Low (Shaded, North-facing)
Medium (Partially shaded, East/West)
High (Full sun, South-facing)
Understanding AC Tonnage and the Calculation
Air conditioning (AC) capacity is measured in tons of refrigeration. One ton of cooling is equivalent to the amount of heat required to melt one ton (2,000 pounds) of ice in 24 hours. This is approximately equal to 12,000 British Thermal Units (BTUs) per hour.
Why Calculate AC Tonnage?
Choosing the right AC tonnage is crucial for effective and efficient cooling. An undersized unit will struggle to cool the space, running constantly and wasting energy. An oversized unit will cool the space too quickly, leading to poor dehumidification, uneven temperatures, and potential short-cycling (damaging the compressor).
Factors Influencing AC Tonnage Calculation
This calculator considers several key factors to provide an estimate of the required AC tonnage:
Square Footage: The primary driver of cooling load. Larger areas require more cooling capacity.
Ceiling Height: Higher ceilings mean more air volume to cool, increasing the required tonnage. The standard assumption is often 8 feet.
Sun Exposure: Rooms receiving direct sunlight, especially on the south or west side, will absorb more heat.
Occupancy: Each person generates body heat, contributing to the cooling load. A typical estimate is 400-600 BTUs per person.
Heat-Generating Appliances: Electronics like TVs, computers, and kitchen appliances all add to the heat load in a room.
The Calculation Formula
This calculator uses a common rule-of-thumb formula, adjusted by factors, to estimate the required BTUs, which are then converted to tons.
The basic calculation starts with a baseline BTU per square foot, often around 20 BTU/sq ft, and then applies adjustments.
Base BTU/sq ft: A common starting point is 20 BTU/sq ft.
Ceiling Height Factor: A multiplier to account for extra air volume. For an 8-foot ceiling, this might be close to 1.0. For higher ceilings, it increases. (e.g., for 10ft ceiling, factor ≈ 1.25)
BTUs/Occupant: Approximately 400-600 BTUs per person. We use 500 BTUs for this calculation.
BTUs/Appliance: Varies, but a typical estimate for a home appliance (like a TV or PC) is 1000-2000 BTUs. We use 1500 BTUs here.
Sun Exposure: The calculated BTUs are multiplied by a factor based on sun exposure (1.0 for low, 1.15 for medium, 1.3 for high).
Finally, the total estimated BTUs are divided by 12,000 to convert them into tons of cooling capacity.
This calculator provides an estimate for general guidance. Professional HVAC technicians should always be consulted for a precise load calculation (Manual J) specific to your building's construction, insulation, climate, and specific layout.
function calculateTonnage() {
var squareFootage = parseFloat(document.getElementById("squareFootage").value);
var ceilingHeight = parseFloat(document.getElementById("ceilingHeight").value);
var sunExposure = parseFloat(document.getElementById("sunExposure").value);
var occupancy = parseFloat(document.getElementById("occupancy").value);
var heatGeneratingAppliances = parseFloat(document.getElementById("heatGeneratingAppliances").value);
var resultDiv = document.getElementById("result");
resultDiv.textContent = ""; // Clear previous result
// Input validation
if (isNaN(squareFootage) || squareFootage <= 0 ||
isNaN(ceilingHeight) || ceilingHeight <= 0 ||
isNaN(occupancy) || occupancy < 0 ||
isNaN(heatGeneratingAppliances) || heatGeneratingAppliances < 0) {
resultDiv.textContent = "Please enter valid positive numbers for all fields (except occupancy and appliances which can be zero).";
return;
}
// Constants for calculation
var BASE_BTU_PER_SQFT = 20;
var BTUS_PER_OCCUPANT = 500;
var BTUS_PER_APPLIANCE = 1500;
var BTUS_PER_TON = 12000;
// Calculate ceiling height factor (simplistic: 1.0 for 8ft, +0.025 for each additional foot)
var ceilingHeightFactor = 1.0 + Math.max(0, ceilingHeight – 8) * 0.025;
// Calculate base load from square footage and ceiling height
var baseLoadBTUs = squareFootage * ceilingHeightFactor * BASE_BTU_PER_SQFT;
// Calculate load from occupants
var occupantLoadBTUs = occupancy * BTUS_PER_OCCUPANT;
// Calculate load from appliances
var applianceLoadBTUs = heatGeneratingAppliances * BTUS_PER_APPLIANCE;
// Total estimated BTUs before sun exposure adjustment
var totalBTUs = baseLoadBTUs + occupantLoadBTUs + applianceLoadBTUs;
// Apply sun exposure adjustment
var adjustedBTUs = totalBTUs * sunExposure;
// Convert BTUs to Tons
var requiredTonnage = adjustedBTUs / BTUS_PER_TON;
// Round to a reasonable precision and add units
var formattedTonnage = requiredTonnage.toFixed(2);
resultDiv.textContent = formattedTonnage + " Tons";
}