Air Conditioner BTU Calculator
Size a window or portable AC for one room — area, ceilings, sun, people and kitchen — using the EnergyStar rule of thumb, with the matching off-the-shelf unit, running cost, and a downloadable PDF.
Based on the DOE / EnergyStar room guideline of ~20 BTU per sq ft with adjustments. Great for room AC; a Manual J is what sizes central systems. For planning purposes only.
| EER | Power (W) | kWh / mo | Cost / mo |
|---|
For a 8,000 BTU unit at 8 hr/day. A higher EER pays back over the years.
How to use the AC BTU calculator
Four quick steps turn a room’s size and conditions into the right window or portable AC — the standard unit size, calculated load, power draw, and running cost — with a downloadable PDF and an efficiency comparison.
Enter the room
Type length and width, or the known area, plus ceiling height and how many people use the room.
Set the conditions
Pick sun exposure and flag whether the room is a kitchen — the two biggest adjustments.
Add efficiency & rate
Enter the unit’s EER, your electricity rate, and daily run hours to estimate the cost.
Read results & save a PDF
Get the recommended BTU, wattage, monthly cost, and a full PDF with an EER comparison.
How the BTU size is calculated
The size comes from the EnergyStar room guideline — about 20 BTU per square foot — scaled for ceiling height and sun, then topped up for extra people and a kitchen. From there it rounds to a real off-the-shelf unit and works out the running cost. Here is the exact math.
Worked example — a 22 × 16 ft room (352 sq ft), 8-ft ceilings, average sun, 2 people
Base load: 352 × 20 = 7,040 BTU
Adjusted: 7,040 × 1.00 (ceiling) × 1.00 (sun) = 7,040 BTU
Add-ons: 0 (2 people) + 0 (no kitchen) = 0
Calculated load: 7,040 BTU → rounds up to 8,000 BTU unit
Power: 8,000 ÷ 11 EER ≈ 730 W, Cost: 730 W × 8 hr × 30 × $0.16 ≈ $28/mo
An 8,000 BTU unit covering ~400 sq ft, drawing about 730 W, ~175 kWh and roughly $28 a month at 8 hr/day.
AC sizing charts
Handy lookups for the questions people ask most — BTU by room size, the EnergyStar coverage guideline, the adjustments that move the load, and what efficiency does to your bill. Use them to sanity-check the unit you’re eyeing.
| Room area | Base load (x20) | Standard unit |
|---|---|---|
| 100–250 sq ft | 2,000–5,000 | 5,000 BTU |
| 300 sq ft | 6,000 | 6,000 BTU |
| 350 sq ft | 7,000 | 8,000 BTU |
| 400 sq ft | 8,000 | 8,000 BTU |
| 450 sq ft | 9,000 | 10,000 BTU |
| 550 sq ft | 11,000 | 12,000 BTU |
| 700 sq ft | 14,000 | 14,000 BTU |
Average room, 8-ft ceilings, 2 people, no kitchen. Sun, tall ceilings, extra occupants, and a kitchen all push the load — and often the unit size — higher.
| Unit size | Approx. coverage | Typical room |
|---|---|---|
| 5,000 BTU | ~250 sq ft | Bedroom, small office |
| 6,000 BTU | ~300 sq ft | Bedroom, den |
| 8,000 BTU | ~400 sq ft | Living room, large bedroom |
| 10,000 BTU | ~500 sq ft | Open living area |
| 12,000 BTU | ~600 sq ft | Great room, studio |
| 14,000 BTU | ~700 sq ft | Large open space |
Coverage is the unit’s BTU divided by 20. It shrinks in bright, tall, or crowded rooms, so treat these as an upper bound for average conditions.
| Condition | Setting | Effect on load |
|---|---|---|
| Sun exposure | Shaded · Average · Sunny | −10% · no change · +10% |
| Ceiling height | Per foot vs. 8 ft | Scales by ceiling ÷ 8 |
| Occupants | Each person over 2 | +600 BTU |
| Kitchen | Room is a kitchen | +4,000 BTU |
Sun and ceiling scale the base load; occupants and the kitchen add on top. These follow the DOE / EnergyStar room-AC guidance.
| EER | Power (8,000 BTU) | kWh / mo | Cost / mo |
|---|---|---|---|
| EER 8 | 1,000 W | 240 | $38 |
| EER 10 | 800 W | 192 | $31 |
| EER 12 | 667 W | 160 | $26 |
| EER 14 | 571 W | 137 | $22 |
An 8,000 BTU unit at 8 hr/day and $0.16/kWh. A higher EER cuts the same cooling’s power draw, so the pricier efficient unit often pays back over a few seasons.
area × 20
sun · people · kitchen
BTU ÷ EER
The size comes from one room load — floor area sets the base, conditions adjust it, and efficiency turns the chosen unit into a monthly bill.
- Room — floor area times 20 BTU per sq ft sets the base load.
- Adjust — sun and ceiling scale it; extra people and a kitchen add on top.
- Unit & cost — round up to a standard size, then divide by EER for the wattage and bill.
