BTU Air Conditioner: What Room Size Can It Cool? (2026)
data-guide • 11 min read
What Size AC for 1,000 Sq Ft? (BTU & Tonnage Guide 2026)
data-guide • 9 min read
What Size AC for 1,500 Sq Ft? (BTU & Tonnage Guide 2026)
data-guide • 10 min read
What Size AC for 2,000 Sq Ft? (BTU & Tonnage Guide 2026)
data-guide • 9 min read