# Air conditioner BTU calculator

> Air conditioner size for a room in BTU/h, tons and kW from floor area, ceiling height, insulation, sun and occupants, beside the Energy Star chart.

Interactive version: https://www.calcopenly.com/construction/btu-calculator
Subject: Construction and home calculators

The cooling load starts at 20 BTU/h per square foot for an 8 ft ceiling, scales with ceiling height and an insulation factor, then takes Energy Star's room air conditioner adjustments: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU/h for each regular occupant beyond two and 4,000 BTU/h for a kitchen. The load is also given in tons (12,000 BTU/h each) and kW, with the next common unit size.

A 300 ft² room with an 8 ft ceiling and average conditions comes to 6,000 BTU/h, half a ton or 1.76 kW. Energy Star's sizing chart, based on 8 ft ceilings, lists 8,000 BTU/h for 300 up to 350 ft², so its figure is shown alongside for rooms of 100 to 2,500 ft².

Both are estimates for one room. For a whole house, ACCA describes its Manual J as the ANSI-recognised method for equipment sizing loads.

## Inputs

- **Room area**
- **Ceiling height**
- **Insulation** (options: Good, Average, Poor)
- **Sun exposure** (options: Heavily shaded, Normal, Very sunny)
- **People who regularly use the room**
- **The room is a kitchen**

## Results

- Cooling load (BTU/h) — main result
- Cooling load (tons)
- Cooling load (kW)
- Next common unit size (BTU/h)
- Energy Star chart for this area (BTU/h)

## Formula

$$
Q = 20\,A\,\frac{h}{8}\,f_{\text{ins}}\,f_{\text{sun}} + 600\max(0, p - 2) + 4000\,[\text{kitchen}]
$$

## Worked examples

### 300 ft² room, 8 ft ceiling, average everything

- Room area: 300 ft²
- Ceiling height: 8 ft
- Insulation: Average
- Sun exposure: Normal
- People who regularly use the room: 2
- **Cooling load: 6,000 BTU/h**
- **Cooling load: 0.5 tons**
- **Cooling load: 1.76 kW**
- **Next common unit size: 6,000 BTU/h**
- **Energy Star chart for this area: 8,000 BTU/h**
- Checked against: Python decimal: 300 × 20 = 6,000 BTU/h ÷ 12,000 = 0.5 ton × 0.29307 = 1.758 kW; Energy Star chart row 300–350 ft² = 8,000

### Sunny 400 ft² kitchen, 10 ft ceiling, poor insulation, 4 people

- Room area: 400 ft²
- Ceiling height: 10 ft
- Insulation: Poor
- Sun exposure: Very sunny
- People who regularly use the room: 4
- The room is a kitchen: yes
- **Cooling load: 18,400 BTU/h**
- **Cooling load: 1.53 tons**
- **Next common unit size: 24,000 BTU/h**
- **Energy Star chart for this area: 16,200 BTU/h**
- Checked against: Python decimal: 400 × 20 × 1.25 × 1.2 × 1.1 + 2 × 600 + 4,000 = 18,400; Energy Star 10,000 × 1.1 + 1,200 + 4,000 = 16,200

### Shaded 20 m² bedroom, 2.4 m ceiling, good insulation, 1 person

- Room area: 20 m²
- Ceiling height: 2.4 m
- Insulation: Good
- Sun exposure: Heavily shaded
- People who regularly use the room: 1
- **Cooling load: 3,433 BTU/h**
- **Next common unit size: 5,000 BTU/h**
- **Energy Star chart for this area: 5,400 BTU/h**
- Checked against: Python decimal: 215.28 ft² × 20 × (7.874/8) × 0.9 × 0.9 = 3,432.6 BTU/h; one occupant adds nothing; chart 6,000 × 0.9

### Edge: 150 ft² sits on a chart boundary

- Room area: 150 ft²
- Ceiling height: 8 ft
- Insulation: Average
- Sun exposure: Normal
- People who regularly use the room: 2
- **Cooling load: 3,000 BTU/h**
- **Energy Star chart for this area: 6,000 BTU/h**
- Checked against: Energy Star chart: 150 ft² falls in the “150 up to 250” row (6,000 BTU/h); rule of thumb 150 × 20 = 3,000

## Questions

### What size air conditioner do I need for my room?

Energy Star's chart for 8 ft ceilings lists 5,000 BTU/h for 100–150 ft², 6,000 for 150–250 ft², 8,000 for 300–350 ft², 10,000 for 400–450 ft², 12,000 for 450–550 ft² and 18,000 for 700–1,000 ft². The 20 BTU/h per ft² rule gives less for small rooms, 6,000 against 8,000 at 300 ft², so the chart figure is shown beside the result.

### How many BTU per square foot do I need?

This calculator uses 20 BTU/h per square foot for an 8 ft ceiling. Energy Star's chart works out higher for small and mid-sized rooms: 8,000 BTU/h for 300 ft² is about 27 per ft², and 12,000 BTU/h for 450 ft² is also about 27. At 1,000 ft² the chart's 21,000 BTU/h is 21 per ft², close to the rule.

### How many BTU is a ton of air conditioning?

One ton of refrigeration is 12,000 BTU/h, which NIST SP 811 lists as 3,516.853 W; one BTU/h is 0.2930711 W. A 6,000 BTU/h room unit is therefore half a ton or 1.76 kW of cooling, and a 24,000 BTU/h split system is 2 tons or 7.03 kW.

### Is a bigger air conditioner better?

No. Energy Star says an oversized unit costs more to buy, wastes energy and does not cool better: it cools the room before it has time to remove the humidity, leaving the air damp and clammy. Choose the unit size that matches the load rather than rounding up by a full size or more for safety.

### How do ceiling height and sunlight change the BTU needed?

Energy Star's chart assumes 8 ft ceilings, and this calculator scales the load in proportion to height, so a 10 ft ceiling adds 25%. Energy Star advises 10% less capacity for a heavily shaded room and 10% more for a very sunny one, plus 600 BTU/h for each regular occupant beyond two and 4,000 BTU/h in a kitchen.

### How accurate is the air conditioner BTU calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 4 worked examples whose answers come from independent sources; for example, “300 ft² room, 8 ft ceiling, average everything” is checked against Python decimal: 300 × 20 = 6,000 BTU/h ÷ 12,000 = 0.5 ton × 0.29307 = 1.758 kW; Energy Star chart row 300–350 ft² = 8,000.

### Where does the method come from?

ENERGY STAR — Room air conditioners: sizing chart and adjustments; ACCA Manual J 8th Edition (ANSI/ACCA 2 Manual J – 2016) — residential load calculation; NIST SP 811 Appendix B.9 — ton of refrigeration and BTU/h to watt.

## Sources

- [ENERGY STAR — Room air conditioners: sizing chart and adjustments](https://www.energystar.gov/products/room_air_conditioners)
- [ACCA Manual J 8th Edition (ANSI/ACCA 2 Manual J – 2016) — residential load calculation](https://www.acca.org/standards/technical-manuals/manual-j)
- [NIST SP 811 Appendix B.9 — ton of refrigeration and BTU/h to watt](https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9)