What size AC do I need for my square footage, and does a contractor's number check out?
Enter your home's size and get a rough cooling ballpark in tons, built on ENERGY STAR's own sizing guidance. Then, if you have a quote, see whether the tonnage they proposed looks right-sized or oversized.
Free. Educational, not a quote. The core figures (roughly 20 BTU/hr per square foot, 12,000 BTU per ton, and a ±10% sun adjustment) come from ENERGY STAR (sources listed at the bottom); the climate and ceiling tweaks are general rules of thumb layered on top, not exact numbers. Important: ENERGY STAR and the U.S. Department of Energy both say a system should be sized with a full ACCA Manual J load calculation, not from square footage. This tool is a sanity-check to help you ask the right question, not a substitute for that calculation.
The rough answer: central cooling runs around 20 BTU per hour per square foot in a moderate climate, about one ton (12,000 BTU/hr) per 500–600 sq ft. So a 1,500 sq ft home lands near 2.5–3 tons as a ballpark. But that is only a sanity-check. ENERGY STAR and the DOE say to size a system with a full Manual J load calculation, because square footage ignores your insulation, windows, and air leakage. Use the number below to spot a wildly wrong quote, then ask to see the load calculation.
By the numbers
~20 BTU/hr per sq ft: rough moderate-climate cooling load; consistent with ENERGY STAR's area-to-capacity chart. ENERGY STAR room AC sizing chart
500–600 sq ft per ton: a ton is 12,000 BTU/hr, so ~20–24 BTU/sq ft works out to roughly this. Derived from the ENERGY STAR chart
±10%: reduce capacity ~10% for a heavily shaded home, add ~10% for a very sunny one. ENERGY STAR sizing adjustments
Oversized = clammy: too-big units cool the air before removing humidity, then short-cycle, wasting energy. ENERGY STAR
Manual J: the real sizing method; never accept sizing from square footage alone. ACCA Manual J / DOE
1. Your home
Rough numbers are fine. This is a ballpark, not a load calculation.
Leave this blank to just see the estimate. Enter it to check whether their number looks right-sized or oversized.
Rough cooling estimate
Enter your square footage
You'll get a ballpark tonnage range to sanity-check against a quote.
Read this before you buy anything. This is a square-footage ballpark, and ENERGY STAR and the DOE are explicit that square footage is not how to size a system. Its real job is to help you catch a number that is obviously off and to ask your contractor the one question that matters: “Can I see the Manual J load calculation for this size?” A contractor who sized it properly will have run one and should be willing to show you.
Why bigger isn't better
The most common sizing mistake is going too big, and it backfires.
An air conditioner only pulls humidity out of the air while it's running. An oversized unit blasts the temperature down and shuts off before it can dry the air, so the house feels cold and clammy. Then it turns back on a few minutes later: that constant short-cycling wastes energy and keeps the system stopping and starting all day. ENERGY STAR puts it plainly: an oversized unit “will cool the room, before it has a chance to remove the humidity.”
That's why the fix isn't a rule of thumb, it's a Manual J load calculation: the ACCA standard that accounts for your insulation, windows, orientation, ceiling height, and air leakage. The number this tool gives you is a starting point for the conversation, not the answer.
Get the one-page “right-sizing” cheat sheet
The questions that tell you whether a contractor actually sized your system, or just eyeballed it, plus what a real Manual J looks like. One email, no spam.
Questions people ask us
What size air conditioner do I need for my square footage?
As a rough starting point, central cooling runs around 20 BTU per hour per square foot in a moderate climate, roughly one ton (12,000 BTU/hr) per 500 to 600 square feet. A 1,500 sq ft home lands near 2.5 to 3 tons as a ballpark. But that's only a sanity-check: ENERGY STAR and the DOE both say a system should be sized with a full ACCA Manual J load calculation, not from square footage alone.
How many square feet does a ton of AC cover?
A common rule of thumb is about 500 to 600 square feet per ton (a ton equals 12,000 BTU per hour) in an average climate. Hot, sunny, or poorly insulated homes need more capacity per square foot; cool, shaded, well-insulated homes need less. Treat it as a ballpark for spotting a wildly wrong quote, not a substitute for a load calculation.
Is a bigger air conditioner better?
No. An oversized AC cools the air fast but shuts off before it removes humidity, so the house feels cold and clammy, and the constant short-cycling wastes energy. ENERGY STAR specifically warns that an oversized unit cools the room before it has a chance to remove the humidity. Right-sized beats oversized.
How do I know if a contractor oversized my system?
Enter your square footage and the tonnage they proposed into the estimator above. If their number is well above the square-footage ballpark, that's your cue to ask to see the Manual J load calculation that justifies it. A contractor who sized it properly will have run one and should be willing to show it to you; sizing by square footage or a rule of thumb is exactly what the DOE tells homeowners not to accept.
Can I use square footage instead of a Manual J calculation?
Only to sanity-check, never to buy. A Manual J load calculation factors in your insulation, windows, orientation, ceiling height, air leakage, and local climate. Things square footage can't capture. This tool gives you a ballpark so you can tell whether a proposed size is in the right neighborhood, then ask the right question: “Can I see the load calculation?”
Educational only. This estimator gives a rough, square-footage-based cooling ballpark to help you sanity-check a quote and ask better questions. It is not a load calculation, a specification, or a quote for your home. ENERGY STAR and the U.S. Department of Energy recommend sizing any system with a full ACCA Manual J load calculation performed by a qualified contractor, which accounts for insulation, windows, orientation, air leakage, and local climate. Real needs vary widely; do not purchase equipment based on this number.