Very Hot (e.g., Phoenix, AZ)
Hot (e.g., Dallas, TX)
Moderate (e.g., Chicago, IL)
Cool (e.g., Seattle, WA)
High (e.g., many windows, south-facing)
Medium (e.g., some windows, mixed orientation)
Low (e.g., few windows, shaded)
None
Low (e.g., computers, TVs)
Medium (e.g., kitchen appliances, office equipment)
High (e.g., server room, multiple heat sources)
Your estimated AC size will appear here.
Understanding Air Conditioner Sizing (BTU)
Choosing the right size air conditioner for your space is crucial for efficient cooling, energy savings, and maintaining optimal comfort. An AC unit that is too small will struggle to cool the room, running constantly and consuming excessive energy without ever reaching the desired temperature. Conversely, an AC unit that is too large will cool the space too quickly, leading to short cycling. This results in poor dehumidification (making the air feel clammy) and uneven temperatures, while also putting unnecessary wear and tear on the unit.
The cooling capacity of an air conditioner is measured in British Thermal Units (BTU) per hour. The higher the BTU rating, the more heat the unit can remove from a space. Our calculator provides an estimate based on several key factors that influence the cooling load of a room.
Factors Considered:
Room Square Footage: The primary factor. Larger rooms require more cooling capacity.
Ceiling Height: Taller ceilings mean a larger volume of air to cool, increasing the BTU requirement.
Climate Zone: Regions with hotter climates require higher BTU ratings to combat ambient heat.
Sun Exposure: Rooms with significant direct sunlight (especially south or west-facing) will gain more heat, necessitating a larger AC.
Occupancy: Each person in a room generates body heat, increasing the cooling load.
Heat-Generating Appliances: Electronics, kitchen appliances, and other devices emit heat, contributing to the overall temperature of the space.
How the Calculation Works (Simplified):
This calculator uses a common rule of thumb, starting with a base BTU per square foot and then adjusting based on the other factors.
Base Calculation:
The calculator starts with a general guideline, often around 20 BTU per square foot.
Base BTU = Room Square Footage × 20 BTU/sq ft
Adjustments:
Ceiling Height: For every foot above 8 feet, an additional adjustment is made. A common adjustment is to increase the BTU by approximately 10% for every foot over 8 feet.
Climate Zone: A multiplier or percentage adjustment is applied based on climate. Hotter zones increase the BTU requirement significantly (e.g., +10-20%), while cooler zones might slightly decrease it or have less impact.
Sun Exposure: High sun exposure increases the requirement (e.g., +10%), medium might have a smaller increase (e.g., +5%), and low exposure might have no significant adjustment.
Occupancy: For every person beyond the first two regularly occupying the room, add approximately 600 BTU.
Heat-Generating Appliances: Depending on the level, a certain amount of BTU is added (e.g., None: +0 BTU, Low: +500 BTU, Medium: +1000 BTU, High: +1500 BTU).
The final estimated BTU is the sum of the base calculation and all applied adjustments. It's important to note that this is an estimation. For precise sizing, especially for entire homes or complex layouts, consulting with an HVAC professional is highly recommended. They can perform a detailed load calculation (Manual J) that considers insulation, window types, air leakage, and ductwork.
function calculateACSize() {
var sqFt = parseFloat(document.getElementById("roomSquareFootage").value);
var ceilingHeight = parseFloat(document.getElementById("ceilingHeight").value);
var climate = parseInt(document.getElementById("climateZone").value);
var sun = document.getElementById("sunExposure").value;
var occupancy = parseInt(document.getElementById("peopleOccupancy").value);
var appliances = document.getElementById("heatGeneratingAppliances").value;
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = 'Your estimated AC size will appear here.'; // Reset
if (isNaN(sqFt) || sqFt <= 0) {
resultDiv.innerHTML = 'Please enter a valid room square footage.';
return;
}
if (isNaN(ceilingHeight) || ceilingHeight <= 0) {
resultDiv.innerHTML = 'Please enter a valid ceiling height.';
return;
}
if (isNaN(occupancy) || occupancy 8) {
var heightAdjustmentFactor = (ceilingHeight – 8) * 0.10; // 10% per foot over 8ft
totalBtu += baseBtu * heightAdjustmentFactor;
}
// Climate Zone Adjustment
var climateAdjustment = 0;
if (climate === 1) climateAdjustment = 0.20; // Very Hot: +20%
else if (climate === 2) climateAdjustment = 0.10; // Hot: +10%
else if (climate === 3) climateAdjustment = 0.05; // Moderate: +5%
// Cool zone has no significant positive adjustment
totalBtu += baseBtu * climateAdjustment;
// Sun Exposure Adjustment
var sunAdjustment = 0;
if (sun === "high") sunAdjustment = 0.10; // High: +10%
else if (sun === "medium") sunAdjustment = 0.05; // Medium: +5%
// Low sun exposure has no significant positive adjustment
totalBtu += baseBtu * sunAdjustment;
// Occupancy Adjustment (add for each person over 2)
if (occupancy > 2) {
var occupancyBtuToAdd = (occupancy – 2) * 600; // 600 BTU per additional person
totalBtu += occupancyBtuToAdd;
}
// Heat Generating Appliances Adjustment
var applianceBtuToAdd = 0;
if (appliances === "low") applianceBtuToAdd = 500;
else if (appliances === "medium") applianceBtuToAdd = 1000;
else if (appliances === "high") applianceBtuToAdd = 1500;
totalBtu += applianceBtuToAdd;
// Round to nearest standard AC size (e.g., 500 BTU increments, or common sizes)
// Common AC sizes: 5000, 6000, 8000, 10000, 12000, 15000, 18000, 24000 BTU
// For simplicity, we'll round to the nearest 500 BTU and then suggest a standard size.
var roundedBtu = Math.round(totalBtu / 500) * 500;
var suggestedSize = "";
if (roundedBtu <= 5000) suggestedSize = "5,000 – 6,000 BTU";
else if (roundedBtu <= 8000) suggestedSize = "8,000 BTU";
else if (roundedBtu <= 10000) suggestedSize = "10,000 BTU";
else if (roundedBtu <= 12000) suggestedSize = "12,000 BTU";
else if (roundedBtu <= 15000) suggestedSize = "15,000 BTU";
else if (roundedBtu <= 18000) suggestedSize = "18,000 BTU";
else if (roundedBtu <= 24000) suggestedSize = "24,000 BTU";
else suggestedSize = "Larger capacity needed (consult professional)";
resultDiv.innerHTML = 'Estimated AC Size Needed: ' + suggestedSize + ' (' + roundedBtu.toLocaleString() + ' BTU)';
}