How many BTUs do I need per square foot?

A general rule of thumb is 20 to 25 BTUs per square foot for average rooms with standard ceilings. However, kitchens, sunny rooms, and rooms with high ceilings require more. Use our calculator for a precise number.

What happens if my AC is too big (oversized)?

An oversized AC will cool the room too quickly and shut off (short cycle) before it can remove humidity. This leaves the room feeling clammy and wears out the unit faster.

How do I convert BTUs to Tons?

1 Ton of cooling is equal to 12,000 BTUs/hr. To convert, divide your total BTUs by 12,000. For example, 18,000 BTU is 1.5 Tons.

BTU Calculator

Accurate BTU/hr estimate for AC & Heating.

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Result will appear after calculation.

Quick Help

Enter your room dimensions and ceiling height. Select the room type (kitchens need more power) and insulation quality. Enter the temperature difference (e.g., if it's 95°F outside and you want 75°F inside, enter 20).

Complete Guide to BTU Calculation for AC & Heating

Choosing the correctly sized air conditioner or heater is the most important factor for home comfort and energy efficiency. BTU stands for British Thermal Unit, the international standard for measuring heat energy. Whether you are installing a mini-split system, a window unit, or central air, getting the BTU calculation right is essential.

Why Accurate Sizing is Critical

A common misconception is that "bigger is better" for air conditioning. This is false. Proper HVAC sizing follows the "Goldilocks principle"—it must be just right.

Under-sized Units: If the unit has too few BTUs, it will run continuously without ever reaching your set temperature. This results in high electric bills and a unit that burns out quickly.
Over-sized Units: If the unit is too powerful, it will cool the room in minutes and shut off. This is called "short cycling." Since air conditioners need time to remove humidity, an oversized unit leaves your home feeling cold but damp and clammy.

How the Calculation Works

The Calculatorbudy BTU Calculator uses a volumetric approach rather than a simple square-footage rule. This provides a much more accurate estimate because it accounts for:

Room Volume: We measure Length × Width × Height. High ceilings (over 8ft) significantly increase the air volume that needs to be treated.
Insulation Factor: A drafty older home (Poor insulation) requires 20-30% more power than a modern, sealed home.
Room Type Usage: Kitchens generate heat from appliances, requiring an additional 4,000 BTU buffer. Bedrooms and living rooms have standard requirements.

BTU Sizing Chart by Room Size

For standard rooms with 8-foot ceilings and average insulation, you can use this reference chart. For precise numbers, always use the calculator above.

Room Area (Sq Ft)	Approximate BTU (Cooling)	Recommended Unit Type
100 - 150 sq ft	5,000 - 6,000 BTU	Small Window Unit
150 - 250 sq ft	6,000 - 7,000 BTU	Window / Portable AC
250 - 350 sq ft	7,000 - 8,000 BTU	Medium Window Unit
350 - 450 sq ft	9,000 - 10,000 BTU	Large Portable / Mini Split
450 - 550 sq ft	12,000 BTU (1 Ton)	Mini Split / Central
550 - 700 sq ft	14,000 BTU	Large Window / Mini Split
700 - 1,000 sq ft	18,000 BTU (1.5 Ton)	Central / Multi-Zone

Key Factors That Affect BTU Needs

When entering your data into the calculator, consider these environmental factors:

Sunlight: Rooms with large, sun-facing windows (South or West facing) gain significant heat. You may need to increase capacity by 10%.
Occupancy: Humans radiate heat. If a room is usually occupied by more than two people, add roughly 600 BTUs per additional person.
Climate: If you live in an extremely hot climate (consistently over 95°F), increase the "Temperature Difference" in the calculator to ensure the unit can keep up on the hottest days.

Frequently Asked Questions (FAQ)

What is the difference between BTU and Tons?

A "Ton" in HVAC refers to cooling capacity, not weight. 1 Ton is equal to 12,000 BTUs per hour. For example, a 24,000 BTU unit is a 2-Ton system.

Does heating require the same BTUs as cooling?

Not always. Heating generally requires more BTUs than cooling because the temperature difference in winter (e.g., raising 30°F outside to 70°F inside = 40° diff) is often larger than in summer (lowering 95°F to 75°F = 20° diff). Use the "Heating" mode on our calculator for winter estimates.

Can I use a portable AC for a large room?

Portable ACs are less efficient than window or split units because they generate heat inside the room that must be exhausted. If you use a portable unit, buy one with a slightly higher BTU rating (Dual Hose models are best) to compensate for this efficiency loss.