HVAC System BTU Sizing Calculator

Estimate the heating and cooling BTU capacity required for your space using the Manual J load calculation methodology.

Room Length (ft) Room Width (ft) Ceiling Height (ft) Total Window Area (sq ft) Number of Exterior Doors

Climate Zone Sun Exposure Floor Type Number of Stories

Insulation Quality Window Type Number of Occupants Kitchen Present?

Cooling / Heating Conduction Load (BTU/hr):

Q_cond = A × U × ΔT

Where A = surface area (ft²), U = overall heat transfer coefficient (BTU/hr·ft²·°F), ΔT = design temperature difference (°F).

Solar Heat Gain (cooling only):

Q_solar = A_window × SHGC × I_solar × SunMultiplier

Where SHGC = Solar Heat Gain Coefficient, I_solar = 150 BTU/hr·ft² (average peak irradiance).

Infiltration Load (BTU/hr):

Q_inf = Volume × ACH × 0.018 × ΔT

Where 0.018 BTU/ft³·°F is the volumetric heat capacity of air at standard conditions, ACH = air changes per hour.

Internal Gains (cooling):

Q_internal = (Occupants × 250) + Kitchen + (Floor Area × 3)

250 BTU/hr per person (sensible), 1,200 BTU/hr for kitchen, 3 BTU/hr·ft² for lighting.

Total Cooling Load:

Q_cool = (Q_cond + Q_solar + Q_inf + Q_internal) × ClimateFactor × StoryFactor

Tonnage:

Tons = BTU/hr ÷ 12,000

1 ton of refrigeration = 12,000 BTU/hr. Equipment is rounded up to the nearest 0.5 ton.

Based on ACCA Manual J (8th Edition) simplified residential load calculation methodology.
Design temperature differences (ΔT) are representative values per ASHRAE climate zone; actual values vary by specific location.
Wall U-values range from 0.04 (excellent, ~R-25) to 0.20 (poor, ~R-5); ceiling from 0.022 (R-45) to 0.15 (R-7).
Window U-values: single pane 1.10, double pane 0.48, triple/Low-E 0.22 (per NFRC standards).
SHGC values: single 0.86, double 0.57, triple/Low-E 0.32.
Infiltration ACH: 0.25 (excellent/tight) to 1.0 (poor/leaky), per ASHRAE 62.2 guidance.
Occupant sensible heat gain: 250 BTU/hr per person (ASHRAE Fundamentals Handbook).
Solar irradiance average: 150 BTU/hr·ft² (representative mid-latitude peak average).
Exterior door area assumed 20 ft² each (standard 3 ft × 6 ft 8 in door).
This calculator does not account for duct losses, humidity (latent) loads, or local code requirements.
Always consult a licensed HVAC engineer for final equipment selection and Manual J compliance.
References: ACCA Manual J, ASHRAE Handbook of Fundamentals, ENERGY STAR HVAC guidelines.

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