Choosing the right Air Conditioner size is critical for both comfort and energy efficiency. An AC unit that is too small won't cool the room effectively, while one that is too large will cycle on and off too frequently, leading to poor dehumidification and high electricity bills.
The Basic BTU Formula
A "BTU" (British Thermal Unit) is a measure of heat energy. In the context of air conditioning, it describes how much heat the unit can remove from a room per hour. The baseline calculation is typically 25 BTUs per square foot of living space.
Step 1: Measure the length and width of the room in feet.
Step 2: Multiply Length x Width to get the Square Footage.
Step 3: Multiply Square Footage by 25.
Critical Adjustment Factors
Real-world conditions require adjustments to the baseline calculation:
Sunlight: If the room is very sunny, increase capacity by 10%. If it is heavily shaded, reduce it by 10%.
Occupancy: The human body generates heat. If more than two people regularly occupy the room, add 600 BTUs for each additional person.
Kitchens: Cooking appliances generate massive heat. If the unit is for a kitchen, add an extra 4,000 BTUs to the total.
Ceiling Height: Standard calculations assume an 8-foot ceiling. If your ceilings are 10 feet or higher, the volume of air increases, requiring more cooling power.
Understanding AC Tonnage
In many regions, AC units are sold by "Tons." One ton of cooling capacity is equal to 12,000 BTUs per hour. For example, a 1.5-ton AC unit provides 18,000 BTUs of cooling capacity.
function calculateAC() {
var length = parseFloat(document.getElementById('roomLength').value);
var width = parseFloat(document.getElementById('roomWidth').value);
var height = parseFloat(document.getElementById('ceilingHeight').value);
var occupants = parseInt(document.getElementById('occupants').value);
var sunlight = document.getElementById('sunlight').value;
var isKitchen = document.getElementById('isKitchen').value;
if (isNaN(length) || isNaN(width) || isNaN(height) || length <= 0 || width 8) {
var heightFactor = height / 8;
btu = btu * heightFactor;
}
// 3. Adjust for Sunlight
if (sunlight === "shady") {
btu = btu * 0.90;
} else if (sunlight === "sunny") {
btu = btu * 1.10;
}
// 4. Adjust for Occupants (Add 600 BTU per person over 2)
if (occupants > 2) {
var extraPeople = occupants – 2;
btu += (extraPeople * 600);
}
// 5. Adjust for Kitchen
if (isKitchen === "yes") {
btu += 4000;
}
// Round for display
var finalBTU = Math.round(btu);
var tonnage = (finalBTU / 12000).toFixed(2);
// Display Results
document.getElementById('btuOutput').innerHTML = finalBTU.toLocaleString();
document.getElementById('tonOutput').innerHTML = tonnage;
var recText = "Based on your inputs, a " + tonnage + " ton unit is the mathematical requirement. ";
if (tonnage <= 0.75) {
recText += "We recommend looking at a 0.75 Ton or 9,000 BTU unit.";
} else if (tonnage <= 1.0) {
recText += "We recommend a 1.0 Ton or 12,000 BTU unit.";
} else if (tonnage <= 1.5) {
recText += "We recommend a 1.5 Ton or 18,000 BTU unit.";
} else if (tonnage <= 2.0) {
recText += "We recommend a 2.0 Ton or 24,000 BTU unit.";
} else {
recText += "For this size, you may need multiple units or a heavy-duty central cooling system.";
}
document.getElementById('recommendationText').innerHTML = recText;
document.getElementById('acResult').style.display = 'block';
}