BTU Calculator
Estimate the BTU and AC tonnage to cool a room or home by square footage, climate zone, insulation, sun, and occupancy. A fast pre-Manual J sizing check.
Conditioned floor area of the space.
Pick the zone whose example city is closest to your climate.
Standard is 8 ft. Taller ceilings need more cooling.
+600 BTU each above 2.
+1000 BTU each above 6.
Recommended cooling size
- Base BTU (before adjustments)
- 30,000BTU
- Total adjustments applied
- +0BTU
- Climate zone
- Zone 4 — Mixed (e.g. Washington DC)
This is a rough sizing estimate. A Manual J load calculation is required for accurate professional sizing.
Oversized AC units short-cycle and remove less humidity. Don’t round up aggressively.
Heating BTU needs differ. For furnace sizing in cold climates, multiply by roughly 1.5–2×.
How this calculator works
This BTU calculator gives you a fast estimate of the cooling capacity — in BTU per hour and in tons — needed to keep a space comfortable. It is built for a quick gut-check before you talk to a contractor, not as a substitute for an engineered load calculation.
Start with square footage and climate
The core of the estimate is floor area multiplied by a BTU-per-square-foot factor that depends on your climate zone. Hot zones like Miami use about 30 BTU per square foot; very cold zones like Duluth use around 14, because the cooling job is smaller where summers are milder. That product is your base load.
Adjust for the things that move the load
A flat per-square-foot number ignores most of what actually drives cooling demand, so this tool applies sensible modifiers. Ceiling height scales the load by height ÷ 8, since you are cooling a volume of air. Poor insulation raises the load by 20% and good insulation cuts it by 15%. A very sunny exposure adds 15%; deep shade subtracts 10%. Each occupant beyond two adds 600 BTU, and each window beyond six adds 1,000 BTU, capturing internal and solar heat gains.
Why Manual J still matters
Rules of thumb are convenient but blunt. The professional standard, ACCA Manual J, models your home in real detail: window orientation and glazing, infiltration, duct losses, and local design temperatures. It routinely lands on a smaller, better-matched system than a square-foot estimate would suggest — which is exactly why right-sizing beats oversizing. Use this calculator to get in the right range, then have a pro run Manual J before buying equipment. Verify against current local codes.
Frequently asked questions
- What is the difference between BTU and tonnage?
- BTU (British Thermal Units per hour) measures how much heat an air conditioner can remove. Tonnage is just BTU in bigger units: 12,000 BTU equals 1 ton of cooling. So a 36,000 BTU system is a 3-ton unit. The term comes from the cooling power of a ton of ice melting over 24 hours.
- Is a bigger AC always better?
- No — oversizing is one of the most common HVAC mistakes. An oversized unit cools the air quickly, then shuts off before it has run long enough to pull humidity out of the space. The result is a cold, clammy room, more wear from frequent on/off short-cycling, and higher energy bills. Right-sizing matters more than raw capacity.
- What is Manual J, and do I need it?
- ACCA Manual J is the industry-standard residential load calculation. It accounts for orientation, window types, infiltration, duct losses, local design temperatures, and more — far more detail than a per-square-foot estimate. For a new install, equipment replacement, or a permit, you need a real Manual J. Use this calculator for a quick ballpark, not a final spec. Verify against current local codes.
- How does ceiling height affect AC size?
- Cooling load tracks the volume of air you are conditioning, not just the floor area. The standard heuristic assumes 8-foot ceilings; this calculator scales the load by ceiling height ÷ 8, so a 10-foot ceiling adds about 25% to the base BTU. Vaulted and cathedral ceilings can push the requirement up substantially.
- What climate zone am I in?
- This calculator uses simplified DOE-style zones from 1 (hot, like Miami) to 7 (very cold, like Duluth). Pick the zone whose example city is closest to your summer climate. Hotter zones use a higher BTU-per-square-foot factor because the indoor-to-outdoor temperature difference the system must overcome is larger.
- Why does this differ from a manufacturer or contractor estimate?
- Per-square-foot estimates are deliberately rough. A manufacturer’s selection tool or a contractor’s Manual J factors in your specific windows, insulation R-values, duct design, and local design temperatures, so their number can land above or below this one. Treat this result as a sanity check, then confirm with a professional load calculation.