How many BTU do I need per square foot? (room size chart)
There is no single BTU-per-square-foot figure: for cooling one room, the ENERGY STAR chart gives about 40 BTU per sq ft in a small room and about 15 in a large one; for heating a whole home, the published rule runs from 30 BTU per sq ft in the warmest US climate zone to 60 in the coldest. The three tables below give the actual figures, produced by the same code as the calculators.
1. Room size to BTU for a room air conditioner
The ENERGY STAR chart (ENERGY STAR, Room Air Conditioners, "Room Air Conditioner Sizing" chart) in full, with a room in the middle of each band worked through the chart's own adjustments: heavily shaded x0.9, very sunny x1.1, four people in the room (+1,200), a kitchen (+4,000). Read at an 8 ft ceiling.
| Area (sq ft) | Chart BTU | BTU per sq ft | Heavily shaded | Very sunny | Four people | Kitchen |
|---|---|---|---|---|---|---|
| 100 up to 150 | 5,000 | 40 | 4,500 | 5,500 | 6,200 | 9,000 |
| 150 up to 250 | 6,000 | 30 | 5,400 | 6,600 | 7,200 | 10,000 |
| 250 up to 300 | 7,000 | 25 | 6,300 | 7,700 | 8,200 | 11,000 |
| 300 up to 350 | 8,000 | 25 | 7,200 | 8,800 | 9,200 | 12,000 |
| 350 up to 400 | 9,000 | 24 | 8,100 | 9,900 | 10,200 | 13,000 |
| 400 up to 450 | 10,000 | 24 | 9,000 | 11,000 | 11,200 | 14,000 |
| 450 up to 550 | 12,000 | 24 | 10,800 | 13,200 | 13,200 | 16,000 |
| 550 up to 700 | 14,000 | 22 | 12,600 | 15,400 | 15,200 | 18,000 |
| 700 up to 1,000 | 18,000 | 21 | 16,200 | 19,800 | 19,200 | 22,000 |
| 1,000 up to 1,200 | 21,000 | 19 | 18,900 | 23,100 | 22,200 | 25,000 |
| 1,200 up to 1,400 | 23,000 | 18 | 20,700 | 25,300 | 24,200 | 27,000 |
| 1,400 up to 1,500 | 24,000 | 17 | 21,600 | 26,400 | 25,200 | 28,000 |
| 1,500 up to 2,000 | 30,000 | 17 | 27,000 | 33,000 | 31,200 | 34,000 |
| 2,000 up to 2,500 | 34,000 | 15 | 30,600 | 37,400 | 35,200 | 38,000 |
Tons = BTU/h / 12,000, so the chart runs from 0.42 tons to 2.83 tons. The AC BTU calculator does the lookup and the adjustments for your own room and shows kW too.
2. Home size and climate zone to heating BTU and tons
The published rule (HVACDirect.com, How to Size Your Air Conditioner or Heater, heating factor): Zone 1 30-35, Zone 2 35-40, Zone 3 40-45, Zone 4 45-50, Zone 5 50-60 BTU per hour per sq ft. Each cell is the midpoint of the range with average insulation; the heat pump / furnace calculator shows the whole range, the insulation factor and the equipment class, and sets the zone from your state.
| Home (sq ft) | Zone 1 | Zone 2 | Zone 3 | Zone 4 | Zone 5 |
|---|---|---|---|---|---|
| 1,000 | 32,500 (2.7 t) | 37,500 (3.1 t) | 42,500 (3.5 t) | 47,500 (4 t) | 55,000 (4.6 t) |
| 1,500 | 48,750 (4.1 t) | 56,250 (4.7 t) | 63,750 (5.3 t) | 71,250 (5.9 t) | 82,500 (6.9 t) |
| 2,000 | 65,000 (5.4 t) | 75,000 (6.3 t) | 85,000 (7.1 t) | 95,000 (7.9 t) | 110,000 (9.2 t) |
| 2,500 | 81,250 (6.8 t) | 93,750 (7.8 t) | 106,250 (8.9 t) | 118,750 (9.9 t) | 137,500 (11.5 t) |
| 3,000 | 97,500 (8.1 t) | 112,500 (9.4 t) | 127,500 (10.6 t) | 142,500 (11.9 t) | 165,000 (13.8 t) |
A 2,000 sq ft home needs 65,000 BTU/h in zone 1 and 110,000 BTU/h in zone 5: the zone matters more than anything else in the rule. Rule of thumb: a Manual J calculation replaces it.
