Air Conditioner Size Calculator
Oversizing is the most common air-conditioner mistake, and it backfires in a specific way. An AC removes heat two ways: sensible cooling (lowering the temperature) and latent cooling (removing moisture). An oversized unit cools the air fast, satisfies the thermostat, and shuts off before it has run long enough to wring any water out of the air. The result is a house that feels cold and clammy, with high humidity, mold risk, and short-cycling that wears the equipment out. That is why ACCA Manual S sizes a single-stage AC at just 100 to 115 percent of the calculated load — this calculator applies that limit and reports the sensible and latent split so you can see the moisture load a bigger unit would fail to handle.
It runs on the same published physics as a professional load calc — conduction (U-value times area times the design temperature difference), window solar gain by orientation, internal gains from people and appliances, and infiltration (1.08 times the leakage airflow times the temperature difference for heat, 0.68 times the humidity difference for moisture), with a duct-loss adder — using the ASHRAE 1 percent cooling design temperature for your climate zone. Windows usually drive 25 to 40 percent of the cooling load, which is why the calculator asks for glass area by direction: west and east glass catch the afternoon and morning sun and dominate the peak.
What it is not is a substitute for a real Manual J. It is a screening estimate to help you right-size before you shop, sanity-check a contractor who wants to sell you a bigger unit, and understand where your cooling load comes from. For the heating side, pair it with the furnace size calculator, and for both together use the full Manual J load calculator. A full ACCA Manual J and Manual S by a professional are still required for a permit and the final install. Free, no signup — part of the HVAC and ventilation section.
AC Size Calculator
What size air conditioner do you need? Tonnage should match your home’s cooling load — not its square footage. This free AC size calculator estimates the cooling load the way a Manual J does, from your home’s size, climate, insulation, windows by direction, and air leakage, then gives the right nominal tonnage under the ACCA Manual S sizing rule. It shows the load-based size next to the “500 sq ft/ton” rule of thumb so you can see how much rules oversize — and why an oversized AC leaves a house cold and clammy. Screening estimate — a full Manual J is still required for a permit. Free, no signup.
Your home
Conditioned floor area, ceiling height, stories, and bedrooms (bedrooms set the default occupant count).
Climate & construction
Climate sets the ASHRAE 1% cooling design temperature and humidity. Insulation sets the envelope U-values.
Windows by direction
Approximate glass area (ft²) facing each direction — sunlight through glass is often 25–40% of the cooling load, and west/east glass drives the afternoon peak.
Air leakage & ducts
Infiltration and duct location are big swing factors. Ducts in a hot attic add 15–30% and pull humid air into the system.
Calculation Formulas
Heat conducted in through each surface equals its U-value times area times the cooling design temperature difference. The ceiling uses a higher "sol-air" ΔT because a vented attic runs 120–140°F.
Example:
2,000 ft² of R-49 ceiling (U ≈ 0.026) at a 41°F attic ΔT = 0.026 × 2,000 × 41 ≈ 2,130 BTU/h.
Indoor design is 75°F. The outdoor value is the ASHRAE 1% summer dry-bulb — the temperature exceeded only 1% of hours — so the AC is sized for a normal hot afternoon, not the record high.
Example:
Houston (Zone 2A) 1% design ≈ 95°F, so ΔT = 95 − 75 = 20°F.
Sunlight through glass is often 25–40% of the cooling load. West and east glass dominate the afternoon and morning peak; north is diffuse only. Low-SHGC glazing and shading cut it sharply.
Example:
50 ft² of west glass, SHGC 0.35, with blinds (0.75): 0.75 × 0.35 × 50 × 85 ≈ 1,100 BTU/h.
People and appliances add heat and moisture to the cooling load. Default occupancy is bedrooms + 1; the kitchen/appliance default is 1,200 BTU/h sensible.
Example:
4 occupants: 4 × 230 = 920 BTU/h sensible + 800 latent, plus 1,200 appliance.
Hot outdoor air leaking in adds sensible heat, and its moisture adds latent load. Dry climates have near-zero grains and essentially no latent load; humid climates carry a large one.
Example:
107 CFM at 20°F ΔT = 2,310 BTU/h sensible; × 30 grains × 0.68 = 2,180 BTU/h latent.
One ton is 12,000 BTU/h. Manual S sizes a single-stage AC at 100–115% of the total cooling load. Two-stage and variable-speed (inverter) systems allow a wider window because they run at part load.
Example:
A 21,000 BTU/h load = 1.75 tons; the 115% limit is 2.0 tons, so a 2-ton unit fits.
An AC needs roughly 400 CFM of air per ton to move its rated capacity. Too little airflow freezes the coil; too much hurts dehumidification. The ductwork must match the equipment.
Example:
A 2-ton system needs about 800 CFM of blower airflow.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Sensible air multiplier | 1.08 | 0.075 lb/ft³ × 0.24 BTU/lb·°F × 60 min/h (standard air). |
| Latent air multiplier | 0.68 | 60 × 0.075 × 1,061 BTU/lb ÷ 7,000 grains/lb. |
| Indoor cooling design | 75 °F / 50% RH | ACCA Manual J default cooling setpoint and humidity. |
| BTU per ton | 12,000 | One ton of cooling = 12,000 BTU/h. |
| Manual S cooling limit | 100–115% | Single-stage nominal capacity vs the calculated load. |
| Airflow per ton | ≈400 CFM | Blower airflow the coil needs per ton of capacity. |
| Occupant gain | 230 + 200 BTU/h | Sensible + latent per person; default occupancy = bedrooms + 1. |
| Attic sol-air bonus | +25 °F | Added to the ceiling cooling ΔT for a vented attic (120–140°F). |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
ACCA Manual J — Residential Load Calculation(ANSI/ACCA 2-2016 (8th ed.))
