What Size System Does Your Home Need?

Answer five questions to get the cooling tonnage, the heating BTU and the size range to shop, with every step of the math shown. Switch tabs to size mini split zones or to check a furnace's output.

Tell Us About The Space

Conditioned square feet

Ceiling height

Climate zone Click the colored region where you live, or choose a state default below.

United States HVAC climate zones, from red in the hottest regions through orange, yellow, and green to blue in the coldest regions.

Insulation and windows

Sun exposure

List The Rooms You Want To Condition

Climate zone Click the colored area where you live, or pick a zone below.

Each room gets its own indoor unit. The outdoor unit is sized to run all of them together.

Check A Furnace Against Your Heating Load

Where is the house? Click the colored area where you live.

Conditioned square feet

Climate zone Click the map or pick a zone

Insulation

Efficiency (AFUE)

Furnace you are considering Input BTU, optional

A furnace is labeled by its input. The heat that reaches the house is the output: input multiplied by AFUE.

Three Things Decide The Size

Square footage sets the starting point. The climate zone scales it. The house itself, through its insulation, sun exposure and ceiling height, pushes the number up or down. The calculator shows each step so you can see why it landed where it did.

Start With Square Footage

Cooling starts from a tonnage band for your floor area. Heating starts from a BTU per square foot figure for your zone, from 30 in the hot South to 60 in the far North.

Adjust For The House

Nine foot ceilings add about 12% more air to condition. Full sun and a lot of glass add load. Newer insulation and windows take load away. These three factors move the number more than anything else.

Do Not Round Up

An oversized air conditioner cools the air before it has dried it, cycles on and off too often, and wears out early. If you land between two sizes, the smaller one is usually right unless the house is drafty.

Find Your Climate Zone

These are the five zones used throughout this guide. Choose the one that matches your winters as well as your summers.

United States HVAC climate zones with state outlines: red for hot, orange for warm, yellow for mild, green for cool, and blue for cold regions.

Cooling Tonnage By Floor Area

Assumes 8 ft ceilings and average insulation. The calculator above adjusts these figures for your house.

Heating BTU Per Square Foot

Zone	Well Insulated	Average	Older Or Drafty

Size Each Room, Then Add Up The Indoor Units

Each indoor unit is sized for its own room. The outdoor unit must be able to run every indoor unit at once, which is why multi zone condensers are rated by total connected capacity.

Room Size	Indoor Unit	Typical Room
Up To 250 Sq Ft	6,000 BTU	Bedroom Or Office
250 To 400 Sq Ft	9,000 BTU	Large Bedroom
400 To 550 Sq Ft	12,000 BTU	Living Room
550 To 1,000 Sq Ft	18,000 BTU	Open Living And Dining Area
1,000 To 1,500 Sq Ft	24,000 BTU	Great Room Or Small Apartment
1,500 To 2,000 Sq Ft	30,000 BTU	Open Floor Plan
2,000 To 2,500 Sq Ft	36,000 BTU	Small Home Served By One Unit

Example: Three Rooms

A 200 sq ft office, a 350 sq ft bedroom and a 500 sq ft living room call for 6,000, 9,000 and 12,000 BTU indoor units. That adds up to 27,000 BTU connected, so the outdoor unit should be a 27,000 or 36,000 BTU three zone condenser, depending on how often all three rooms run at the same time. Use the Mini Split Zones tab above to build your own list.

The Label Shows Input. Output Is What Heats The House.

Output Equals Input Multiplied By AFUE

A 100,000 BTU input furnace delivers 80,000 BTU at 80% AFUE and 96,000 BTU at 96% AFUE. Compare the output with the heating BTU the calculator gives you, not the input printed on the box.

Count Every Conditioned Floor

Square footage means every floor the system heats, including a finished basement if it is ducted. Leave out the garage and any room that has its own mini split.

Before You Buy

Is this enough to order a system?

It is enough to shop the right size range and request a quote. A Manual J calculation by your installer confirms the size using your windows, orientation, duct layout and local design temperatures.

Why does the heat pump result show both tons and BTU?

A heat pump is sized for cooling in tons and then checked against the heating BTU. In cold zones the heating number can be the larger one. That is when a cold climate model or auxiliary heat strips come into the picture.

My current unit is a 4 ton. Should I replace it with another 4 ton?

Only if the original was sized correctly. Run the numbers first. Many older homes were oversized, and any insulation or window upgrades made since then will have lowered the load.

What about the ductwork?

Ducts are sized to the airflow of the new unit, roughly 400 CFM per ton. If you move up or down a size, have the installer check the trunk lines and returns.

Want A Second Opinion On The Size?

Send us the numbers from the calculator and a photo of your current unit's label. We will confirm the size and quote the system.

Request A Sizing Check

Estimates only. Final sizing requires a Manual J calculation by your installer.