Note: This is an estimate based on standard BTU formulas. Always consult a licensed HVAC professional for a Manual J Load Calculation before purchasing equipment.
How to Calculate the Right AC Unit Size
Choosing the correct size for your air conditioning system is critical for both comfort and energy efficiency. An undersized unit will run constantly without cooling your home effectively, while an oversized unit will "short cycle," turning on and off too quickly, which leads to high humidity and premature wear on the compressor.
The standard unit of measurement for cooling capacity is the BTU (British Thermal Unit). In the HVAC world, 12,000 BTUs is equal to "one ton" of cooling capacity.
The BTU Sizing Formula Used in This Calculator
Our calculator uses a modified residential load formula to provide a realistic estimate:
Base Load: Square footage multiplied by 20 BTU per square foot.
Ceiling Adjustment: If ceilings are higher than 8 feet, the load increases by 10% for every additional foot of height.
Insulation Factor: We multiply the total by a factor based on how well your home retains air.
Window & Occupant Load: Each window adds approximately 1,000 BTUs of heat gain, and each person adds roughly 400 BTUs.
The Kitchen Factor: Cooking appliances generate significant heat; we add 4,000 BTUs if a kitchen is included in the cooled area.
HVAC Tonnage Reference Table
Home Size (Sq Ft)
Required BTUs
Tons
600 – 1,000
18,000
1.5 Tons
1,000 – 1,500
24,000
2.0 Tons
1,500 – 2,000
30,000
2.5 Tons
2,000 – 2,500
36,000
3.0 Tons
2,500 – 3,000
42,000
3.5 Tons
3,000 – 3,500
48,000
4.0 Tons
Why an Accurate Calculation Matters
If you install a 5-ton unit in a home that only needs a 3-ton unit, the air will cool so fast that the thermostat will shut the unit off before it has a chance to pull moisture (humidity) out of the air. This results in a "clammy" environment and potential mold growth. Conversely, a unit that is too small will rack up massive electricity bills as it struggles to reach the target temperature on a hot afternoon.
function calculateHVAC() {
var sqFt = parseFloat(document.getElementById("sqFootage").value);
var ceilingHeight = parseFloat(document.getElementById("ceilingHeight").value);
var insulation = parseFloat(document.getElementById("insulationQuality").value);
var windows = parseFloat(document.getElementById("windowCount").value);
var occupants = parseFloat(document.getElementById("occupants").value);
var kitchen = parseFloat(document.getElementById("kitchenPresence").value);
if (isNaN(sqFt) || sqFt 8) {
var heightExtra = (ceilingHeight – 8) * 0.10;
baseBTU = baseBTU * (1 + heightExtra);
}
// 3. Add Window Load (Approx 1000 BTU per window)
var windowLoad = windows * 1000;
// 4. Add Occupant Load (Approx 400 BTU per person)
var personLoad = occupants * 400;
// 5. Calculate Total and apply Insulation Multiplier
var subTotal = (baseBTU + windowLoad + personLoad + kitchen) * insulation;
// Final BTU result
var finalBTU = Math.round(subTotal);
// Calculate Tonnage (12,000 BTUs = 1 Ton)
var tonnage = (finalBTU / 12000).toFixed(2);
// Round to nearest 0.5 ton for commercial availability
var suggestedTonnage = Math.ceil(tonnage * 2) / 2;
// Display results
document.getElementById("btu-output").innerHTML = "Estimated Load: " + finalBTU.toLocaleString() + " BTUs";
document.getElementById("tonnage-output").innerHTML = "Recommended Unit Size: " + suggestedTonnage + " Tons (" + tonnage + " exact)";
document.getElementById("hvac-result").style.display = "block";
}