When you’re outfitting a space with a PTAC unit—whether that’s a hotel room, studio apartment, or commercial office—it’s easy to overlook just how critical proper sizing is. Install a unit that’s too small, and it’ll constantly run without ever achieving the comfort you’re looking for. Go too large, and you’ll pay more than necessary upfront while dealing with short cycling and poor humidity control. So, what’s the right fit?
In this guide, I’ll walk you through how to size a PTAC based on square footage, climate, insulation, and a few other real-world factors. We’ll keep it simple and practical—no fluff, just facts.
What Is BTU and Why Does It Matter?
BTU stands for British Thermal Unit, and it’s the standard measurement of cooling or heating capacity for HVAC systems. The higher the BTU rating, the more powerful the unit. The goal is to match the BTU output of your PTAC with the specific needs of your space.
Square Footage vs. BTU: The General Rule
Here’s a helpful starting point to estimate your BTU needs:
| Room Size (sq ft) | Recommended BTU Rating |
|---|---|
| 100–250 | 5,000–6,000 BTUs |
| 250–350 | 7,000–8,000 BTUs |
| 350–450 | 9,000–10,000 BTUs |
| 450–550 | 11,000–12,000 BTUs |
| 550–700 | 13,000–14,000 BTUs |
If your space falls between these thresholds, round up slightly for comfort and efficiency.
Adjustments Based on Real-World Conditions
The square footage guide is a great starting point, but it doesn’t tell the whole story. Here are several adjustments you should factor in:
1. Climate Zone
If you live in a hot and humid region (like Florida or Texas), you’ll want to add 10–20% more BTUs to the standard recommendation. Conversely, in mild climates, you might get away with a slightly smaller unit.
2. Ceiling Height
Standard calculations assume 8-foot ceilings. If your ceilings are higher, multiply your BTU needs by a factor of 1.2–1.3.
3. Insulation Quality
Poor insulation? You’ll need more BTUs. Add 15–20% if you’re working with older buildings or single-pane windows.
4. Sun Exposure
Rooms with large south- or west-facing windows that get direct sunlight will require more cooling capacity. Add 10% if your space gets strong afternoon sun.
5. Occupancy and Electronics
If the room consistently holds multiple people or includes appliances that generate heat (like mini-fridges or office equipment), increase your BTU needs by another 500–1,000 BTUs.
Common BTU Sizes for PTAC Units
Most packaged terminal air conditioners fall within this BTU range:
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7,000–9,000 BTUs: Ideal for small rooms, dorms, or studio apartments.
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9,000–12,000 BTUs: Best for average hotel rooms and small suites.
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12,000–15,000 BTUs: Larger suites, open studios, and commercial use.
Keep in mind that some PTACs also offer electric heat or heat pump features, which may influence your sizing choices during the winter months.
Matching BTUs to Application: Examples
Here are a few examples from the field:
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Hotel Room, 325 sq ft, moderate insulation – 9,000 BTU unit
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Studio Apartment, 450 sq ft, poor insulation, south-facing windows – 12,000 BTU unit
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Office Room, 300 sq ft, 2 people working + desktop electronics – 10,000 BTU unit
Oversizing vs Undersizing: The Consequences
Oversizing leads to:
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Short cycling (causes wear and tear)
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Inadequate dehumidification
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Higher initial and long-term costs
Undersizing results in:
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Constant running and poor cooling
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Increased energy consumption
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Shortened equipment life
Considerations for Multi-Room or Long Hallway Areas
If your layout involves irregular shapes, open doorways, or adjoining rooms, consider either a higher BTU model or separate units to cover different zones effectively.
When in Doubt, Consult the Pros
There’s no substitute for a site assessment by a licensed HVAC technician, especially in multi-unit buildings. They can also help you confirm voltage and amperage compatibility.
Final Thoughts
Choosing the right PTAC size ensures energy efficiency, longer system life, and consistent comfort. Take the time to measure, calculate, and consider the unique conditions of your space. For a more in-depth look at how installation and electrical considerations tie into sizing, be sure to read PTAC Installation 101 and Electrical Requirements for PTACs.
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