Determine the appropriate BTUs needed for your space.
Low (Shady rooms, few windows)
Medium (Average sunlight)
High (Very sunny rooms, many windows)
Recommended AC Size:
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BTUs
Understanding Air Conditioner Sizing (BTUs)
Choosing the correct size for your air conditioner 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 failing to reach the desired temperature. Conversely, an AC unit that is too large will cool the room too quickly but also dehumidify it excessively, leading to a clammy feeling and inefficient energy use. This calculator helps you estimate the British Thermal Units (BTUs) required for a room.
What are BTUs? A British Thermal Unit (BTU) is a unit of energy. In the context of air conditioning, it measures the amount of heat an AC unit can remove from a room in one hour. A higher BTU rating means the AC can cool a larger area or overcome more heat.
The Calculation Logic
Our calculator uses a common method for estimating AC size based on several factors:
Base BTU for Area: The primary factor is the square footage of the room. A general guideline often starts at 20 BTUs per square foot.
Formula Component: Room Area (sq ft) * 20 BTUs/sq ft
Ceiling Height Adjustment: Standard calculations assume an 8-foot ceiling. Taller ceilings mean more air volume to cool, so an adjustment is made.
Formula Component: (Ceiling Height (ft) – 8 ft) * (Room Area (sq ft) * 20 BTUs/sq ft) / 8
Sun Exposure Factor: Rooms with significant sun exposure will require more cooling capacity. This is applied as a multiplier.
Formula Component: Base BTU * Sun Exposure Multiplier
Occupancy Adjustment: Each person in a room adds heat. The calculator adds BTUs for occupants beyond the first two.
Formula Component: Number of Occupants * 400 BTUs/person
Heat-Generating Items Adjustment: Appliances like TVs, computers, and lamps also generate heat. A standard estimate is added for these.
Formula Component: Number of Heat Generating Items * 100 BTUs/item
The calculator sums these components to provide a recommended BTU range.
When to Use This Calculator
This calculator is ideal for homeowners and renters who are:
Purchasing a new window air conditioning unit or portable AC.
Considering a ductless mini-split system for a specific room.
Trying to understand why a room isn't cooling effectively.
Planning for new AC installations.
Disclaimer: This calculator provides an estimation. For precise sizing, especially for whole-house central air systems or complex room layouts, consult with a qualified HVAC professional. Factors like insulation quality, climate zone, and specific room usage can significantly impact ideal AC sizing.
function calculateACSize() {
var roomArea = parseFloat(document.getElementById("roomArea").value);
var ceilingHeight = parseFloat(document.getElementById("ceilingHeight").value);
var sunExposure = parseFloat(document.getElementById("sunExposure").value);
var occupancy = parseFloat(document.getElementById("occupancy").value);
var heatGeneratingItems = parseFloat(document.getElementById("heatGeneratingItems").value);
var resultDiv = document.getElementById("result");
var resultValueDiv = document.getElementById("result-value");
var resultUnitDiv = document.getElementById("result-unit");
if (isNaN(roomArea) || roomArea <= 0 ||
isNaN(ceilingHeight) || ceilingHeight <= 0 ||
isNaN(occupancy) || occupancy < 0 ||
isNaN(heatGeneratingItems) || heatGeneratingItems 8) {
ceilingHeightAdjustment = (ceilingHeight – 8) * (roomArea * baseBtuPerSqFt) / 8;
}
var occupancyBtu = 0;
if (occupancy > 2) {
occupancyBtu = (occupancy – 2) * 400;
}
var itemsBtu = heatGeneratingItems * 100;
var totalBtu = baseBtu + ceilingHeightAdjustment + occupancyBtu + itemsBtu;
totalBtu = totalBtu * sunExposure;
// Round to nearest hundred BTUs for practical sizing
var roundedBtu = Math.ceil(totalBtu / 100) * 100;
resultValueDiv.textContent = roundedBtu.toLocaleString(); // Use toLocaleString for comma separation
resultValueDiv.style.color = "#28a745";
resultUnitDiv.textContent = "BTUs";
}