Free BTU calculator
Enter your room's square footage, insulation quality, ceiling height, and occupants — this BTU calculator estimates the cooling or heating capacity needed to size your AC or heater, updated live, as you type.
On this page10 sections
Rule-of-thumb estimate. For accurate HVAC sizing use a Manual J load calculation. 1 ton = 12,000 BTU/hr.
Results are estimates. Consult a professional.
How the BTU calculator works
The calculator multiplies the floor area of the space by a cooling factor — a rule-of-thumb that bundles climate, insulation, and sun exposure into a single BTU-per-square-foot number. Adjustment values are then added for high ceilings, extra occupants, and kitchen heat loads. The result is the estimated cooling capacity needed in BTU per hour (BTU/hr), which you can convert directly to AC tonnage or use to select a window unit.
Worked example: open-plan kitchen/living room
David is sizing a window AC for a 300 sq ft open-plan kitchen and living area in a 1960s home. The insulation is average, the climate is moderate, there are normally 3 people in the space, and it contains a kitchen — so all three adjustments apply.
BTU recommendations by room size
Use this table to find the base cooling requirement before applying adjustments for occupants, kitchen use, or ceiling height. Values are in BTU per hour. To convert to AC tonnage, divide by 12,000.
| Room size (ft²) | Excellent insulation (20 BTU/ft²) | Average insulation (30 BTU/ft²) | Poor insulation / hot climate (50 BTU/ft²) |
|---|---|---|---|
| 100 | 2,000 | 3,000 | 5,000 |
| 150 | 3,000 | 4,500 | 7,500 |
| 200 | 4,000 | 6,000 | 10,000 |
| 300 | 6,000 | 9,000 | 15,000 |
| 400 | 8,000 | 12,000 | 20,000 |
| 500 | 10,000 | 15,000 | 25,000 |
| 600 | 12,000 | 18,000 | 30,000 |
Base values only — add adjustments for occupants above 2 (+600 BTU each), kitchen use (+1,200 BTU), and ceilings above 8 ft (+10%). Source: Energy Star / DOE guidelines.
Tips for sizing room cooling correctly
Both undersized and oversized AC units create problems. Getting the BTU number right is the single most impactful decision you can make before purchase.
- Bigger is not better — an oversized unit cools the air quickly, shuts off, and restarts repeatedly. This short-cycling prevents the unit from running long enough to remove humidity, leaving the room cold and clammy. Excess moisture also promotes mold growth.
- Account for sun exposure — a south- or west-facing room with large windows in a hot climate can behave like a 'poor insulation' space even in a well-insulated home. When in doubt, bump up the cooling factor by 20–30%.
- Measure the conditioned area only — if the room has a connected hallway or open staircase that will share the cooled air, include that square footage in your estimate. If the space is isolated, measure only the room.
- Window units vs. mini-splits — window ACs are sized by BTU/hr and are practical up to roughly 24,000 BTU/hr (2 tons). For larger spaces or whole-home cooling, a ductless mini-split or central system is more efficient. Mini-splits are rated in BTU/hr and tons, so this calculator applies directly.
- For new HVAC equipment, get a Manual J — this BTU calculator is a planning tool. For central air systems and heat pumps, ask your contractor for a full ACCA Manual J load calculation, which accounts for window area, insulation R-values, infiltration rates, and local design temperatures.
Accuracy and limitations
This is a rule-of-thumb sizing tool, not a Manual J load calculation. The BTU/ft² factors (20, 30, 50) are industry-standard planning values, but they simplify a complex picture. A Manual J from a certified HVAC technician is the industry standard for new equipment sizing and is required by code in many jurisdictions.
Variables the calculator cannot capture include: the exact R-value of walls, ceilings, and floors; the number, size, and glazing type of windows; the building's air-infiltration rate; internal heat gains from appliances and lighting; and the local 1% design temperature. Each of these can shift the true cooling load by 10–25%. Use this estimate to understand the ballpark and to ask better questions when shopping or consulting a contractor.
Cooling and BTU terms defined
About this BTU calculator
This calculator runs entirely in your browser — nothing you enter is sent to any server.
Browse more in our construction calculators, or explore the complete library on the free calculators page.