AC Unit Size Chart & BTU Calculator

Read across the row, not down the column

Look at the 2,000 square foot line in the first chart. The same house, in the same climate, ranges from 2 tons to 5 tons depending only on how it was built.

That is a two-and-a-half times spread from a single variable that no square-footage chart can see. Insulation levels, air tightness, window area and orientation, duct location, and whether the ductwork leaks into an attic — those determine the load. Floor area only scales it.

So use the chart the way it's actually useful: as a sanity check, not a specification. If your house is 1,800 square feet and someone quoted you a 5-ton system, the chart tells you to ask why. If they quoted 3 tons, the chart says that's plausible and the conversation moves on to the real calculation.

Why oversizing is the more common failure — and the worse one

Contractors oversize far more often than they undersize, and the incentives explain it. An undersized system generates a callback in the first heat wave. An oversized one generates a comfort complaint that most people never connect to equipment capacity. One of those failure modes is visible and the other isn't.

But in a humid climate, oversizing is the one that actually hurts.

An air conditioner does two jobs, and only gets credit for one

Cooling equipment removes sensible heat (temperature) and latent heat (moisture). The thermostat only measures the first one. Moisture removal happens because air spends time on a cold coil — water condenses out and drains away — and that requires runtime.

An oversized unit satisfies the thermostat on temperature fast, then shuts off. The coil never gets properly cold and wet, the run cycle is too short, and moisture stays in the house.

If that describes a house you're looking at, the relative humidity calculator shows why turning the thermostat down makes it feel worse, and the dew point calculator will tell you whether the registers are actually below the dew point.

The other costs

• Short cycling. Five to ten minute run cycles instead of fifteen to twenty. Compressor starts are the hardest thing a compressor does, and more of them means shorter life.
• Uneven temperatures. Short cycles don't run long enough to move conditioned air to the far end of the duct system. The rooms furthest from the air handler never catch up.
• Worse efficiency than the label. SEER ratings assume steady-state operation. Efficiency during startup is poor, so a system that spends its life starting and stopping never reaches its rated performance.
• Higher cost, twice. More money for the equipment, and then more money running it badly.

Manual S: the rule that stops this

ACCA Manual S governs equipment selection once Manual J has given you a load. Its central constraint:

That ceiling is the part routinely ignored. Here's what it means in practice, given that equipment only comes in half-ton steps:

When two sizes both comply, take the smaller one in a humid climate. The smaller unit runs longer, and longer runtime is what removes moisture. In a dry climate the argument is weaker and either is defensible.

Note also the gap between 4 and 5 tons. A house with a 4.4 ton load has no compliant option — 4 tons is 91% (fine) but 5 tons is 114%, technically inside the window and in practice a meaningfully oversized machine. This is where a second system, zoning, or variable-capacity equipment earns its cost.

Variable-capacity equipment changes this calculus

Inverter-driven and two-stage systems can modulate down, so a nominal 4-ton unit might run at 1.5 tons on a mild day. That largely solves the short-cycling problem and is genuinely more forgiving of imperfect sizing. It does not make sizing irrelevant — a badly oversized variable system still has a minimum capacity it can't go below — but it widens the acceptable window considerably.

What a real load calculation accounts for that a chart cannot

ACCA Manual J is the standard, and it's referenced by the IRC. It looks at:

• Each facade separately — window area, glazing type and shading coefficient, by orientation. West-facing glass in the afternoon is a completely different load from north-facing glass.
• Each assembly's actual R-value — walls, ceiling, floor, slab edge.
• Infiltration, ideally measured with a blower door rather than assumed.
• Duct location and leakage. Ducts in a vented attic in Florida can add 20–30% to the load by themselves. Ducts inside conditioned space add nearly nothing.
• Internal gains — occupants, appliances, lighting.
• Local design conditions — the 1% design dry bulb and coincident wet bulb for your specific location, not a regional guess.
• Sensible and latent load separately — which is what actually determines whether equipment will control humidity, and which no square-footage chart can produce at all.

That last point is the real gap. This page can estimate capacity. It cannot tell you the sensible heat ratio, and in a humid climate the sensible heat ratio is the number that decides whether the house is comfortable.

Checking a system you already have

Switch the tool above to "Check if my current system is oversized" and it will report your square feet per ton against the typical band. Treat that as a screening result — but it pairs well with two things you can observe directly:

1. Time a run cycle on a hot afternoon. Fifteen to twenty minutes is healthy. Consistent five to eight minute cycles when it's 90 °F outside strongly suggests oversizing.
2. Put a hygrometer in the main living space. If the system is holding temperature but indoor RH sits above 55–60%, the equipment is not getting enough runtime to dehumidify. A cheap digital hygrometer costs about fifteen dollars and settles the argument.

Those two observations together are worth more than any chart on this page.

The honest bottom line

If you're replacing a system, the right sequence is: Manual J for the load, Manual S for the equipment, Manual D for the ducts. Skipping to equipment selection is how houses end up cold and clammy with a brand-new system in them.

A contractor who runs a real load calculation and shows you the output is telling you something meaningful about how they work. One who sizes from square footage, or simply matches whatever was there before, is repeating whatever mistake was made last time.

If the stakes justify an independent calculation — a new system, a major renovation, or a comfort problem nobody has been able to solve — that's what my practice does.