What size AC unit do you need?
By the HVAC Service Call editorial team · Sizing guidance cross-referenced from ACCA and manufacturer data
Most homes need roughly 20 BTU of cooling per square foot — about 2 tons per 1,000 sq ft — but climate zone, insulation, windows, and ceiling height shift the answer by up to 30%. A Manual J load calculation is the only accurate size; everything else is a starting estimate. This page sizes central systems; window and room units use a different chart.
A ballpark for budgeting. A Manual J is the number you actually buy with.
Tons, BTU, and reading your nameplate
AC size is measured in tons — 1 ton equals 12,000 BTU per hour of heat removal (nothing to do with weight). Residential units come in half-ton steps from 1.5 to 5 tons. Size isn't the same as efficiency: size is capacity, while SEER2 and EER2 measure how cheaply the unit delivers it — both are printed on the label.
To find your current size, read the model number on the outdoor unit and find the two- or three-digit code divisible by 12, then divide by 12:
...24ABC36W... → 36 ÷ 12 = 3 tons
Tonnage by square footage
A 2,000 sq ft home lands around 3–4 tons; here's the full range, plus what each size covers.
| Home size | Tonnage | Capacity | Inverse: what it covers |
|---|---|---|---|
| 600–1,000 sq ft | 1.5 tons | 18,000 BTU | A 1.5-ton unit cools a small home or condo |
| 1,000–1,500 sq ft | 2–2.5 tons | 24,000–30,000 BTU | A 2-ton unit covers roughly 1,000–1,300 sq ft |
| 1,500–2,000 sq ft | 2.5–3 tons | 30,000–36,000 BTU | A 3-ton unit covers roughly 1,500–1,800 sq ft |
| 2,000–2,500 sq ft | 3–4 tons | 36,000–48,000 BTU | A 4-ton unit covers roughly 2,000–2,400 sq ft |
| 2,500–3,000 sq ft | 4–5 tons | 48,000–60,000 BTU | A 5-ton unit covers roughly 2,500–3,000 sq ft |
Assumes a mixed climate and 8-foot ceilings. Adjust below.
Climate and ceilings shift the number
| Factor | Adjustment |
|---|---|
| Hot-humid Southeast | Size up ~one notch |
| Hot-dry Southwest | Baseline to +½ ton |
| Cold North | Size down ~one notch |
| Ceilings over 8 ft | +10% per extra 2 ft |
Your city's design temperature — the local hot-day target a system is sized for — is what ultimately decides the number.
Why wrong sizing costs you
Too big
An oversized AC cools the air fast, then shuts off before it pulls out humidity — a clammy 72° that feels like 78°. The constant short-cycling wears the compressor and pushes bills up. Bigger is the belief that sells the wrong unit.
Too small
An undersized AC never reaches setpoint on a hot day, so it runs a marathon and ages its parts fast, while the house never quite gets comfortable.
Three sizing mistakes to avoid: sizing from the old unit, sizing from square footage alone, and letting the biggest quote win.
Manual J
A Manual J is a room-by-room load calculation, and it's the number you buy with. A contractor who quotes a size without one is guessing. It reads your insulation, your windows and their orientation, air infiltration, room volumes, local design temperatures, and occupancy.
Online calculators — including the one on this page — are a ballpark for budgeting. The Manual J is what confirms it. Choosing who runs it: how to choose an HVAC company.
Sizing a replacement — don't just copy the old unit
Your envelope has changed since the last install — new windows, added insulation, air sealing — and the original unit may have been mis-sized from day one. Don't default to the old tonnage. Tell the contractor what actually happens in your home: last summer's hot-room map, how long it ran to keep up, and any additions or renovations. That's what makes the replacement right, not a repeat of an old mistake.
Still deciding whether to replace at all? See repair or replace.
One call routes you to a licensed local contractor: (888) 810-2291.
Common questions
What size AC do I need for a 2,000 sq ft house?
Roughly 3 to 4 tons (36,000–48,000 BTU) as a starting estimate. A hot-humid climate, high ceilings, or lots of west-facing glass push it toward 4 tons; a well-insulated home in a mild climate toward 3. A Manual J calculation settles it.
How many square feet does a 2-ton AC cool?
About 1,000–1,300 square feet in a typical climate with 8-foot ceilings — less in hot, humid, or poorly insulated homes.
How many square feet does a 3-ton AC cool?
Roughly 1,500–1,800 square feet under average conditions, adjusted for climate, insulation, and ceiling height.
How many square feet does a 4-ton AC cool?
Around 2,000–2,400 square feet in a typical setup, shifting with your climate zone and home envelope.
Is it better to oversize the AC slightly?
No. An oversized AC short-cycles — it cools the air fast but shuts off before it removes humidity, leaving the house cool but clammy, and the frequent starts wear the compressor. Right-sized beats oversized.
Does ceiling height matter?
Yes. The rule of thumb assumes 8-foot ceilings; every extra 2 feet adds roughly 10% to the cooling load because there's more air volume to condition.
How do I find my current AC's size?
Read the model number on the outdoor unit's nameplate and look for a two- or three-digit number divisible by 12 — 24, 30, 36, 48, 60. Divide it by 12 for the tonnage: 36 means 3 tons.
What sizes do AC units come in?
Residential central ACs come in half-ton steps from 1.5 to 5 tons — 1.5, 2, 2.5, 3, 3.5, 4, 4.5, and 5.