Low (Shaded rooms, few windows)
Medium (Average exposure)
High (Direct sun, large windows)
Understanding Air Conditioning Sizing (BTU)
Choosing the right size air conditioner for a room or a home is crucial for both comfort and energy efficiency. An AC unit that is too small will struggle to cool the space effectively, running constantly and consuming excessive energy. Conversely, an AC unit that is too large will cool the space too quickly without adequately removing humidity, leading to a cold, clammy feeling and short-cycling, which is also inefficient and can cause premature wear.
The cooling capacity of an air conditioner is measured in British Thermal Units (BTU). BTU represents the amount of heat an AC unit can remove from a room per hour. The following calculator uses a common set of factors to estimate the BTU requirement for a specific space.
Factors Influencing AC Size:
Room Size (Square Footage): This is the primary factor. Larger rooms require more cooling capacity.
Ceiling Height: Taller ceilings mean a larger volume of air to cool, increasing the BTU need.
Windows and Sun Exposure: Windows, especially those facing south or west, allow solar heat to enter. The more windows and the more direct sunlight they receive, the higher the BTU requirement.
Occupancy: Each person in a room adds body heat, increasing the cooling load.
Heat-Generating Appliances: Electronics like TVs, computers, and even lamps generate heat, contributing to the overall cooling demand.
Insulation and Climate: While not directly inputted into this simplified calculator, factors like the quality of insulation, climate zone, and whether the space is on a top floor or gets a lot of ambient heat are important considerations for professional sizing.
The Calculation Logic:
This calculator uses a simplified approach to estimate BTU. The general formula starts with a base BTU per square foot and then adjusts for other factors.
Base BTU Calculation:
A common starting point is 20 BTU per square foot.
Base BTU = Square Footage × 20
Adjustments:
Ceiling Height Adjustment: For every foot of ceiling height above 8 feet, we add approximately 10% to the Base BTU.
Window Adjustment: For each window, a certain amount of BTU is added, varying slightly by sun exposure. A rough estimate might be 1000 BTU per window, increased for high sun exposure.
Sun Exposure: High sun exposure can increase the BTU need by an additional 10-15% over the base calculation.
Occupancy Adjustment: For each occupant beyond two, add approximately 600 BTU.
Appliance Adjustment: For each heat-generating appliance, add approximately 400 BTU.
Example Calculation:
Let's consider a room that is 200 sq ft with 8 ft ceilings, 2 windows, medium sun exposure, 3 occupants, and 1 appliance.
In this example, a 7000 BTU air conditioner would be a suitable choice.
Important Note: This calculator provides an estimate. For precise sizing, especially for entire homes or complex layouts, consult with an HVAC professional. Factors like insulation quality, air leakage, climate, and the specific heat load of appliances can significantly impact the actual cooling needs.
function calculateACSize() {
var sqFt = parseFloat(document.getElementById("squareFootage").value);
var ceilingHeight = parseFloat(document.getElementById("ceilingHeight").value);
var windows = parseFloat(document.getElementById("windows").value);
var sunExposure = document.getElementById("sunExposure").value;
var occupancy = parseFloat(document.getElementById("occupancy").value);
var appliances = parseFloat(document.getElementById("heatGeneratingAppliances").value);
var resultDiv = document.getElementById("result");
var recommendationDiv = document.getElementById("result-recommendation");
// Clear previous results
resultDiv.innerHTML = "";
recommendationDiv.innerHTML = "";
// Input validation
if (isNaN(sqFt) || sqFt <= 0 ||
isNaN(ceilingHeight) || ceilingHeight <= 0 ||
isNaN(windows) || windows < 0 ||
isNaN(occupancy) || occupancy < 0 ||
isNaN(appliances) || appliances 8) {
ceilingAdjustment = (ceilingHeight – 8) * (baseBtu * 0.10);
totalBtu += ceilingAdjustment;
}
// Window adjustment (approx. 1000 BTU per window, more for high sun)
var windowAdjustment = 0;
var windowBtuPerWindow = 1000;
if (sunExposure === "high") {
windowBtuPerWindow = 1500; // Increase BTU for high sun exposure
} else if (sunExposure === "medium") {
windowBtuPerWindow = 1000;
} else { // low
windowBtuPerWindow = 750; // Slightly less for low sun exposure
}
windowAdjustment = windows * windowBtuPerWindow;
totalBtu += windowAdjustment;
// Occupancy adjustment (add 600 BTU for each occupant over 2)
var occupancyAdjustment = 0;
if (occupancy > 2) {
occupancyAdjustment = (occupancy – 2) * 600;
totalBtu += occupancyAdjustment;
}
// Appliance adjustment (add 400 BTU per appliance)
var applianceAdjustment = appliances * 400;
totalBtu += applianceAdjustment;
// Round to nearest 500 BTU for standard AC unit sizes
var roundedBtu = Math.ceil(totalBtu / 500) * 500;
resultDiv.innerHTML = "Estimated Required BTU: " + roundedBtu.toLocaleString() + " BTU";
// Provide a general recommendation based on rounded BTU
var recommendation = "";
if (roundedBtu = 5000 && roundedBtu 8000 && roundedBtu 12000 && roundedBtu <= 18000) {
recommendation = "For larger rooms or multiple zones, look for systems in the " + roundedBtu.toLocaleString() + " BTU range. A mini-split system or central AC might be more efficient.";
} else {
recommendation = "For very large areas or whole-home cooling, this suggests a more complex system. Central air conditioning or multiple ductless mini-splits are likely necessary. Consult an HVAC professional.";
}
recommendationDiv.innerHTML = recommendation;
}