AC Sizing Calculator
Estimate the cooling load for your ZIP code and house, split into sensible heat and moisture, and see the tonnage range that fits it. Estimate only; a room-by-room Manual J is what you size equipment and ducts from.
AC sizing calculator
Northeast US (default)Zone 5A · 5°F heating / 88°F cooling design
Estimate only. Not a substitute for a room-by-room Manual J.
Estimated cooling load
24,500 BTU/h
20,800 BTU/h – 28,200 BTU/h at 88°F outdoor
Sensible share
91%
22,300 BTU/h sensible / 2,200 BTU/h latent
Tonnage range (90–115%)
1.8 – 2.3 tons
of total cooling load
Nominal size to shop for
2 tons
nearest common size at or above the load
Humid climate (zone 5A): about 2,200 BTU/h of this load is moisture. Pick equipment whose sensible capacity covers the sensible load and whose latent capacity covers the rest at your design conditions. An oversized unit cools the air fast, shuts off and leaves the house clammy.
Nominal tons are a shopping shortcut. Equipment is matched from manufacturer data for the specific condenser, coil and airflow at your design temperature.
Assumptions behind this estimate
- Climate
- Northeast US (default) (zone 5A)
- Design temperature
- 88°F outdoor / 75°F indoor (approx. 1%)
- Insulation level
- average
- Window area / SHGC
- 349 sq ft / 0.55
- Solar gain
- Average of all orientations, no shading
- Occupants
- 3
- Size limits
- 90% to 115% of total cooling load (Manual S, cooling-only equipment)
- Duct gains
- Not included
Do this room by room in Tradewright
Real equipment matches, a 15-year cost comparison and a signed proposal, all from one visit.
Why “a ton per 500 square feet” misleads
Square-foot rules treat every house in every city the same. A shaded, well-insulated house near the coast and a glassy house in the desert can share a floor area and differ hugely in cooling load. Rules of thumb also drift toward bigger, because nobody gets a callback for a house that is too cold.
Oversizing hurts most in humid climates. An AC removes moisture only while the coil runs cold for a while. An oversized unit hits the thermostat setpoint quickly and shuts off before it has pulled much water out, which leaves the house cool and clammy.
The sizing rule for cooling-only equipment is tight: total capacity between 90% and 115% of the total cooling load, with sensible and latent capacity checked at design conditions (ACCA Manual S). If the next nominal size is outside that window, look at manufacturer data before rounding up.
Sources
How the estimate works
Step 1: Cooling design temperature
Your ZIP sets an approximate 1% cooling design temperature and tells us whether the climate is humid (zones ending in A).
Step 2: Sensible gains
Conduction through walls, windows and a hot attic, solar gain through glass, air leakage, people and appliances.
Step 3: Latent gains
Moisture from people and from outdoor air leaking in. Humid zones carry a much larger latent share.
Step 4: Manual S window
Total load × 90% and × 115% sets the tonnage range. The suggested nominal size is the nearest common size at or above the load.
Questions
What size AC do I need?
One whose capacity at your design temperature falls between 90% and 115% of the total cooling load. The calculator estimates that load and range from your ZIP, size and insulation.
How many square feet per ton of AC?
There is no reliable figure. Climate, insulation, windows, shading and air leakage change the cooling load per square foot so much that a fixed ratio will be wrong for most houses.
Is a bigger AC better?
No. Oversized units short cycle, remove less moisture and cost more. In humid climates that often means a cold, damp house.
What is sensible versus latent cooling?
Sensible cooling lowers air temperature. Latent cooling removes moisture. Equipment has to cover both at design conditions, and humid climates need more latent capacity.
Why does my climate zone letter matter?
IECC zones ending in A are moist. Outdoor air leaking in carries more water there, so the latent load is higher and equipment selection has to account for it.
Should I just replace with the same tonnage?
Not without checking. Insulation, windows and air sealing may have changed since the old unit went in, and the original size may have been a guess. Run a load calculation first.
Is this a real Manual J?
No. It is a whole-house estimate with averaged solar gain and generalized insulation values. A Manual J calculates each room including orientation and duct gains.
Get a room-by-room Manual J and real equipment matches in Tradewright.
A 20-minute demo on a real home in your territory.