HVAC BTU Calculator
Estimate cooling BTU from room size, insulation, sun, occupants, and kitchen, plus heating BTU by climate zone.
Processed locally in your browser — nothing is uploaded.
HVAC BTU Calculator
+600 BTU per person beyond 2
Enter the room area to estimate BTU.
About HVAC BTU Calculator
The cooling estimate starts from the common 20 BTU per square foot rule, scales for ceiling height, insulation quality, and sun exposure, then adds 600 BTU per occupant beyond two and 4,000 BTU for a kitchen. Heating uses a climate-zone rate from 30 (hot climates) to 60 BTU/sq ft (very cold).
Common uses
- Pick a window or portable AC size for a bedroom
- Get a ballpark furnace or mini-split heating load by climate
Example
Input
300 sq ft, 8 ft ceiling, average insulation, 4 people, kitchen
Output
11,200 BTU/hr cooling · 12,000 BTU/hr heating (zone 4)
Limitations
- Rule-of-thumb sizing only — a Manual J calculation is the real standard for installed equipment
- Windows, air sealing, and ductwork are not modeled
Frequently asked questions
How many BTU do I need per square foot?
About 20 BTU/hr per square foot is the standard cooling rule of thumb this calculator starts from, before adjusting for ceiling height, insulation, and sun. Heating runs 30–60 BTU per square foot depending on climate zone — 30 in hot climates, 60 in very cold ones.
What size air conditioner do I need for a 300 square foot room?
Roughly 6,000 BTU/hr as a baseline (300 × 20), before adjustments. Poor insulation or strong sun pushes it up 10–20%, more than two occupants add 600 BTU each, and a kitchen adds 4,000 BTU for appliance heat.
What is a Manual J calculation?
The ACCA's room-by-room load calculation standard that HVAC contractors use to size installed equipment, accounting for windows, orientation, air leakage, and ductwork. This calculator's rule-of-thumb estimate is fine for window units and ballparks, but a furnace or central AC purchase deserves a Manual J.
Do high ceilings change the BTU requirement?
Yes — there's more air volume to condition, so this calculator scales the load by your ceiling height relative to a standard 8 ft. A 10 ft ceiling raises the estimate by 25%.
Is my home's information transmitted when I size an AC?
No. The BTU estimate is computed in your browser from the numbers you enter — nothing about your home is sent anywhere or retained.