Your contractor should run a Manual J load calculation. Square footage charts give a rough start only. Insulation, windows, air leaks, and local winter temps move the number more than floor area.
Start With Manual J, Not Square Footage
Manual J is the room by room math in ACCA Standard Manual J. It counts heat loss through walls and windows, air leakage, people and appliances inside, plus your local design temps.
The Department of Energy Building America program points to Manual J as the sizing basis for heating and cooling equipment (DOE HVAC sizing guidance (https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/strategy_guide_hvac_sizing.pdf)). Pacific Northwest National Lab says the same for cold regions, run Manual J first, then pick equipment (PNNL cold climate sizing guide (https://basc.pnnl.gov/resource-guides/cold-climate-heat-pump-sizing-and-selection)).
Ask for a copy of the report. It should list each room, the heating load in BTU per hour, the cooling load, and the outdoor design temps used.
What Tonnage Means In Plain Words
One ton equals 12,000 BTU per hour. A 3 ton unit moves about 36,000 BTU per hour.
Most single family homes land between 2 and 5 tons. That range is wide on purpose. A tight 2,000 square foot ranch with new windows can need less capacity than a drafty 1,400 square foot colonial with old storms.
ENERGY STAR publishes county level sizing tables as a rough check (ENERGY STAR sizing guidelines (https://www.energystar.gov/sites/default/files/asset/document/SizingGuidelines_0.pdf)). Use them to sanity check a bid. Do not buy off them alone.
Why Bigger Is Not Better
An oversized unit short cycles. It turns on, blasts, shuts off, repeats. You get clammy rooms in summer, temp swings in winter, extra wear, higher bills.
An undersized unit runs long and still misses setpoint on the coldest nights. Backup heat fills the gap and bills climb.
Right size means long steady runs. Rooms stay even. Humidity stays in check in summer. The unit lives longer.
What Moves The Number In Your Home
Insulation in attic and walls. Window age and count. Air sealing around rim joists, attic hatches, and duct joints. Duct location and leaks. Ceiling height. Shade and sun exposure. How many people live there. Your ZIP winter design temp.
A good tech measures rooms, peeks in the attic, counts windows, looks at ducts, and asks about cold spots. A fast quote that only asks square footage skips most of this.
Ductless And Ducted Size Differently
Ducted whole home systems get one central size from the whole house load. Duct condition matters. Leaky ducts in a cold attic change delivered heat.
Ductless gets sized zone by zone. Each indoor head has its own load. A living room head might be 12,000 BTU while a small bedroom needs 7,000. Your installer should show the per room plan.
Cold Places Need A Heating Check
In warm areas cooling often sets the size. In cold areas heating sets it.
Ask what outdoor temp the unit still meets your full heating load. Ask what happens below that point. Some plans carry backup heat. Some add a second head downstairs. Get that plan in writing before install day.
Questions To Ask Before You Sign
Did you run Manual J for my house. What design temps did you use. What heating and cooling loads did you find. Why did you pick this capacity. How does my ductwork change delivered output. What is the backup heat plan on design nights.
If a bid cannot answer those, get a second bid.