HVAC sizing has a vocabulary problem: tons that weigh nothing, BTUs nobody defines, and a rule of thumb — "500 square feet per ton" — that gets repeated as if your house were an empty box in an unspecified climate. It isn't. Here's what the units actually mean, what really determines the size you need, and why the most common sizing mistake is buying too much.
Tons and BTUs, in plain English
A BTU (British thermal unit) is a unit of heat energy — roughly the heat from one lit match. Air conditioning capacity is measured in BTUs moved out of your house per hour, and a ton is shorthand for 12,000 BTU/hr. (The name is a leftover from the ice era: melting one ton of ice over 24 hours absorbs heat at about that rate.) Residential systems run from 1.5 tons (18,000 BTU/hr) to 5 tons (60,000 BTU/hr), usually in half-ton steps. Furnaces use BTU/hr directly — typically 40,000 to 120,000 — and the same sizing logic applies.
The square-footage starting point — by climate
Square footage is where sizing starts, and climate is the first thing that bends it. A 2,000 sq ft home in Phoenix carries a very different cooling load than the same floor plan in Minneapolis:
| Climate | Rough rule | 2,000 sq ft home lands near |
|---|---|---|
| Hot-dry (Phoenix, Las Vegas) | ~500–600 sq ft per ton | 3.5–4 tons |
| Hot-humid (Houston, Atlanta) | ~500–650 sq ft per ton | 3–4 tons |
| Mixed (Denver, Kansas City) | ~600–700 sq ft per ton | 3–3.5 tons |
| Cold (Minneapolis, Boston) | ~700–800 sq ft per ton | 2.5–3 tons |
Treat the table the way an installer should: as the first estimate a real calculation then corrects. Two same-size houses on the same street can legitimately need different equipment.
What a real load calculation looks at
The industry-standard method is called Manual J, and it exists because square footage ignores almost everything that determines heat gain and loss:
- Insulation levels — attic, walls, and floor. A well-insulated home can need a full ton less than a leaky one of the same size.
- Windows — how many, how big, single- or double-pane, and which way they face. West-facing glass is a heat engine every summer afternoon.
- Orientation and shading — a shaded north-facing home and a sun-baked corner lot are different math.
- Ceiling height — you cool volume, not floor plan. Vaulted ceilings add load the square footage never shows.
- Air leakage — older homes trade air with the outdoors constantly; tight new construction doesn't.
- Ductwork — leaky or undersized ducts waste capacity before it reaches a room, and a duct problem can masquerade as a sizing problem.
None of this requires standing in your kitchen. It requires knowing about your house — age, insulation, windows, stories — which is exactly what a good online sizing process asks, and what a bad in-home one skips on the way to reading your old unit's nameplate.
The oversizing trap
The instinct is understandable: if 3 tons is good, 4 must be safer. It isn't. An oversized air conditioner reaches the thermostat's set point too fast and shuts off — a pattern called short cycling — and that quick blast is the problem:
- Humidity stays. Dehumidification happens while the system runs. Short cycles pull the temperature down without pulling the moisture out, leaving the house cold and clammy — so you lower the thermostat further, chasing comfort the equipment can't deliver.
- Bills rise. Startup is the least efficient moment of every cycle, and an oversized unit lives in startups.
- Equipment wears early. Compressors age by starts as much as by hours. Short cycling is a lifetime of starts compressed into fewer years.
Undersizing has symptoms too
The rarer mistake, but a miserable one: an undersized system runs continuously on hot days and never reaches the set point, the rooms farthest from the equipment never get comfortable, and the bills are high because the system literally never rests. If your current AC runs nonstop every July afternoon and still loses ground after 4 p.m., it may be undersized — or its real capacity may have decayed with age and a slow refrigerant leak, which looks identical from the couch.
Why "same as the old one" can be wrong
The fastest way to size a replacement is to read the old unit's nameplate and match it. It's also how sizing mistakes get inherited for decades, because it assumes two things that are often false:
- That the original was sized right. Plenty weren't. Builders and past contractors oversized routinely — rules of thumb, safety margins on top of safety margins — and matching the nameplate photocopies the error into your next 15 years.
- That your house hasn't changed. New windows, added attic insulation, air sealing, a new roof — every envelope improvement since the old unit went in reduces the load. A home that needed 4 tons in 2008 may genuinely need 3 today, and the smaller system costs less to buy and to run.
A replacement is the one moment you get to correct the record. It's worth the extra questions.
Size is half the decision
Getting the tonnage right sets the ceiling on comfort; the equipment tier and installation quality decide whether you reach it. From here, the useful next reads are what a new system actually costs by size and what a proper install day looks like — because a correctly sized system installed right is the whole game, and neither half can rescue the other.