3. Square metres to kW at three insulation levels
The UK heat loss rule (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area): poor 130 W/m², average 100 W/m², good 80 W/m². Heat loss in kW = m² x W/m² / 1,000. The boiler size calculator turns your floor area, bedrooms and bathrooms into a combi or system boiler kW range and sizes one room's radiator.
| Floor area (m²) | poor (130 W/m²) | average (100 W/m²) | good (80 W/m²) |
|---|---|---|---|
| 50 | 6.50 kW | 5.00 kW | 4.00 kW |
| 70 | 9.10 kW | 7.00 kW | 5.60 kW |
| 90 | 11.70 kW | 9.00 kW | 7.20 kW |
| 120 | 15.60 kW | 12.00 kW | 9.60 kW |
| 150 | 19.50 kW | 15.00 kW | 12.00 kW |
| 200 | 26.00 kW | 20.00 kW | 16.00 kW |
The heat loss is the heating side only; a combi boiler is sized above it because it also heats the hot water on demand, which is why the installers' tables go by bedrooms and bathrooms rather than by m². Both figures are on the boiler calculator, with the sources.
Frequently asked questions
- How many BTU do I need for 1,000 square feet?
- To heat a 1,000 sq ft home: 32,500 BTU/h in zone 1, 42,500 BTU/h in zone 3, 55,000 BTU/h in zone 5 at the midpoint of the published range with average insulation (HVACDirect.com, How to Size Your Air Conditioner or Heater, heating factor). To cool one 1,000 sq ft open space with a room unit: the chart's "1,000 up to 1,200" band, 21,000 BTU/h before adjustments.
- What size air conditioner do I need for a 1,500 sq ft house?
- Room by room with the chart: a single 1,500 sq ft space is the "1,500 up to 2,000" band at 30,000 BTU/h (2.50 tons), but a house is several rooms with their own sun, people and kitchen, so the Pro report adds them up and names a central class in tons. The AC calculator does one room at a time.
- How many BTU per square foot for heating?
- 30-35 in zone 1 (hot) up to 50-60 in zone 5 (cold), HVACDirect.com, How to Size Your Air Conditioner or Heater, heating factor; poor insulation adds 10 %, good takes 10 % off. Table 2 multiplies it out for 1,000 to 3,000 sq ft.
- How many kW do I need per square metre?
- The UK rule of thumb is 80 W/m² for a new build, 100 W/m² for an average double-glazed home and 130 W/m² for an older, poorly insulated one (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area). A 90 m² house is 7.20 kW, 9.00 kW or 11.70 kW of heat loss; table 3 has the other sizes, and the boiler calculator turns it into a boiler kW range.
- Why is the cooling figure per square foot lower for big rooms?
- The reason usually given is that heat comes in through walls, windows and the roof, and a room twice the floor area does not have twice the exposed wall. Whatever the reason, the chart steps from 5,000 BTU for 100-150 sq ft to 34,000 BTU for 2,000-2,500 sq ft, and a flat BTU-per-square-foot rule would oversize the large rooms.
- Is this what an installer would size?
- No. These are published charts and rules of thumb, quoted with their sources, for a first estimate and a check on a quote. An installer's Manual J (US) or room-by-room heat loss calculation (UK) counts windows, orientation, air leakage and the local design temperature; that is the figure equipment is bought on.
Last updated 2026-09-20. Sizing estimates from published charts and rules of thumb (ENERGY STAR, manufacturer guides), each with its source and date: a Manual J load calculation by an installer is the final word.