BTU and AC Size Calculator
Estimate the BTUs and air conditioner tonnage a room or home needs from its size, sun, shade and occupants, as a rule of thumb before a load calculation. Room sizes use the ENERGY STAR chart; a central system needs a Manual J load calculation by a contractor.
Your numbers
The chart assumes two. Each extra person adds 600 BTU.
Adds 4,000 BTU.
Air conditioner size
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- In tons
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- Next size up in the chart
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- Chart capacity for 0 sq ft
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- Adjustments
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A rule of thumb for a room unit from the ENERGY STAR chart, which assumes an 8-foot ceiling. Higher ceilings may need more capacity.
For a central system or a whole home, the size comes from a Manual J load calculation, not from this chart.
The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.
The ENERGY STAR sizing chart
ENERGY STAR's sizing chart for room air conditioners, with tons worked out at 12,000 BTU per hour to the ton. "100 to 150" means at least 100 and under 150 sq ft. The capacities assume an 8-foot ceiling.
| Area to cool | BTU per hour | Tons |
|---|---|---|
| 100 to 150 sq ft | 5,000 | 0.42 |
| 150 to 250 sq ft | 6,000 | 0.5 |
| 250 to 300 sq ft | 7,000 | 0.58 |
| 300 to 350 sq ft | 8,000 | 0.67 |
| 350 to 400 sq ft | 9,000 | 0.75 |
| 400 to 450 sq ft | 10,000 | 0.83 |
| 450 to 550 sq ft | 12,000 | 1 |
| 550 to 700 sq ft | 14,000 | 1.17 |
| 700 to 1,000 sq ft | 18,000 | 1.5 |
| 1,000 to 1,200 sq ft | 21,000 | 1.75 |
| 1,200 to 1,400 sq ft | 23,000 | 1.92 |
| 1,400 to 1,500 sq ft | 24,000 | 2 |
| 1,500 to 2,000 sq ft | 30,000 | 2.5 |
| 2,000 to 2,500 sq ft | 34,000 | 2.83 |
How the adjustments work
ENERGY STAR adjusts the chart for four things: a heavily shaded room needs 10% less, a very sunny room 10% more, each person over two who regularly uses the room adds 600 BTU, and a kitchen adds 4,000 BTU. ENERGY STAR does not say in which order to apply them; this calculator takes the 10% off or on the chart capacity first, then adds people and the kitchen.
A 12 × 20 ft bedroom is 240 sq ft, which the chart puts at 6,000 BTU per hour. If it is very sunny, add 10%: 6,600. With three people using it, add 600: 7,200. The next size up in the chart is 8,000 BTU, so that is the unit to look at.
The chart has no factor for climate. A very sunny room is the one heat adjustment it makes; a Manual J load calculation is where the local design temperatures come in.
BTU to tons
Central systems are sold in tons. One ton of cooling is 12,000 BTU per hour, so tons = BTU ÷ 12,000.
| Tons | BTU per hour |
|---|---|
| 1.5 | 18,000 |
| 2 | 24,000 |
| 2.5 | 30,000 |
| 3 | 36,000 |
| 3.5 | 42,000 |
| 4 | 48,000 |
| 5 | 60,000 |
Why bigger is not better
An oversized air conditioner cools the air quickly and shuts off before it has removed the moisture. ENERGY STAR puts it plainly: the room ends up feeling damp and clammy, and the bigger unit costs more to buy and wastes energy without cooling any better.
For whole-house systems the U.S. Department of Energy warns that rules of thumb too often lead to oversized systems, with more cost, wasted energy and too-frequent on-and-off cycling that hurts comfort and efficiency.
Room unit or central system: where Manual J comes in
For a window, wall or portable unit in one room, the ENERGY STAR chart and its adjustments are a fair guide, and that is what this calculator does.
For a central air conditioner or heat pump, the size should come from a load calculation. ACCA's Manual J is the method contractors use: it works room by room from the house itself and uses the design temperatures for your location. This page is not a substitute for that calculation; use it to get a first number and to know what to ask the contractor.
For HVAC reps: what to write down on the visit
The tonnage comes from the load calculation. The sale comes from what the homeowner says: which rooms are hot, what the summer bills look like, how old the current system is, what they want to spend and who else decides. Write it down while you are in the house, and read it back before you send the quote.
Sources, checked September 2026:
More for HVAC and home-services reps: HVAC sales training, HVAC sales, Door-to-door sales.
Size the unit, then close the sale
Sizing is the easy part of an HVAC sale. What the homeowner said about hot rooms, bills and budget is what closes it: record the visit with ParrotNotes and get the summary and next steps.
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