AC Tonnage Calculator

A ton of refrigeration is 12,000 Btu per hour, by definition, from the heat needed to melt a short ton of ice in a day. It has nothing to do with the weight of the equipment.

It is a rate of heat moved, and that is the distinction people trip on. 12,000 Btu/h is about 3,517 watts of heat shifted out of the house. The electricity the compressor draws to do it is far less, because a heat pump moves heat rather than making it, and the ratio between the two is the efficiency rating.

So a 3 ton system is not a 10.5 kW electrical load. It moves 36,000 Btu/h and draws whatever its efficiency requires to do so, which is a different number entirely and the one that matters for the electrical side.

And equipment comes in half-ton steps. Which means the answer to a load calculation is almost never a size you can buy, and rounding is a decision: up, and you are into the oversizing problem above; down, and you have a system that runs continuously on the hottest afternoon, which is what it was designed for.

Heating is one job. Cooling is two, and the second one is the reason oversizing an air conditioner fails rather than merely wasting money.

Sensible load is heat you can measure with a thermometer. Latent load is moisture, and removing it takes energy that never shows up as a temperature change at all: roughly 1,000 Btu per pound of water condensed out of the air.

• Cooling and drying are separate jobs. A coil removes heat from the air and, once its surface is below the dew point, water as well. Both happen only while the coil is running.
• An oversized system stops before it has dried anything. It pulls the temperature to setpoint quickly, satisfies the thermostat and shuts off. The sensible job is done and the latent job is barely started.
• Which is why the failure is a cold, clammy house. Not a warm one. A house that feels damp at 72 degrees usually has too much equipment rather than too little, and no amount of extra capacity fixes it.
• And why short cycling is the symptom to look for. Frequent short runs are what oversizing produces, and they are visible without any instrument.

Sensible heat ratio is the share of a system's capacity that goes to lowering temperature rather than removing moisture. It is a property of the equipment at a stated condition, not a constant, and it appears on the expanded performance data rather than the box.

Which is why this page asks for it rather than assuming, and why the sensible figure a conduction calculation produces is not the whole answer even when it is exactly right.

Residential equipment comes in half-ton steps: 1.5, 2, 2.5, 3 and so on. A load calculation produces a number like 2.67 tons, which is not on the list.

So every job ends in a rounding decision, and the calculator prints both options with the percentage each is off by, because that is the information the decision actually needs.

The size down runs longer on the hottest afternoon. That is not a fault: design conditions are deliberately exceeded a few hours a year, and a system running continuously on the worst day is a system that is correctly sized. It also dehumidifies better, because it runs longer.

The size up idles. On a mild day it satisfies the thermostat in minutes and stops, which is where the moisture problem and the short cycling live.

Variable capacity equipment changes this calculation entirely, because it can modulate down to match a part load rather than cycling. That is a different product decision and not a sizing one.

Twelve thousand Btu per hour is about 3,517 watts of heat moved. It is not 3,517 watts of electricity drawn, and the gap between those two is the entire point of a heat pump.

At SEER 16 a three ton system moving 36,000 Btu/h draws roughly 2,250 W. That is about 16 watts of heat shifted for every watt consumed, which is why cooling is cheap to run relative to what it achieves and why resistance heating is expensive relative to the same.

The practical consequence is on the electrical side. Sizing a circuit or a generator for an air conditioner means using the electrical draw from the nameplate, not converting the tonnage. Converting the tonnage overstates it by more than an order of magnitude.

The watt calculator on this site does that conversion and shows the definition behind it.

It does not produce a load. It converts one. If you arrived with floor area and nothing else, the rule-of-thumb mode exists to show you what that method produces, and the page argues against using it.

It does not select equipment. That is ACCA Manual S, which matches specific equipment to a load at design conditions and accounts for how the capacity splits between sensible and latent at those conditions. It is a separate standard and it is not free.

It does not size ducts. Manual D, also not free. A correctly sized system on undersized ducts performs like an incorrectly sized one.

And it cannot see your ducts' location. Ductwork running through an unconditioned attic loses a substantial fraction of the capacity before it reaches a room, and that loss is part of a real load calculation rather than a correction applied afterwards.

The cooling version of the rule of thumb is square feet per ton: pick a number between 400 and 600, divide, and buy that. It has the same defect as the heating version and the same source objects to it:

“Rules of thumb” are too often used to size comfort systems, which results in excessively oversized systems. This leads to increased cost, wasted energy, and too-frequent on and off cycling, which can lead to comfort and efficiency issues. A trained contractor or qualified designer will determine a home's specific heating and cooling loads to establish the right size equipment.

Source: Proper Sizing HVAC System, Building America Solution Center, Pacific Northwest National Laboratory for the US Department of Energy. Read 2026-09-08.

The reason it is worse for cooling is the latent half. A square-foot rule cannot see glazing, which is where solar gain enters and which is almost entirely a cooling load. It cannot see how many people are in the house, and people are a latent load. It cannot see whether the ducts run through a hot attic.

So the calculator on this page takes a load in Btu per hour, from a calculation rather than a rule, and converts it. If all you have is floor area, the honest position is that you do not yet have a load.

Read 2026-09-08.

The design temperature difference is not published here either. Winter design temperature is a local value, it appears in IRC Table R301.2(1) as a jurisdiction-by-jurisdiction figure, and the calculator asks you for it rather than assuming a climate.