Everything the calculator works out
One room profile gives you a complete picture — the unit to buy, the load behind it, and what it will cost to run day to day and month to month.
Key figures every room AC depends on
Built for any room you need to cool
From a bedroom window unit to a sunny home office, the same calculator turns the room into the right BTU size and a clear running cost.
Shopping for a window or portable AC and wants the right size without over- or under-buying.
- Measure the room and set the sun
- Match the BTU to a shelf unit
- Check the monthly cost before buying
Already has a unit or two and wants to know what they cost to run and where to save.
- Enter EER and run hours
- Compare cost across EER ratings
- Trim hours or upgrade efficiency
Spec’ing room ACs across bedrooms and studios and needs a fast, consistent size per room.
- Size each room the same way
- Standardize on a few BTU sizes
- Save PDFs for the file
7 tips for sizing a room AC
Small habits keep your unit efficient, your room comfortable, and your electric bill in check.
AC BTU calculator FAQ
The sizing, efficiency, and running-cost questions people ask most before buying a window or portable air conditioner.
Start with the DOE / EnergyStar rule of thumb: about 20 BTU per square foot of room. Multiply your room’s length by width to get the area, then multiply by 20. Adjust for tall ceilings, sun exposure, extra occupants, and whether the room is a kitchen, then round up to the nearest standard AC size.
For example, a 350 sq ft room needs about 7,000 BTU (350 × 20), which rounds up to a standard 8,000 BTU unit. A very sunny room adds roughly 10%, and a kitchen adds 4,000 BTU.
A 300 sq ft room needs about 6,000 BTU under the 20-BTU-per-sq-ft rule, which matches a standard 6,000 BTU window or portable unit. That covers an average room with two people and 8-foot ceilings.
If the room is very sunny, west-facing, or has a kitchen, the load climbs and you may need an 8,000 to 12,000 BTU unit instead. Add 600 BTU for each person over two and 4,000 BTU for a kitchen before choosing the size.
An oversized room AC cools the air quickly but shuts off before it removes enough humidity, leaving the room cold and clammy. Those short on-off cycles — called short-cycling — also waste energy and wear the unit out faster.
Bigger is not better. A right-sized unit runs longer, steadier cycles that pull moisture from the air and hold a comfortable temperature, which is why the rule of thumb is meant to match the room rather than overshoot it.
EER (Energy Efficiency Ratio) is the cooling output in BTU per watt of power. A unit’s power draw equals its BTU rating divided by its EER, so a higher EER means less electricity for the same cooling. CEER is a newer version that also counts standby power.
For example, an 8,000 BTU unit at EER 10 draws about 800 watts, while the same cooling at EER 14 draws about 570 watts. Over a season of daily use, that gap adds up on your electric bill, which is why the higher-EER unit often pays back its price premium.
Running cost depends on the unit’s wattage, how many hours it runs, and your electricity rate. Divide the BTU by the EER to get watts, multiply by hours to get watt-hours, divide by 1,000 for kWh, then multiply by your rate per kWh.
For example, an 8,000 BTU unit at EER 11 draws about 730 watts. Running it 8 hours a day at 16 cents per kWh costs roughly $28 a month. Higher efficiency, fewer run hours, and a cheaper rate all lower that figure.
No. This calculator uses the EnergyStar room guideline, which is designed for sizing a window or portable unit for a single room. Central and whole-home systems need an ACCA Manual J load calculation that accounts for the whole envelope, ductwork, orientation, and local design temperatures.
Use this tool to pick a room AC and estimate its running cost. For central air or a heat pump, have an HVAC professional run a Manual J before buying or installing anything.
Because the rule of thumb is about 20 BTU per square foot, each standard unit covers roughly its BTU rating divided by 20. A 5,000 BTU unit suits about 250 sq ft, an 8,000 BTU unit about 400 sq ft, and a 12,000 BTU unit about 600 sq ft.
Sun, ceiling height, occupants, and a kitchen all shrink that coverage, so a bright, crowded, or tall room needs a larger unit than its floor area alone suggests.
Yes. The EnergyStar guideline adjusts the base load for real conditions. Increase capacity about 10% for a very sunny or west-facing room and reduce it about 10% for a heavily shaded one. Add 600 BTU for each person over two who regularly occupies the room.
If the unit cools a kitchen, add 4,000 BTU to cover the heat from cooking. Tall ceilings also raise the load, since the unit conditions more air volume than the floor area alone implies.
General Estimating Notice: This calculator provides a simplified room-AC estimate — recommended BTU, power draw, and running cost — using the DOE / EnergyStar guideline of roughly 20 BTU per square foot with adjustments for ceiling height, sun, occupants, and kitchens. It is intended for sizing window and portable units in a single room, not central or whole-home systems, which require an ACCA Manual J load calculation. Oversizing hurts comfort and humidity control, so aim to match the room rather than overshoot it. Running costs are estimates that vary with weather, usage, and local electricity rates. This is a planning aid only — confirm equipment selection with a qualified professional where appropriate. For planning purposes only.