HVAC BTU Calculator Formula

Understand the math behind the hvac btu calculator. Each variable explained with a worked example.

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Formulas Used

Recommended BTU

total_btu = ceil((base_btu * ceiling_factor * sun_exposure + occupant_btu) / 1000) * 1000

AC Tonnage

tonnage = total_btu / 12000

Variables

VariableDescriptionDefault
room_sqftRoom Size(sq ft)300
ceiling_heightCeiling Height(ft)8
sun_exposureSun Exposure1
num_occupantsTypical Occupants2
base_btuDerived value= room_sqft * 20calculated
ceiling_factorDerived value= ceiling_height > 8 ? 1 + (ceiling_height - 8) * 0.05 : 1calculated
occupant_btuDerived value= (num_occupants - 2) * 600calculated

How It Works

BTU Calculation

Base BTU = Square Footage × 20

Adjustments:

  • High ceilings: +5% per foot above 8ft
  • Sun exposure: ±10%
  • Extra occupants: +600 BTU each beyond 2
  • Kitchen: add 4,000 BTU

    • 1 ton of AC = 12,000 BTU

    Worked Example

    300 sq ft room, 8ft ceiling, average sun, 2 occupants.

    room_sqft = 300ceiling_height = 8sun_exposure = 1num_occupants = 2
    1. 01Base BTU = 300 × 20 = 6,000
    2. 02Ceiling factor = 1.0 (standard height)
    3. 03Sun factor = 1.0 (average)
    4. 04Occupant BTU = 0 (2 is baseline)
    5. 05Total = 6,000 BTU
    6. 06Tonnage = 6,000 / 12,000 = 0.5 ton

    Frequently Asked Questions

    What if I have a kitchen?

    Add 4,000 BTU for a kitchen due to heat from cooking appliances.

    What about insulation?

    Poorly insulated rooms may need 20-30% more BTU. Well-insulated rooms may need 10% less.

    Learn More

    Guide

    Understanding HVAC Sizing: How to Calculate Heating and Cooling Loads

    Learn how to size HVAC systems using Manual J load calculations. Covers BTU requirements, climate factors, insulation values, ductwork sizing, and equipment selection.

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