AC BTU Calculator
Estimate the cooling capacity a single room needs and see the result in BTU/h, tons and kW. The inputs are designed for ordinary users: measure the room, tell us about sunlight and people, and we do the rest.
Tell us about the room
Enter simple measurements. Each measurement shows its unit beside the number.
—
Enter your room details to calculate.
How to calculate AC BTU, formulas, worked example, limitations & FAQs
How to use the AC BTU calculator
- Measure the inside length and width of the room.
- Measure the ceiling height from the finished floor to the ceiling.
- Choose the sunlight condition that best describes the room.
- Enter the number of people normally in the room.
- Select kitchen if the air conditioner is cooling a kitchen space.
- Read the estimated load and the next standard equipment size.
What BTU means
BTU/h is a unit of cooling capacity: it describes how much heat an air conditioner can remove per hour. AC equipment is commonly sold by cooling capacity in BTU/h, while tonnage and kW are alternative ways to express capacity.
The ToolVault calculation
The first step uses the published ENERGY STAR room-air-conditioner capacity chart. That chart is based on an 8-foot ceiling and maps room floor area to a room-AC capacity. ToolVault then applies the published adjustments for shade/sun, occupancy and kitchen use. For ceilings higher than 8 ft, ToolVault applies a transparent planning factor equal to ceiling height ÷ 8 ft; this higher-ceiling factor is a ToolVault approximation because the ENERGY STAR chart itself only states that higher ceilings may require more capacity rather than prescribing this exact equation.
Worked example
Floor area = 15 × 12 = 180 ft².
The ENERGY STAR chart puts a 150–250 ft² room at 6,000 BTU/h for an 8 ft ceiling.
Sun factor = 1.00 and extra occupants = 0, kitchen = 0.
Estimated load = 6,000 BTU/h.
The next standard room-AC size from the chart is 6,000 BTU/h = 0.50 ton ≈ 1.76 kW of cooling capacity.
Why air-conditioner size should not be too large
A unit that is too large can cool the room quickly without running long enough to remove as much humidity. ENERGY STAR specifically warns that proper sizing matters for comfort and energy efficiency.
Accuracy and limitations
This calculator is intended for a single room, room air conditioner, window unit, portable unit or single-zone cooling planning. It does not model window area and orientation, insulation, air leakage, infiltration, floor/roof construction, adjacent-zone temperatures, duct losses, equipment performance curves, solar heat gain calculations or full internal heat loads.
For whole-home systems, large glazed spaces, commercial buildings or permit/design work, use a full HVAC load calculation appropriate to the project and local requirements.
ENERGY STAR reference chart
The room-area portion of this calculator follows the published ENERGY STAR room-air-conditioner chart: 100–150 ft² → 5,000 BTU/h; 150–250 → 6,000; 250–300 → 7,000; 300–350 → 8,000; 350–400 → 9,000; 400–450 → 10,000; 450–550 → 12,000; 550–700 → 14,000; 700–1,000 → 18,000; 1,000–1,200 → 21,000; 1,200–1,400 → 23,000; 1,400–1,500 → 24,000; 1,500–2,000 → 30,000; 2,000–2,500 → 34,000 BTU/h.
ENERGY STAR also recommends a 10% reduction for heavily shaded rooms, a 10% increase for very sunny rooms, 600 BTU/h for each regular occupant above two, and a 4,000 BTU/h increase when the unit is used in a kitchen.
Source: ENERGY STAR — Room Air Conditioners.
Frequently asked questions
How many BTUs does my room need?
Measure the room's floor area, then use the room-AC capacity chart and apply the room conditions. This calculator does those steps automatically.
How many BTUs is one ton of AC?
One ton of cooling is 12,000 BTU/h.
What size AC do I need for a 300 square foot room?
For an 8 ft ceiling, the ENERGY STAR chart assigns 7,000 BTU/h to a room up to 300 ft² and 8,000 BTU/h to the next 300–350 ft² band. Sun, occupancy and kitchen use can change the result.
Does ceiling height matter?
Yes. The published ENERGY STAR chart is based on an 8 ft ceiling and notes that higher ceilings may require more capacity. ToolVault makes its higher-ceiling approximation explicit rather than hiding it inside the result.
Can I use this for a whole house?
No. This is a room/zone sizing estimate. Whole-home HVAC sizing should use a detailed load calculation.