Find the recommended BTU rating for an air conditioner or heater based on your room's size, ceiling height, sun exposure, and occupants.
Enter your room details and click Calculate BTU to see the recommended rating
This free BTU calculator estimates the recommended cooling (or heating) capacity for a room, measured in British Thermal Units (BTU) per hour, based on the room's square footage, ceiling height, sun exposure, number of occupants, and whether it's a kitchen. It's built to help you avoid the two most common sizing mistakes — buying an air conditioner that's too small to keep up, or one that's oversized and cycles on and off without properly removing humidity.
The calculator starts with a widely used baseline of 20 BTU per square foot of floor area for a standard 8-foot ceiling, then adjusts that baseline for your specific room: it scales up for taller ceilings (since a taller room has more air volume to cool), applies a 10% reduction for shady rooms or a 10% increase for sunny rooms, adds 600 BTU for each occupant beyond the first two, and adds 4,000 BTU if the room is a kitchen. The final figure is rounded up to the nearest 500 BTU and matched against a table of standard AC unit sizes.
Homeowners and renters shopping for a window, portable, or single-zone mini-split air conditioner; anyone sizing a space heater for a specific room; and DIYers who want a quick sanity check before buying HVAC equipment. It's a planning tool for single rooms and small spaces, not a substitute for a professional Manual J load calculation on a whole-house central HVAC system.
The exact adjustments this calculator applies to the baseline square-footage estimate
Floor area is the strongest single predictor of cooling load for a typical residential room, which is why it's used as the starting baseline before finer adjustments.
Scaling by ceiling height ÷ 8 approximates the room's actual air volume, since a 10-foot ceiling holds 25% more air than an 8-foot ceiling over the same floor area.
Sun exposure, occupants, and kitchen appliances are all extra heat sources beyond the room's basic size, which is why each gets its own adjustment.
From measuring your room to reading your recommended BTU rating
Type your room's length and width in feet to get its floor area.
Enter the ceiling height in feet — the default is 8 feet, a standard residential ceiling.
Select Shady, Average, or Sunny based on how much direct sunlight the room gets during the day.
Enter how many people typically use the room and check the box if it's a kitchen.
See your recommended BTU rating, room area, room volume, and the closest standard AC unit size.
A standard bedroom and a sunny kitchen, calculated step by step
A 20 ft × 15 ft bedroom, 8-foot ceiling, average sun exposure, 2 occupants, not a kitchen.
A 25 ft × 20 ft open kitchen with a 10-foot ceiling, sunny exposure, 4 occupants, and it is a kitchen.
A widely used reference chart for room air conditioner sizing (8 ft ceiling, average conditions)
| Room Area | Recommended BTU |
|---|---|
| 100–150 sq ft | 5,000 BTU |
| 150–250 sq ft | 6,000 BTU |
| 250–300 sq ft | 7,000 BTU |
| 300–350 sq ft | 8,000 BTU |
| 350–400 sq ft | 9,000 BTU |
| 400–450 sq ft | 10,000 BTU |
| 450–550 sq ft | 12,000 BTU |
| 550–700 sq ft | 14,000 BTU |
| 700–1,000 sq ft | 18,000 BTU |
| 1,000–1,200 sq ft | 21,000 BTU |
| 1,200–1,400 sq ft | 23,000 BTU |
This chart reflects the same 20 BTU per square foot baseline this calculator uses, and is a commonly published starting point in HVAC buying guides. Your calculated value above already factors in your specific ceiling height, sun exposure, occupants, and kitchen use, so it may land between two rows of this simplified chart — round up to the next standard unit size available from the manufacturer.
This calculator provides a planning estimate using a widely used square-footage-based method. It is not a substitute for a professional Manual J load calculation, especially for whole-house central HVAC systems or rooms with unusual construction.
Where getting the BTU rating right saves money and comfort
Find the right BTU rating before buying a window unit for a bedroom, office, or apartment.
Size a portable air conditioner for a room without existing central air.
Get a starting BTU estimate for a single-zone ductless mini-split installation.
Use the same square-footage approach as a starting point for sizing a room heater.
Check whether a home office with extra electronics needs a bigger unit than the room size alone suggests.
Account for cooking-appliance heat load when sizing cooling for a kitchen or combined kitchen-living area.
Quickly estimate the right unit size for a rental property room without a professional assessment.
Double-check a salesperson's or online listing's BTU recommendation against your own room's specifics.
What this BTU calculator does well, and where a professional assessment is still worthwhile
Get a more accurate BTU estimate and avoid common AC-buying pitfalls
Summary: This BTU calculator turns your room's size, ceiling height, sun exposure, occupancy, and kitchen use into a recommended BTU rating, matched against a standard AC unit size chart — a fast, free planning tool for window, portable, and single-zone mini-split units. Pair it with the Electricity Bill Calculator to estimate what running your new unit will cost.
Common questions about sizing an air conditioner or heater
Official guidance to complement this calculator — not a substitute for a licensed HVAC contractor
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