What Size Mini Split Do I Need? (The Ultimate Sizing Guide + Calculator)
You are ready to buy a mini split, but you are stuck on one question:
“How many BTUs do I actually need?”
If you buy a unit that is too small, it will run forever and never cool the room.
If you buy one that is too big, it will turn on and off constantly (short-cycling), leaving the room humid and clammy.
Most people just guess. Don’t guess.
In this guide, I will show you the “Goldilocks Rule” for sizing and give you a simple chart to find the perfect unit for your square footage.
The “Rule of Thumb” (Quick Chart) 📏
For a standard room with 8-foot ceilings and average insulation, you can use this quick cheat sheet based on square footage.
| Room Size (Sq. Ft.) | Recommended Size (BTU) | Common Use Case |
| 100 – 400 sq. ft. | 9,000 BTU | Bedroom / Office |
| 400 – 650 sq. ft. | 12,000 BTU | Master Bedroom / Living Room |
| 650 – 1,000 sq. ft. | 18,000 BTU | Large Living Area / Garage |
| 1,000 – 1,300 sq. ft. | 24,000 BTU | Open Concept Floor / Shop |
| 1,300+ sq. ft. | 36,000 BTU | Whole Floor / Large Garage |
⚠️ Warning: This chart is just a starting point. Your specific room might need more power based on the factors below.
The 3 Hidden Factors That Change Everything ☀️
Square footage is not the only thing that matters. If you ignore these three things, you will buy the wrong size.
1. Ceiling Height 🏠
The chart above assumes standard 8-foot ceilings.
If you have Cathedral Ceilings (12ft+), you have 50% more air volume to cool.
- The Fix: Upgrade to the next size up (e.g., if the chart says 12k, buy 18k).
2. Insulation Quality 🧱
- Good Insulation: New windows, sealed walls. (Stick to the chart).
- Poor Insulation: Old windows, drafty doors, or an uninsulated garage.
- The Fix: You need roughly 30% more power to fight the heat loss.
3. Sunlight & Kitchens 🍳
- Sun: Does the room have big south-facing windows? The sun acts like a heater.
- Kitchen: If the unit is near an oven or stove, you have a massive internal heat source.
- The Fix: Add 4,000 BTUs to your calculation.
Why “Bigger” is NOT Better ❌
You might think: “I’ll just buy the biggest one (24k) for my small bedroom to be safe.”
This is a huge mistake.
An oversized unit will cool the air too fast (in 5 minutes) and shut off.
- The Result: The air is cold, but the walls are still warm.
- The Humidity: It didn’t run long enough to remove moisture. You end up with a cold, damp, moldy room.
You want a unit that runs at low power for a long time. That is the secret to comfort.
👉 Before you size or buy anything, review this mini split overview
Stop Guessing: Use The Calculator 🧮
Why risk spending $1,500 on the wrong unit?
We built a specialized tool that takes your room size, insulation, and sun exposure into account to give you the exact number you need.
It takes 30 seconds.
👉 Click Here to Calculate Your Perfect Mini Split Size
Conclusion
Sizing is the most critical step in your project.
- Too Small: Won’t cool.
- Too Big: Clammy and inefficient.
- Just Right: Perfect comfort and low bills.
Start with the square footage, adjust for your ceiling height, and verify with our calculator before you order.
