Understanding your result
The headline is the verdict, with the installed capacity, the estimated load and the ratio in the sub-line. The bars put the load and the installed capacity side by side. The table shows both, the ratio, the range ACCA Manual S accepts (90 to 115% for cooling, 90 to 140% for a furnace), the right size by this estimate and the verdict.
The verdict has five possibilities: oversized, undersized and about right, plus about right on paper but with oversizing symptoms (short cycles or clamminess at a ratio slightly above 100%), and about right on paper but with undersizing symptoms. The last two mean the size is probably not the cause and something else, often the ducts or the thermostat, deserves a look.
The load comes from the same climate-zone method as the central AC size calculator and the furnace size calculator. If the system is oversized and old, the lifespan planner shows when it is due for replacement; if the complaint is humidity, the dew point calculator shows how humid the air really is.
How we calculate this
The loads use the climate-zone figures on the BTU per square foot table with the insulation and ceiling factors used across the site. The accepted ranges follow ACCA Manual S, which allows cooling equipment up to 115% of the calculated load (up to 125% for heat pumps in heating-dominated climates) and furnace output up to 140%. A furnace’s output is its input multiplied by its AFUE, which the efficiency ratings table explains.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Cooling and heating loads per sq ft | By climate zone | Rule-of-thumb climate-zone charts; see the BTU per square foot table |
| People and kitchen | 600 BTU/hr per person beyond two; 4,000 for a kitchen | ENERGY STAR room AC guidance |
| Cooling range | 90 to 115% of the load | ACCA Manual S |
| Furnace range | 90 to 140% of the load | ACCA Manual S |
| Undersized threshold | Below 90% | Our threshold; a little short is tolerable on design days |
Assumptions last reviewed October 8, 2026.
The estimate cannot see the house’s actual windows, orientation, air leakage or duct losses, which a Manual J captures, and a 20% error either way is possible. It also takes the nameplate at face value, while an old unit’s real capacity may have fallen through refrigerant loss or a dirty coil. The guide on why oversized systems cost comfort and money covers the symptoms in detail, and the guide on how to size an air conditioner explains what a Manual J adds.
Two worked examples
A 4-ton air conditioner on a modest house
4 tons installed, 1,800 sq ft in zone 4, average insulation, 8 ft ceilings, three people and a kitchen.
- Load: 1,800 × 18 = 32,400 + 600 + 4,000 = 37,000 BTU/hr
- Installed: 4 × 12,000 = 48,000 BTU/hr
- Ratio: 130%, above the 115% limit
- Verdict: Oversized; the estimate’s size is 3.5 tons
A 4-ton unit on a 3-ton load short-cycles on most summer days. When it is replaced, a 3.5-ton or a variable-speed unit will be quieter, drier and cheaper to run; a Manual J will settle between 3 and 3.5 tons.
A 100,000 BTU furnace that short-cycles
100,000 BTU input at 80% AFUE, 1,600 sq ft in zone 4, average insulation; the owner reports short cycles.
- Load: 1,600 × 38 = 60,800 BTU/hr
- Installed output: 100,000 × 0.80 = 80,000 BTU/hr
- Ratio: 132%, inside the 140% furnace limit
- Verdict: About right on paper, with oversizing symptoms
By Manual S the furnace is acceptable, so the short cycles probably have another cause: a dirty flame sensor, a high-limit trip from a clogged filter or closed registers, or a thermostat in a draft. A technician should check those before anyone proposes a smaller furnace, though a 76,000 BTU input unit would be the right size at replacement time.
Where to find your inputs
Installed size. For an AC or heat pump, the model number on the outdoor unit: the two digits after the series letters are thousands of BTU per hour (18, 24, 30, 36, 42, 48, 60). For a furnace, the input and AFUE on the nameplate inside the cabinet.
House. The conditioned floor area, climate zone by county, and insulation by the age and condition of the house.
Symptoms. Time a cooling or heating cycle on a hot or cold day with a watch; under 10 minutes is short.
Common mistakes
- Assuming bigger is safer. Oversized air conditioners leave houses humid and wear out sooner.
- Reading the wrong number. The model number’s digits are capacity; the nameplate’s amps and volts are not.
- Using furnace input as output. Multiply by the AFUE.
- Blaming the size for every problem. Ducts, filters and thermostats cause most comfort complaints.
- Replacing like for like. Check the size before the old one is copied.
- Trusting one hot day. Running nonstop on the hottest afternoon of the year is what a correct size does.
Questions people ask
- How do I know if my AC is oversized?
- Compare its size with the house's cooling load. The model number usually encodes the capacity in thousands of BTU per hour (36 = 3 tons, 48 = 4 tons). If the load estimated for your house and climate is 37,000 BTU per hour and the unit is 4 tons (48,000), it is 30% oversized, beyond the 15% that ACCA Manual S allows. The symptoms confirm it, cooling cycles under 10 minutes on hot days, a house that is cool but clammy, and wide temperature swings.
- What are the signs of an undersized air conditioner?
- It runs nonstop on the hottest afternoons and the house drifts a few degrees above the setting, then catches up in the evening. That is normal on two or three design days a year; if it happens on ordinary summer days, the unit may be undersized, or more often the ducts leak, the filter is clogged, the refrigerant charge is low or the attic insulation is poor. Check those before buying a bigger unit.
- How much bigger than the load can a furnace be?
- ACCA Manual S allows a furnace's output to be up to about 140% of the calculated heating load, because furnaces come in coarse 20,000 BTU steps and some oversizing does little harm. A 100,000 BTU input furnace at 80% AFUE delivers 80,000 BTU per hour; on a 60,800 BTU per hour load that is 132%, inside the range. Two-stage and modulating furnaces make even larger margins harmless.
- Why does an oversized air conditioner make the house humid?
- An air conditioner removes moisture only while its coil is cold and wet, and it takes several minutes of running before the coil starts draining water. An oversized unit satisfies the thermostat in a few minutes and stops, then the moisture on the coil evaporates back into the air. The house reaches the temperature without losing the humidity, which feels cold and clammy. Correct sizing or variable-speed equipment fixes it.
- Should I replace my AC with the same size?
- Not without checking. Many existing systems were sized by rule of thumb or replaced like for like for decades, and houses have often been insulated, re-windowed or air-sealed since. A Manual J load calculation is the standard; this calculator gives a first estimate. If the existing unit is well over the estimate and shows oversizing symptoms, ask the contractor to size from a load calculation.
- Can a correctly sized system still short cycle?
- Yes. A thermostat near a supply register or in the sun, a dirty furnace flame sensor, a high-limit switch tripping on poor airflow, a refrigerant problem or a failing control can all cause short cycles on a correctly sized system. If the size check says about right but the system short cycles, a technician should look for those causes before anyone blames the size.