BTU Calculator

Calculate the BTUs needed to heat or cool a room based on its dimensions, insulation quality, and climate conditions. Proper BTU sizing ensures efficient heating and cooling without wasting energy on an oversized or undersized system.

Enter Your Measurements

Room Length(ft)

Room Width(ft)

Ceiling Height(ft)

Insulation Level (1=Poor, 2=Average, 3=Good)

Sun Exposure (1=Shaded, 2=Average, 3=Sunny)

Number of Occupants

Results

Cooling BTU Needed

3,600

BTU/hr

Heating BTU Needed

5,100

BTU/hr

AC Tonnage

0.3

tons

Room Area

180

sq ft

Formula

Cooling BTU = Room Area × 20 × Height Factor × Insulation Factor × Sun Factor + Occupant BTU

How to Use This Calculator

How to Use

1
Enter the room length and width in feet.
2
Enter the ceiling height (standard is 8 feet).
3
Rate the insulation quality: 1 for older homes with poor insulation, 3 for new construction with good insulation.
4
Rate sun exposure: 1 for heavily shaded rooms, 3 for rooms with large south/west-facing windows.

Frequently Asked Questions

How many BTU per square foot do I need?

A general rule is 20 BTU per square foot for cooling. However, this varies with ceiling height, insulation, sun exposure, climate, and number of occupants. Heating typically requires 25-30 BTU per square foot.

What happens if my AC is too many BTUs?

An oversized AC will cool the room quickly but short-cycle, meaning it turns on and off frequently. This wastes energy, increases wear, and fails to properly dehumidify the air, making the room feel clammy.

How do I convert BTU to tons?

One ton of air conditioning equals 12,000 BTU per hour. So a 24,000 BTU unit is 2 tons, and a 36,000 BTU unit is 3 tons.

In-Depth Guides

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How to Size Your HVAC System: BTU & Tonnage Guide

Learn how to properly size an air conditioner or furnace for your home. Covers BTU calculations, Manual J basics, SEER ratings, heat pumps, and common sizing mistakes.

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Water Heater Sizing Guide: Tank Size, BTU & First Hour Rating

Choose the right water heater size for your home. Calculate tank size by family size, compare tankless vs tank, and understand energy efficiency ratings.

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About This Calculator

This calculator uses the formula: Cooling BTU = Room Area × 20 × Height Factor × Insulation Factor × Sun Factor + Occupant BTU. All calculations follow industry-standard methods. Results are estimates — always verify with a licensed professional for structural or code-compliant work.

Built and maintained by the CalcSmart team. Last updated March 2026.

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