How this calculator works
Enter a room's length and width (or its area directly, with width set to 1), ceiling height, sun exposure, room type, and how many people regularly use the space, and this calculator estimates the cooling and heating capacity, in BTU per hour, that a room air conditioner or space heater would need to handle it. It also converts the cooling figure into tons of air conditioning, the unit central and mini-split systems are usually sized in (1 ton = 12,000 BTU/hr).
The starting point is a widely published rule of thumb: about 20 BTU/hr per square foot of floor area at a standard 8 ft ceiling. This is a simplification of ENERGY STAR's own room air conditioner sizing chart, which is deliberately non-linear rather than a flat rate — its published brackets work out to roughly 40 BTU per sq ft for a small 100-150 sq ft room, down to around 20 BTU per sq ft only in the 700-1,200 sq ft range, and lower still for very large rooms. A flat 20 BTU/sq ft is a reasonable planning average but will tend to undersize small rooms compared to ENERGY STAR's own chart — check that chart directly for a room under about 400 sq ft. From there, standard adjustments are layered on — ±10% for a very sunny or heavily shaded room, +4,000 BTU/hr if the space is a kitchen, +600 BTU/hr for each occupant beyond two, and a proportional scale-up for ceilings taller than 8 ft. Heating capacity uses a separate, much more climate-dependent rule of thumb, since heat loss depends heavily on insulation and outdoor design temperature.
The formula
Baseline cooling load = room area (sq ft) × climate-zone BTU/sq ft factor × (ceiling height ÷ 8)Adjusted cooling load = baseline × sun exposure factor (0.9 shaded, 1.0 normal, 1.1 sunny) + 4,000 if kitchen + 600 × occupants beyond 2Heating load = room area × climate-zone heating BTU/sq ft factor × (ceiling height ÷ 8); Tons of AC = cooling BTU/hr ÷ 12,000The climate-zone BTU/sq ft factors used here (roughly 18-24 for cooling and 18-55 for heating, depending on zone) are typical published planning figures, not a substitute for a room-by-room heat-loss/heat-gain calculation.
Worked example: a 300 sq ft living room
- 20 ft × 15 ft room = 300 sq ft, 8 ft ceiling (height factor = 1), normal sun, not a kitchen, 2 occupants, moderate climate (20 BTU/sq ft cooling, 35 BTU/sq ft heating).
- Cooling: 300 × 20 × 1 × 1.0 = 6,000 BTU/hr, plus 0 for room type and 0 for occupants (2 is the baseline) = 6,000 BTU/hr.
- Tons of AC: 6,000 ÷ 12,000 = 0.5 tons.
- Heating: 300 × 35 × 1 = 10,500 BTU/hr.
Frequently asked questions
Is 20 BTU per square foot always accurate?
It is a widely used flat approximation, not ENERGY STAR's actual chart, which is deliberately non-linear: small rooms need noticeably more than 20 BTU per sq ft (its published brackets work out to roughly 40 BTU per sq ft around 100-150 sq ft), and only rooms in the 700-1,200 sq ft range land close to 20. For a small room, lean on ENERGY STAR's own sizing table rather than this flat rate. Sun exposure, room type, occupancy, ceiling height, and your climate all shift the real number further, which is why this calculator layers those adjustments on top of the baseline.
What is a Manual J calculation, and do I need one?
Manual J is the industry-standard, room-by-room heating and cooling load calculation method used by HVAC professionals, accounting for actual insulation, window area and orientation, air leakage, and local design temperatures. For a single portable or window unit, a rule-of-thumb estimate like this one is usually good enough; for sizing a central system or a whole-house heat pump, a contractor's Manual J calculation is the authoritative method and is worth insisting on.
Why does an oversized air conditioner hurt performance?
An oversized unit cools the air quickly but shuts off before it has run long enough to remove much humidity, leaving the room cold but clammy, and it cycles on and off more often, which wastes energy and wears out components faster. Right-sizing, not maximizing, capacity is the goal.
Why do kitchens and occupants add fixed BTU amounts instead of a percentage?
Cooking appliances and extra people are relatively fixed heat sources regardless of room size, so sizing guidance adds them as flat BTU/hr amounts (about 4,000 for a kitchen, 600 per person beyond two) on top of the area-based baseline rather than scaling them with square footage.
Does this calculator account for humidity or altitude?
No — it estimates sensible cooling and heating load from area, ceiling height, sun, room type, occupants and a general climate zone only. Very humid climates and high-altitude installations both affect real-world equipment selection and are best confirmed with a professional load calculation.