Excellent (New Build)
Average (Standard)
Poor (Old House)
Shady / North Facing
Normal Exposure
Very Sunny / Large Windows
No
Yes (+4,000 BTU)
Recommended System Capacity:
0 BTU
function calculateBTU() {
var length = parseFloat(document.getElementById('roomLength').value);
var width = parseFloat(document.getElementById('roomWidth').value);
var height = parseFloat(document.getElementById('ceilingHeight').value);
var insulation = parseFloat(document.getElementById('insulation').value);
var sunlight = parseFloat(document.getElementById('sunlight').value);
var kitchenAdd = parseFloat(document.getElementById('isKitchen').value);
var occupants = parseInt(document.getElementById('occupants').value);
if (isNaN(length) || isNaN(width) || isNaN(height) || length <= 0 || width 8) {
heightFactor = 1 + ((height – 8) * 0.125);
}
// Occupant adjustment: Add 600 BTU for every person above 2
var occupantAdd = 0;
if (occupants > 2) {
occupantAdd = (occupants – 2) * 600;
}
// Cumulative calculation
var totalBTU = (baseBTU * heightFactor * insulation * sunlight) + kitchenAdd + occupantAdd;
// Rounding up to the nearest hundred
totalBTU = Math.ceil(totalBTU / 100) * 100;
document.getElementById('btuValue').innerText = totalBTU.toLocaleString() + " BTU";
var recommendation = "";
if (totalBTU <= 6000) recommendation = "Recommended: 6,000 BTU Mini Split";
else if (totalBTU <= 9000) recommendation = "Recommended: 9,000 BTU Mini Split";
else if (totalBTU <= 12000) recommendation = "Recommended: 12,000 BTU Mini Split (1 Ton)";
else if (totalBTU <= 18000) recommendation = "Recommended: 18,000 BTU Mini Split (1.5 Tons)";
else if (totalBTU <= 24000) recommendation = "Recommended: 24,000 BTU Mini Split (2 Tons)";
else if (totalBTU <= 30000) recommendation = "Recommended: 30,000 BTU Mini Split (2.5 Tons)";
else if (totalBTU <= 36000) recommendation = "Recommended: 36,000 BTU Mini Split (3 Tons)";
else recommendation = "Recommended: Multiple Units or Commercial Multi-Zone System";
document.getElementById('systemRecommendation').innerText = recommendation;
document.getElementById('btuResult').style.display = "block";
}
Understanding Mini Split BTU Sizing
Choosing the correct BTU (British Thermal Unit) rating for your ductless mini-split is the most critical step in ensuring home comfort and energy efficiency. A unit that is too small will run constantly, failing to cool the room, while a unit that is too large will "short-cycle," leading to high humidity and premature wear on the compressor.
How We Calculate Your BTU Needs
The standard rule of thumb is 20 BTU per square foot of living space. However, real-world conditions require more precise adjustments:
Ceiling Height: Standard calculations assume 8-foot ceilings. If your ceilings are taller, the volume of air increases, requiring more cooling power.
Insulation Quality: Poorly insulated older homes lose cooled air rapidly. We apply a 30% multiplier for homes with poor insulation to compensate for thermal loss.
Sun Exposure: If your room faces south or has large windows, solar heat gain can significantly increase the temperature. Conversely, heavily shaded rooms require 10% less power.
Kitchen Loads: Appliances generate significant heat. If the indoor unit is located in a kitchen, an additional 4,000 BTU is added to the base calculation.
Mini Split Sizing Chart
Square Footage
Average BTU Capacity
150 – 250 sq ft
6,000 BTU
250 – 400 sq ft
9,000 BTU
400 – 650 sq ft
12,000 BTU
650 – 1,000 sq ft
18,000 BTU
1,000 – 1,500 sq ft
24,000 BTU
Example Calculation
Imagine a 15′ x 20′ living room (300 sq ft) with 10-foot ceilings and average insulation.
1. Base: 300 sq ft × 20 = 6,000 BTU.
2. Height Adjustment: +25% for 10ft ceilings = 7,500 BTU.
3. Final: You would likely install a 9,000 BTU unit to ensure the system isn't overstressed on hot days.