Calculate Mini Split Size

Mini-Split Size Calculator

Determine the ideal BTU/hr for your mini-split unit to ensure optimal comfort and efficiency.

Average (Standard) Poor (Old homes, little insulation) Good (Newer homes, well insulated) Excellent (Energy-efficient homes)

Average (Standard number of windows) Few windows or mostly North-facing Many windows or mostly South/West-facing

No Yes

function calculateMiniSplitSize() { var roomArea = parseFloat(document.getElementById('roomArea').value); var ceilingHeight = parseFloat(document.getElementById('ceilingHeight').value); var insulationQuality = document.getElementById('insulationQuality').value; var windowExposure = document.getElementById('windowExposure').value; var numOccupants = parseInt(document.getElementById('numOccupants').value); var hasAppliances = document.getElementById('hasAppliances').value; if (isNaN(roomArea) || roomArea <= 0 || isNaN(ceilingHeight) || ceilingHeight <= 0 || isNaN(numOccupants) || numOccupants < 0) { document.getElementById('miniSplitResult').innerHTML = 'Please enter valid positive numbers for all fields.'; return; } // Base BTU per square foot (standard assumption for 8ft ceiling) var baseBTUPerSqFt = 20; // A common starting point // Adjust base BTU for ceiling height if significantly different from 8ft // This is a simplified linear adjustment. More complex calculations exist. var heightFactor = ceilingHeight / 8; baseBTUPerSqFt = baseBTUPerSqFt * heightFactor; var totalBTU = roomArea * baseBTUPerSqFt; // Adjust for insulation quality if (insulationQuality === 'poor') { totalBTU *= 1.20; // +20% for poor insulation } else if (insulationQuality === 'good') { totalBTU *= 0.90; // -10% for good insulation } else if (insulationQuality === 'excellent') { totalBTU *= 0.80; // -20% for excellent insulation } // Adjust for window exposure if (windowExposure === 'few_north') { totalBTU *= 0.95; // -5% for fewer/north-facing windows } else if (windowExposure === 'many_south_west') { totalBTU *= 1.10; // +10% for many/south/west-facing windows } // Add BTU for occupants (approx. 600 BTU per person) totalBTU += numOccupants * 600; // Add BTU for heat-generating appliances (approx. 1000 BTU) if (hasAppliances === 'yes') { totalBTU += 1000; } // Round up to the nearest 1000 BTU for common mini-split sizes var recommendedBTU = Math.ceil(totalBTU / 1000) * 1000; // Common mini-split sizes are 9,000, 12,000, 18,000, 24,000, 30,000, 36,000 BTU // Let's snap to the closest standard size if it's not too far off, or just provide the calculated value. // For simplicity, we'll just provide the calculated and rounded value. var tonnage = recommendedBTU / 12000; var resultHTML = 'Based on your inputs:'; resultHTML += 'Recommended Mini-Split Size: ' + recommendedBTU.toLocaleString() + ' BTU/hr'; resultHTML += 'This is approximately ' + tonnage.toFixed(2) + ' Tons of cooling/heating capacity.'; document.getElementById('miniSplitResult').innerHTML = resultHTML; }

Understanding Mini-Split Sizing: Why It Matters

Choosing the right size mini-split system is one of the most critical decisions you'll make when installing new heating and cooling. Unlike central HVAC systems, mini-splits are designed to condition specific zones or rooms. An improperly sized unit can lead to significant problems, impacting both your comfort and your energy bills.

What is BTU/hr and Tonnage?

The capacity of a mini-split (or any HVAC system) is measured in British Thermal Units per hour (BTU/hr). One BTU is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. For air conditioning, it represents the amount of heat a unit can remove from a space in an hour.

Another common measurement is "tons" of cooling. One ton of cooling capacity is equivalent to 12,000 BTU/hr. So, a 1-ton mini-split can remove 12,000 BTUs of heat per hour.

The Dangers of Incorrect Sizing

• Oversized Unit: A mini-split that's too large for your space will cool or heat the room too quickly, then shut off. This "short cycling" prevents the unit from running long enough to effectively dehumidify the air, leaving your room feeling clammy and uncomfortable. It also causes more wear and tear on the compressor, shortening the unit's lifespan and consuming more energy due to frequent starts and stops.
• Undersized Unit: Conversely, a mini-split that's too small will struggle to reach your desired temperature, especially on extreme weather days. It will run continuously, consuming excessive energy, yet still fail to provide adequate comfort. This constant operation also puts undue stress on the system, leading to premature failure.

Key Factors Influencing Mini-Split Size

Our calculator takes several crucial factors into account to provide an accurate BTU/hr recommendation:

1. Room Area (Square Feet): This is the primary determinant. Larger rooms naturally require more heating and cooling capacity.
2. Ceiling Height: Taller ceilings mean a larger volume of air to condition, increasing the required BTU/hr. Our calculator adjusts the base BTU/sq ft accordingly.
3. Insulation Quality: Well-insulated rooms retain heat in winter and keep it out in summer more effectively, reducing the demand on your mini-split. Poor insulation means more heat gain/loss, requiring a larger unit.
4. Window Exposure & Quantity: Windows are significant sources of heat gain (especially south and west-facing windows in summer) and heat loss (in winter). Rooms with many windows or those facing direct sunlight will need more capacity.
5. Number of Occupants: Every person in a room generates body heat, adding to the cooling load.
6. Heat-Generating Appliances: Appliances like refrigerators, ovens, computers, and even bright lighting can contribute significantly to a room's heat load, especially in kitchens or home offices.

How to Use the Calculator

Simply input the details of the room you wish to condition into the fields above. Be as accurate as possible with your measurements and assessments of insulation and window exposure. Click "Calculate Mini-Split Size," and the tool will provide a recommended BTU/hr and approximate tonnage for your mini-split unit.

Example Calculation:

Let's say you have a:

• Room Area: 300 sq ft
• Ceiling Height: 8 feet
• Insulation Quality: Average
• Window Exposure: Average
• Number of Occupants: 2
• Heat-Generating Appliances: No

Using our calculator's logic:

• Base BTU: 300 sq ft * 20 BTU/sq ft = 6,000 BTU
• Insulation/Windows: No significant adjustment for average conditions.
• Occupants: 2 people * 600 BTU/person = 1,200 BTU
• Appliances: 0 BTU
• Total Estimated BTU: 6,000 + 1,200 = 7,200 BTU
• Recommended Mini-Split Size (rounded up): 8,000 BTU/hr (often you'd choose a 9,000 BTU unit as the smallest standard size)

This example demonstrates how various factors contribute to the final sizing recommendation. Always consider consulting with a professional HVAC technician for a precise load calculation, especially for complex spaces or whole-home systems.