Based on 0 sq. ft. area and specific environmental factors.
Pro Tip: If your calculated value is between standard sizes (e.g., 6,000 and 8,000), it's generally safer to size up for sunny rooms or kitchens, and size down for bedrooms to ensure better dehumidification.
How to Size Your Air Conditioner Correctly
Choosing the right size air conditioner is critical for comfort and energy efficiency. An AC unit that is too small will run continuously without cooling the room, driving up your electric bill. Conversely, a unit that is too powerful will cool the room too quickly, shutting off before it has a chance to dehumidify the air, leaving your space feeling damp and clammy.
Understanding BTUs
BTU stands for British Thermal Unit. It is a measure of heat energy. In the context of air conditioning, the BTU rating indicates how much heat the unit can remove from the air in one hour. The higher the BTU rating, the more powerful the air conditioner.
Factors Affecting BTU Calculation
While square footage is the primary factor, our calculator considers several critical variables:
Room Dimensions: We calculate the base thermal load based on area (Length x Width).
Ceiling Height: Standard calculations assume 8ft ceilings. Higher ceilings increase the volume of air that needs cooling.
Sunlight Exposure: A room with direct sunlight requires roughly 10% more cooling power, while a shaded room requires 10% less.
Occupancy: Human bodies generate heat. If more than two people regularly occupy the room, the EPA recommends adding 600 BTUs per additional person.
Kitchens: Appliances like ovens and stoves generate significant heat. It is standard practice to add 4,000 BTUs for kitchens.
Standard AC Sizes Reference
Use this quick reference to match your calculated BTU to common commercial unit sizes:
Room Size (Sq. Ft.)
Approximate BTU
100 – 150
5,000 BTU
150 – 250
6,000 BTU
250 – 350
7,000 – 8,000 BTU
350 – 450
9,000 – 10,000 BTU
450 – 550
12,000 BTU
function calculateBTU() {
// 1. Get input values
var length = parseFloat(document.getElementById('roomLength').value);
var width = parseFloat(document.getElementById('roomWidth').value);
var ceiling = parseFloat(document.getElementById('ceilingHeight').value);
var insulation = parseFloat(document.getElementById('insulation').value);
var sun = parseFloat(document.getElementById('sunExposure').value);
var occupants = parseInt(document.getElementById('occupants').value);
var kitchenBtu = parseInt(document.getElementById('isKitchen').value);
// 2. Validation
var errorMsg = document.getElementById('errorMsg');
var resultDiv = document.getElementById('result');
if (isNaN(length) || isNaN(width) || length <= 0 || width 8ft, add percentage increase based on volume
if (ceiling > 8) {
var volumeFactor = (ceiling – 8) * 0.10; // Add 10% for every foot over 8 roughly?
// Better logic: (Volume / BaseVolume) adjustment implies linear.
// Let's simplified: add 10% per foot over 8 is aggressive. Let's do 5% per foot.
var extraHeight = ceiling – 8;
var heightMultiplier = 1 + (extraHeight * 0.05);
subTotal = subTotal * heightMultiplier;
}
// Sun Exposure Adjustment
subTotal = subTotal * sun;
// Occupant Adjustment
// EPA: If more than 2 people, add 600 BTU per person.
if (occupants > 2) {
var extraPeople = occupants – 2;
subTotal += (extraPeople * 600);
}
// Kitchen Adjustment
subTotal += kitchenBtu;
// Rounding to nearest 100 or 500
var finalBtu = Math.ceil(subTotal / 500) * 500;
// 4. Output
document.getElementById('btuResult').innerText = finalBtu.toLocaleString() + ' BTU';
document.getElementById('areaResult').innerText = area.toFixed(0);
resultDiv.style.display = 'block';
}