❄️ Free BTU Calculator

BTU Load Calculator

Find the right AC size for any room — BTUs, tonnage, and unit type in seconds.

❄️ Room BTU Sizing Calculator

📊 AC Sizing Results

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How to Use This Calculator

  1. Enter the square footage of the single room you want to cool — not your whole house. Measure length times width for a rectangular room.
  2. Select ceiling height. Taller ceilings mean more air volume to cool, which increases the BTU load proportionally.
  3. Choose your climate zone. A Phoenix, AZ room needs dramatically more cooling capacity than the same-sized room in San Francisco.
  4. Count your sun-facing windows — south- and west-facing windows admit the most solar heat gain, especially in the afternoon.
  5. Rate your insulation quality. A drafty older home with single-pane windows loses cool air faster and needs more BTUs to compensate.

The Formula Explained

BTU = (Sq Ft × 20 × Ceiling Factor × Climate Factor + Windows × 1,000) × Insulation Factor

The industry baseline of 20 BTU per square foot comes from ACCA Manual J — the standard load calculation method used by HVAC engineers. It assumes average ceiling height, moderate climate, and standard insulation. The multipliers adjust for real-world conditions: a very hot climate zone adds 45% to the load, while modern insulation reduces it by 10%. Sun-facing windows add 1,000 BTU each because south and west glass admits significant solar radiation during peak hours. Tonnage is simply BTUs divided by 12,000 — the "ton" unit dates to the era of ice cooling, when one ton of ice melting over 24 hours removed 12,000 BTU/hour of heat.

AC Unit Buying Guide

Frequently Asked Questions

What does BTU stand for?
BTU stands for British Thermal Unit — the amount of heat energy needed to raise the temperature of one pound of water by 1°F. For air conditioners, BTU/hour measures how much heat the unit can remove from a room per hour. Higher BTU ratings mean more cooling power. A 12,000 BTU AC removes 12,000 BTU of heat every hour, which is why 12,000 BTU equals one ton of cooling.
Is bigger always better for AC?
No — and this is one of the most common mistakes homeowners make. An oversized AC unit short-cycles: it cools the room quickly and shuts off before it has run long enough to remove humidity from the air. The result is a room that feels cold and clammy, not comfortable. It also causes excessive wear on the compressor from frequent starts and stops. Always size your AC to match the calculated load, not the largest unit you can afford.
What's a "ton" of AC?
One ton of cooling equals 12,000 BTU per hour. A 2-ton unit delivers 24,000 BTU/hr; a 3-ton unit delivers 36,000 BTU/hr. The term originates from the era before mechanical refrigeration, when buildings were cooled by large blocks of ice. It takes 12,000 BTU to melt one ton of ice over 24 hours — so a system that cools as effectively as one ton of ice melting per day became known as "one ton" of cooling capacity.
Should I size for the hottest day?
Yes — but carefully. ACCA Manual J recommends sizing for the 1% design day temperature for your climate zone, which represents the hottest conditions that occur about 1% of the year. This ensures your AC can handle extreme heat without running constantly. However, avoid the temptation to go significantly larger "just in case" — the short-cycling problems from an oversized unit outweigh the marginal benefit of extra capacity on the rare extreme-heat days.
Does insulation really affect BTU needs?
Significantly. A well-insulated modern home with double-pane windows and proper air sealing can need 10–20% fewer BTUs than a drafty older home of the same square footage. Good insulation slows the rate at which outdoor heat enters and indoor cool air escapes. Air sealing (caulking gaps, weatherstripping doors) is often more cost-effective than upgrading insulation alone, since warm air infiltration accounts for a large share of the cooling load in older homes.

💡 Pro Tip: Don't forget to account for heat-generating appliances. A kitchen with a stove may need 4,000 extra BTUs. A room with multiple computers or a home gym may need 2,000–3,000 extra.