HVAC BTU / Room Sizing Calculator
Enter room dimensions and a few building factors to estimate the BTU/h load for a window AC, mini-split, or heating unit. This is a planning rule of thumb — not a full Manual J load calculation.
Calculate
Calculate
Measurements convert when you switch units
Result
The formula
Convert floor area to square feet, multiply by a climate BTU/ft² factor, then apply sun, insulation, and ceiling-height multipliers. Add occupancy, window, and kitchen extras. Divide by 12,000 for tons (and convert to kW).
Worked example
- Room 5 m × 4 m → area = 20 m² ≈ 215.3 ft²
- Moderate climate (25 BTU/ft²) → base = 215.3 × 25 ≈ 5,382 BTU/h
- Average sun & insulation, 2.4 m ceiling (no height boost), 2 people, 1 large window → +1,000 BTU/h
- Total ≈ 6,382 → round to 6,400 BTU/h ≈ 0.53 tons ≈ 1.88 kW
Result: ≈ 6,400 BTU/h · suggested 7,000 BTU class
How room BTU sizing is estimated
Residential contractors still use square-footage rules of thumb for quick room AC and heater screens. This calculator follows that transparent approach and states where it stops short of ACCA Manual J.
Area and climate factor
Floor area is length × width. Metric inputs are converted to square feet because common BTU/ft² tables are published in imperial units. Climate selects a base factor: cooler regions need less cooling capacity per square foot; hotter regions need more. Heating mode uses a separate factor set that rises in cold climates.
Default cooling factors are 20 / 25 / 30 / 35 BTU per ft² for cool, moderate, hot, and very hot climates. Heating defaults are 45 / 35 / 25 / 20 BTU per ft² for the same labels.
Sun, insulation, and ceiling height
Shaded rooms trim cooling load (~10%); sunny exposures add ~10%. Poor insulation increases the estimate (~15%); good insulation reduces it (~10%). Ceilings taller than 8 ft (≈2.44 m) scale the base load by height÷8 because more air volume must be conditioned.
People, windows, kitchens, and sunrooms
In cooling mode, the first two occupants are included in the base factor; each additional person adds 600 BTU/h (sensible body heat), and kitchens add 4,000 BTU/h for cooking appliances — classic room-AC chart conventions. Each large window or glass door adds 1,000 BTU/h as a simple fenestration proxy in both modes. A sunroom / glass-heavy room type adds 10% after the sun and insulation multipliers.
In heating mode, occupant and kitchen extras are omitted (internal gains reduce heat need, they do not increase it). Sun exposure is inverted: shaded rooms raise the heating estimate and sunny rooms lower it.
What this calculator does not model
It is not ACCA Manual J. Missing pieces include wall/roof R-values, window U-factor and SHGC by orientation, infiltration CFM, duct losses, design outdoor temperatures, latent (humidity) load, and equipment Manual S selection. Use a Manual J–compliant tool or an HVAC professional before buying or replacing whole-home equipment.
Interesting facts
One refrigeration ton is 12,000 BTU per hour
The “ton” comes from the heat needed to melt one ton of ice in 24 hours. Room units are often sold in BTU/h; central systems are quoted in tons (0.5, 1, 1.5, 2, …).
Oversizing short-cycles and raises humidity
An oversized AC cools the air quickly and shuts off before it removes enough moisture. Comfort suffers even when the thermostat looks fine — a common complaint after rule-of-thumb oversizing.
Manual J replaced “one ton per 500 ft²”
That 1970s rule assumed leaky walls and single-pane glass. Applied to tight, well-insulated homes it routinely oversizes equipment by tens of percent. Codes increasingly require Manual J for permitted work.
BTU/h converts to about 0.293 watts
Electrical and European catalogs often list capacity in kilowatts. Multiply BTU/h by 0.000293 to get kW (or divide kW by 0.000293 for BTU/h).
Sunrooms and west glass dominate cooling
Large west-facing windows can add more afternoon cooling load than several feet of insulated wall. Orientation and shading matter as much as floor area on sunny façades.
Frequently asked questions
Start from floor area in square feet times a climate factor (often ~20–35 BTU/ft²), then adjust for sun, insulation, people, windows, and kitchens. This calculator applies those steps and rounds to a practical BTU/h estimate.
Divide BTU/h by 12,000. Example: 18,000 BTU/h ÷ 12,000 = 1.5 tons. Room units may still be labeled only in BTU/h.
No. Manual J (ACCA) is a room-by-room envelope load calculation using insulation, fenestration, infiltration, and design temperatures. Square-footage BTU tools are quick screens only.
Slightly upsizing within the next common class (for example 6,400 → 7,000 BTU) is normal for product availability. Jumping multiple sizes invites short-cycling — prefer the closest adequate class.
Yes. Rooms taller than about 8 ft have more air volume. This tool multiplies the base load by height÷8 when the ceiling exceeds 8 ft.
Switch the calculator to heating mode. Cold climates use a higher BTU/ft² factor than mild ones. Heat pumps and electric heaters still need a proper heat-loss calculation for whole-home sizing.
Usually yes. Cooking appliances add heat; this estimate adds 4,000 BTU/h when room type is kitchen, matching classic room-AC sizing charts.
Yes. Enter metres (or switch to feet). Area is converted to square feet internally so the BTU/ft² factors stay consistent.
References
- Manual J® Residential Load Calculation ANSI-recognized standard for residential heating and cooling load calculations.
- Room air conditioners Consumer guidance on room AC selection and efficient operation.
- Room air conditioners Efficiency labeling and sizing context for room cooling products.