Most homes require 20–25 BTU per square foot. To calculate AC size, multiply your square footage by the appropriate BTU factor and divide by 12,000 to determine required tonnage.

AC size calculator formula showing BTU per square foot and tonnage conversion chart

AC Size Calculator (BTU & Tonnage Estimator for Any Home – 2026)

Choosing the correct air conditioner size is one of the biggest factors in indoor comfort, energy efficiency, and long-term HVAC performance. This AC size calculator helps you estimate BTU capacity, AC tonnage, climate adjustments, and the airflow your system will need.

If you are asking what size AC do I need, this page gives you a fast starting point before requesting contractor quotes or a full load calculation.

Quick AC Size Calculator

You can estimate AC size in two simple steps:

Step 1: Multiply your home’s square footage by the appropriate BTU factor:

20 BTU per sq ft for cool climates
22–25 BTU per sq ft for moderate climates
25–30 BTU per sq ft for hot and humid climates

Step 2: Divide the total BTU by 12,000 to estimate tonnage.

Example:

2,000 sq ft home in a hot climate:
2,000 × 28 BTU = 56,000 BTU
56,000 ÷ 12,000 = 4.6 tons

Recommended size: 4.5 to 5 tons

If you want the broader sizing framework behind this estimate, read Air Conditioner Sizing Guide.

AC Size Calculator Chart

Home Size	Mild Climate	Moderate Climate	Hot Climate
600 sq ft	12,000 BTU	14,000 BTU	15,000–18,000 BTU
1,000 sq ft	20,000 BTU	22,000–25,000 BTU	25,000–30,000 BTU
1,500 sq ft	30,000 BTU	33,000–36,000 BTU	37,000–45,000 BTU
2,000 sq ft	36,000–40,000 BTU	40,000–48,000 BTU	50,000–60,000 BTU
2,500 sq ft	48,000 BTU	50,000–60,000 BTU	62,000–75,000 BTU
3,000 sq ft	60,000 BTU	66,000–72,000 BTU	75,000–90,000 BTU

For detailed size-specific guides, see:

How BTU Converts to Tonnage

In HVAC, 1 ton = 12,000 BTU per hour. That makes it easy to convert your estimated BTU load into system size.

24,000 BTU = 2 tons
36,000 BTU = 3 tons
48,000 BTU = 4 tons
60,000 BTU = 5 tons

If these terms are new to you, read What Is BTU in Air Conditioning? and What Is a Ton in HVAC?.

Climate Adjustment Guide

Climate has a major effect on AC sizing. Two homes with the same square footage can need very different systems depending on heat and humidity.

Cool / Northern climates: 18–22 BTU per sq ft
Moderate climates: 20–25 BTU per sq ft
Hot & humid climates: 25–30 BTU per sq ft

Homes in high-humidity regions need extra cooling capacity because the system must remove both heat and moisture.

For climate-specific examples, compare What Size AC Do I Need in Texas?, What Size AC Do I Need in Florida?, and What Size AC Do I Need in California?.

Ceiling Height Adjustment

Most online size calculators assume standard 8-foot ceilings. If your home has more air volume than that, the estimate needs to be adjusted upward.

9–10 ft ceilings: add about 10–15% capacity
Vaulted ceilings: add about 15–25%

That is why open great rooms, loft layouts, and tall ceilings often need more cooling than the square footage alone would suggest.

Airflow Requirement (CFM Calculator)

Correct airflow is just as important as correct tonnage. A common HVAC rule is:

400 CFM per ton

That means:

3 ton AC → about 1,200 CFM
4 ton AC → about 1,600 CFM
5 ton AC → about 2,000 CFM

If the duct system cannot handle that airflow, cooling performance drops and the blower has to work harder. For more on this, read How Many CFM Per Ton?.

What Happens If You Choose the Wrong Size?

An AC calculator is useful because incorrect sizing creates expensive problems on both sides.

If the AC is too big

An oversized air conditioner may:

short cycle
leave humidity behind
create uneven room temperatures
raise electricity bills

Learn more in Oversized AC Symptoms and AC Short Cycling Explained.

If the AC is too small

An undersized unit may:

run almost continuously
struggle in peak afternoon heat
stress the compressor
cost more over time than expected

For that side of the problem, read Undersized AC Symptoms and Is My AC Too Small?.

Manual J vs Online AC Calculator

This AC size calculator is a strong starting point, but it is still an estimate. HVAC professionals use a Manual J load calculation to choose the final system size.

Manual J considers:

insulation levels
window efficiency
home orientation
air leakage
duct losses
local climate data

If you are installing a new central air system, replacing equipment near the edge of a size range, or dealing with comfort issues already, a Manual J calculation is worth requesting.

When One Large Unit Isn’t the Best Answer

Homes above about 2,500 to 3,000 sq ft may perform better with:

dual systems
zoned HVAC setups
two-stage or variable-speed compressors

That is especially true in two-story homes or floor plans with hot upstairs rooms. In those cases, layout and zoning can matter more than simply choosing the biggest available unit.

Bottom Line

Use this AC size calculator to get into the right range fast:

multiply square footage by the right climate-based BTU factor
divide by 12,000 for tonnage
adjust for ceiling height and humidity
confirm the final result with Manual J

Correct AC sizing improves indoor comfort, lowers energy waste, protects equipment life, and helps the system control humidity properly. If you are not sure where to start, begin with your home’s square footage, climate zone, and whether the house has standard or high ceilings.

AC Size Calculator (BTU & Tonnage Estimator)