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BTU calculator: how to size an AC for any room
A room’s BTU requirement is a function of its size, its sun exposure, its ceiling height, and how many people use it. The baseline rule is 20 BTU per square foot for standard rooms. A 200 sq ft bedroom needs about 4,000 BTU to cool effectively. A heavily sunlit room of the same size needs roughly 4,400. The exact number matters: an undersized AC runs constantly and never reaches the set temperature; an oversized AC short-cycles and fails to remove humidity.
\[\text{Base BTU} = \text{Room Sq Ft} \times 20\]Square footage to BTU baseline
Standard cooling capacity for typical residential spaces with 8 ft ceilings:
| Room size (sq ft) | BTU needed | Common AC size |
|---|---|---|
| 100-150 | 5,000 | Small window unit |
| 150-250 | 6,000 | Window unit |
| 250-300 | 7,000 | Window unit |
| 300-350 | 8,000 | Window unit |
| 350-400 | 9,000 | Window or portable |
| 400-450 | 10,000 | Window or portable |
| 450-550 | 12,000 | Larger window or mini-split |
| 550-700 | 14,000 | Mini-split |
| 700-1,000 | 18,000 | Mini-split (1.5-ton) |
| 1,000-1,200 | 21,000 | Mini-split (2-ton) |
| 1,200-1,400 | 23,000 | Central AC (2-ton) |
| 1,400-1,500 | 24,000 | Central AC (2-ton) |
| 1,500-2,000 | 30,000 | Central AC (2.5-ton) |
| 2,000-2,500 | 34,000 | Central AC (3-ton) |
| 2,500-3,000 | 38,000 | Central AC (3-ton) |
These are baseline numbers. Apply the adjustments below for real-world conditions.
Adjustments for real conditions
The base 20 BTU/sq ft assumes an average room. Adjust for variations:
Sun exposure
- Heavily sunny room (south or west facing, large windows): +10%
- Heavily shaded room (north facing, no direct sun): -10%
Ceiling height
- Standard 8 ft: no adjustment
- 9 ft ceilings: +12%
- 10 ft ceilings: +25%
- Vaulted or cathedral: +30-50%
Occupancy
- Each person beyond 2 regular occupants: +600 BTU
- Crowded space (kitchen, dining): +1,000 BTU minimum
Heat-generating spaces
- Kitchen (stove, oven, refrigerator): +4,000 BTU
- Server room or workshop: +2,000-4,000 BTU
- Heavy electronics use (gaming, multiple monitors): +1,000 BTU
Worked examples
Bedroom (12 × 14, normal sun, 8 ft ceiling, 2 people)
- Square feet: 168
- Base BTU: 168 × 20 = 3,360
- No adjustments needed
- Required: 4,000 BTU (smallest standard window unit)
Sunny living room (16 × 20, west-facing, 9 ft ceiling, 4 people)
- Square feet: 320
- Base BTU: 320 × 20 = 6,400
- Sun adjustment: +10% = 7,040
- Ceiling adjustment: +12% = 7,885
- Occupancy: +1,200 (2 extra people) = 9,085
- Required: 10,000 BTU (round up to standard size)
Kitchen (12 × 16, normal sun, 8 ft ceiling)
- Square feet: 192
- Base BTU: 192 × 20 = 3,840
- Kitchen adjustment: +4,000 = 7,840
- Required: 8,000 BTU
Open floor plan (kitchen + dining + living, total 700 sq ft)
- Square feet: 700
- Base BTU: 700 × 20 = 14,000
- Kitchen heat: +4,000 = 18,000
- Sun (west-facing): +1,800 = 19,800
- Required: 20,000 BTU (mini-split or large window unit)
Why correct sizing matters
Undersized: The unit runs continuously without reaching the set temperature. It uses more electricity than it should because it never cycles off. Components wear faster from constant operation.
Oversized: The unit cools the air quickly to the set temperature, then shuts off. Air conditioners only remove humidity when they run for sustained periods. An oversized unit cycles on and off in short bursts, leaving the air cold but clammy. Comfort suffers, and electricity costs are also higher than necessary because of frequent startup loads.
The right-sized unit runs in roughly 15-20 minute cycles, removing both heat and humidity, then resting for 10-15 minutes between cycles.
Tons vs BTU
Central AC and mini-split systems are sold in “tons” rather than BTU:
| AC size | BTU per hour |
|---|---|
| 0.5 ton | 6,000 |
| 0.75 ton | 9,000 |
| 1 ton | 12,000 |
| 1.5 ton | 18,000 |
| 2 ton | 24,000 |
| 2.5 ton | 30,000 |
| 3 ton | 36,000 |
| 3.5 ton | 42,000 |
| 4 ton | 48,000 |
| 5 ton | 60,000 |
The conversion: 1 ton = 12,000 BTU/hr. For a whole-house calculation totaling 28,000 BTU, you would size between 2 and 2.5 tons; usually rounding to 2.5 tons for safety.
Whole-house sizing involves more variables (insulation level, window quality, climate zone, infiltration rate). Professional Manual J load calculations account for all of these. The 20 BTU/sq ft rule is a reasonable starting point for individual rooms or rough estimates.
Energy efficiency: SEER rating
Once size is right, efficiency determines operating cost. SEER (Seasonal Energy Efficiency Ratio) measures cooling output per watt of electricity over a typical season:
| SEER | Efficiency level |
|---|---|
| 13-14 | Federal minimum (older units) |
| 15-17 | Standard new units |
| 18-21 | High efficiency |
| 22+ | Premium efficiency |
A 15 SEER unit uses about 25% more electricity than a 20 SEER unit for the same cooling output. For a 24,000 BTU unit running 8 hours per day at $0.16/kWh:
- 15 SEER: 24,000 / 15 = 1,600 W × 8 hr = 12.8 kWh/day = $2.05/day
- 20 SEER: 24,000 / 20 = 1,200 W × 8 hr = 9.6 kWh/day = $1.54/day
Over a 4-month cooling season, that is about a $60 difference. Over 15 years of unit life, $900. The premium for a higher-SEER unit is usually $500-1,500.
Use the BTU calculator for sizing and the electricity cost calculator to estimate operating costs at your local rate.
Quick decision rules
- Single bedroom up to 200 sq ft: 5,000-6,000 BTU window unit
- Master bedroom or small living room (200-400 sq ft): 8,000-10,000 BTU
- Large open space (400-700 sq ft): 12,000-18,000 BTU mini-split
- Whole-house under 1,500 sq ft: 2-ton (24,000 BTU) central AC
- Whole-house 1,500-2,500 sq ft: 2.5-3 ton (30,000-36,000 BTU) central AC
If between two sizes, the smaller one is usually better unless the room has a major heat source (kitchen, sun, electronics) or unusual ceiling height.