AC Size Calculator

AC Size is evaluated from Room or House Area, Ceiling Height and Room Conditions. The calculation reports BTU/hr Needed, Tons Needed and Recommended Unit Size.

Results

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About the AC Size Calculator

AC Size is treated here as a quantitative relation between Room or House Area, Ceiling Height, Room Conditions and Number of Occupants in Room and BTU/hr Needed, Tons Needed, Recommended Unit Size and Room AC Recommendation.

The calculator uses a multi formula configuration. Each reported value is read as a direct evaluation of the stored rules with the declared field formats and units.

Formula basis:
BTU needed = floor area x BTU per sq ft factor x room condition adjustment + occupant load adjustment
Tons = BTU / 12,000
Always round UP to the nearest standard equipment size - but avoid grossly oversizing, as an AC that's too large will cool too quickly without properly dehumidifying the space (short-cycling).
For best humidity control in hot/humid climates: select equipment no larger than 15% above the calculated load.

Interpret the outputs in the order shown by the result fields. Optional inputs affect only the outputs that depend on those variables.

Formula & How It Works

The calculation applies the following relations exactly as recorded in the metadata:

BTU needed = floor area x BTU per sq ft factor x room condition adjustment + occupant load adjustment
Tons = BTU / 12,000
Always round UP to the nearest standard equipment size - but avoid grossly oversizing, as an AC that's too large will cool too quickly without properly dehumidifying the space (short-cycling).
For best humidity control in hot/humid climates: select equipment no larger than 15% above the calculated load.

Each output field is produced by substituting the supplied inputs into the relevant relation and then applying the declared rounding or text format.

Worked Examples

Example 1: Master bedroom: 250 sq ft, hot/humid climate, sunny west-facing

Inputs

room_area: 250 ceiling_ht: 8 room_type: Sunny room (south/west facing windows) occupants: 2 climate: Hot/Humid (South, Southeast)
BTU/hr Needed: 6,900 BTU/hr. Tons Needed: 0.6 tons. Recommended Unit Size: 8,000 BTU/hr. Room AC Recommendation: 6,900

With Room or House Area = 250, Ceiling Height = 8, Room Conditions = Sunny room (south/west facing windows) and Number of Occupants in Room = 2 as the stated inputs, the result is BTU/hr Needed = 6,900 BTU/hr, Tons Needed = 0.6 tons and Recommended Unit Size = 8,000 BTU/hr. Each value corresponds to the declared output fields.

Example 2: Open living/kitchen: 600 sq ft combined, hot/humid, kitchen heat

Inputs

room_area: 600 ceiling_ht: 9 room_type: Kitchen (add for cooking heat) occupants: 3 climate: Hot/Humid (South, Southeast)
BTU/hr Needed: 19,800 BTU/hr. Tons Needed: 1.6 tons. Recommended Unit Size: 21,000 BTU/hr. Room AC Recommendation: 19,800

With Room or House Area = 600, Ceiling Height = 9, Room Conditions = Kitchen (add for cooking heat) and Number of Occupants in Room = 3 as the stated inputs, the result is BTU/hr Needed = 19,800 BTU/hr, Tons Needed = 1.6 tons and Recommended Unit Size = 21,000 BTU/hr. Each value corresponds to the declared output fields.

Example 3: Small apartment: 450 sq ft, moderate climate (San Diego), normal exposure

Inputs

room_area: 450 ceiling_ht: 8 room_type: Normal bedroom/living room occupants: 2 climate: Moderate (Pacific Coast, Mountain)
BTU/hr Needed: 8,100 BTU/hr. Tons Needed: 0.7 tons. Recommended Unit Size: 10,000 BTU/hr. Room AC Recommendation: 8,100

With Room or House Area = 450, Ceiling Height = 8, Room Conditions = Normal bedroom/living room and Number of Occupants in Room = 2 as the stated inputs, the result is BTU/hr Needed = 8,100 BTU/hr, Tons Needed = 0.7 tons and Recommended Unit Size = 10,000 BTU/hr. Each value corresponds to the declared output fields.

Example 4: Whole house: 2,200 sq ft, cool/humid (Boston), basement included

Inputs

room_area: 2200 ceiling_ht: 8 room_type: Basement (usually cooler) occupants: 4 climate: Cool/Humid (Northeast, Midwest)
BTU/hr Needed: 38,600 BTU/hr. Tons Needed: 3.2 tons. Recommended Unit Size: 42,000 BTU/hr. Room AC Recommendation: 38,600

With Room or House Area = 2,200, Ceiling Height = 8, Room Conditions = Basement (usually cooler) and Number of Occupants in Room = 4 as the stated inputs, the result is BTU/hr Needed = 38,600 BTU/hr, Tons Needed = 3.2 tons and Recommended Unit Size = 42,000 BTU/hr. Each value corresponds to the declared output fields.

Common Use Cases

  • Determine AC size for a bedroom or room air conditioner
  • Size a central air conditioner for a house
  • Compare window AC options for cooling a specific room