BTU / Air Conditioner Size Calculator
Work out the cooling capacity a room needs, and the closest standard air conditioner size to match it — based on room size, sun exposure and occupancy.
How this calculator works
The calculator starts from a baseline of 20 BTU per square foot, adjusts for sun exposure, then adds fixed amounts for extra occupants and kitchen heat load:
base BTU = area × 20 × sun multiplier sun multiplier: shaded 0.9, average 1.0, sunny 1.1 extra occupants = max(0, occupants − 2) BTU needed = base BTU + (extra occupants × 600) + (kitchen ? 4000 : 0)
The result is then rounded up to the nearest standard air conditioner size, since units are sold in fixed increments — 5,000 / 6,000 / 8,000 / 10,000 / 12,000 / 14,000 / 18,000 / 21,000 / 24,000 BTU — not arbitrary values.
Worked example
A 15 ft × 12 ft room, average sun, 3 typical occupants, not a kitchen:
- Area: 15 × 12 = 180 sq ft.
- Base BTU: 180 × 20 × 1.0 = 3,600 BTU.
- Extra occupants: 3 − 2 = 1 → +600 BTU.
- Total: 3,600 + 600 = 4,200 BTU needed → recommended unit: 5,000 BTU.
Why this is a simplified estimate
20 BTU per square foot is a widely used starting baseline, but it's a simplification. A full professional room load calculation (often called a Manual J calculation) also accounts for window count and orientation, insulation quality, local climate, ceiling height, and air leakage — all of which can shift the real number meaningfully. Use this calculator to get in the right range for a single-room unit; for a whole-home system or an unusual space, get a proper load calculation from an HVAC contractor.
Occupancy and kitchen adjustments
| Factor | Adjustment | Why |
|---|---|---|
| Each occupant beyond 2 | +600 BTU | Body heat adds to the cooling load |
| Kitchen | +4,000 BTU | Cooking appliances generate significant heat |
| Sunny room | ×1.1 | Direct sun through windows adds heat gain |
| Shaded room | ×0.9 | Less solar heat gain to offset |
Frequently asked questions
How many BTUs per square foot do I need?
A commonly used starting baseline is about 20 BTU per square foot for a room with average sun exposure and a standard ceiling height. This is a simplified rule of thumb, not a full room load calculation, so treat it as a starting point rather than a precise figure.
Does ceiling height matter for sizing an air conditioner?
Yes. This calculator assumes a standard 8 foot ceiling. A room with noticeably higher ceilings has more air volume to cool for the same floor area, so the actual requirement is higher than this estimate for a room with vaulted or unusually tall ceilings.
Why does sun exposure change the BTU calculation?
A room with large south- or west-facing windows and little shade absorbs significantly more heat during the day than a shaded room, increasing the cooling load needed to stay comfortable. This calculator adjusts the baseline up for a sunny room and down for a shaded one.
When should a kitchen or high-occupancy room size up?
Kitchens generate extra heat from cooking appliances, so it's common to add roughly 4,000 BTU to the baseline. Rooms that regularly hold more than two people also need more cooling capacity, since each additional occupant adds body heat — a common rule of thumb is about 600 BTU per extra person beyond two.