View StandardThe standard for the cooling load an AC must meet, separating sensible and latent loads. Required for most permits. This tool uses a simplified whole-house version.
Key Requirements:
- •Use the ASHRAE 1% cooling design temperature and coincident humidity
- •Account for conduction, solar, internal, infiltration, and duct gains
- •Report sensible and latent loads separately
ACCA Manual S — Equipment Selection(ANSI/ACCA 3)
View StandardMatches the AC to the load. A single-stage system is sized at 100–115% of the total cooling load; oversizing beyond that short-cycles and fails to dehumidify. Latent capacity must meet the latent load in humid climates.
Key Requirements:
- •Total capacity ≥ 100% and ≤ 115% of the load (single-stage)
- •Verify latent (moisture) capacity against the latent load
- •Two-stage/variable-speed systems allow a wider window
IECC — mechanical sizing (R403.7)(IECC 2021 R403.7.1.1)
View StandardRequires equipment to be sized from a Manual J load and a Manual S selection, and limits cooling capacity to no more than 1.15× the design cooling load for a single-stage system.
Key Requirements:
- •Cooling capacity ≤ 1.15 × the calculated cooling load (single-stage)
- •Equipment selected per Manual S
AHRI / SEER2 efficiency ratings(AHRI 210/240 (SEER2))
View StandardAir conditioners are rated for efficiency by SEER2 and for capacity in tons/BTU. Capacity (tonnage) is what this calculator sizes; SEER2 is a separate efficiency choice that affects running cost, not the load.
Key Requirements:
- •Match nominal tonnage to the Manual S selection
- •Higher SEER2 lowers operating cost but does not change the required tonnage
International Energy Conservation Code — envelope(IECC 2021 R402)
View StandardPrescriptive envelope U-factors by climate zone (used by the "current code" insulation tier) that determine how much heat conducts and radiates into the home.
Key Requirements:
- •Envelope U-factors and window SHGC vary by climate zone
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Hot-humid climates: latent load is king
Zones 1A–3A, Gulf & Southeast
Humid climates carry a large moisture load, so the sensible heat ratio drops toward 0.70–0.80. An oversized AC short-cycles and never removes that moisture, leaving the house cold and clammy. Right-sizing and a variable/2-stage unit matter more than raw tonnage.
Regional Examples:
Hot-dry climates: almost no latent load
Zones 2B/3B, Southwest
In arid climates the design grains are near zero, so the load is almost all sensible. High design temperatures and roof/west-glass solar drive the size, and evaporative cooling can be an efficient alternative.
Regional Examples:
Cold and marine climates: small AC
Zones 5–8, 4C
Cooling loads are modest in cold and marine climates, and heating usually dominates. A small AC — or the cooling side of a heat pump — is typically right; don’t upsize the AC because of the heating equipment.
Regional Examples:
Duct location changes the size
All zones
Ducts in a hot vented attic add 15–30% to the cooling load and pull humid attic air into the system. Sealing and insulating them — or moving them into conditioned space — can drop the size by half a ton.
Regional Examples:
Rules of thumb oversize everywhere
National
The 400–600 ft²/ton contractor rule ignores the envelope, climate ΔT, and the sensible/latent split. Real Manual J results average around 1,400 ft²/ton, so rules of thumb typically oversize by 2–3× — the leading cause of comfort and humidity complaints.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Related Calculators
Furnace Size Calculator
Free furnace size calculator: the heating load and furnace BTU (input, output, and AFUE) your home needs by size, climate, and insulation. No signup.
Manual J Calculator
Free Manual J calculator: estimate your home's heating and cooling load in BTU and tons from size, climate, insulation, and windows. No signup.
HVAC BTU / Manual J Calculator
Cooling + heating BTU, AC tonnage, furnace size, heat-pump balance point — per ACCA Manual J / S, ASHRAE 1%/99%. DOE 2023 SEER2. Free.
Ceiling Fan Size Calculator
Free ceiling fan size calculator: the right blade span for your room, plus mount type and downrod length by ceiling height. Per ENERGY STAR. No signup.
Insulation Calculator
Free insulation calculator for fiberglass batts, blown-in cellulose, spray foam & rigid board. Bags by square footage, R-value by climate zone, code check.
How to Use This Calculator
- Enter your conditioned floor area, ceiling height, stories, and bedrooms (bedrooms set the default occupant count).
- Pick your climate zone — it sets the ASHRAE cooling design temperature and the design humidity that drives the latent load.
- Choose your insulation level, window type, and foundation.
- Enter approximate glass area facing each direction — west and east glass drive the cooling peak — plus interior shading and the number of doors.
- Set air tightness and where the ducts run, then click Calculate.
- Read the cooling load, the Manual S tonnage, the sensible/latent split, and the rule-of-thumb comparison; use the breakdown to see what is driving the load.
Why bigger is worse, and what tonnage really means
A "ton" of air conditioning is 12,000 BTU per hour of heat removal — the name comes from the cooling power of a ton of melting ice. Residential systems run from 1.5 to 5 tons in half-ton steps. The instinct to buy a bigger AC "to be sure it keeps up" is exactly backwards: an oversized unit reaches the thermostat setpoint quickly on temperature but shuts off before it dehumidifies, so on a humid day the house is cold and sticky. It also short-cycles — frequent on/off bursts that stress the compressor and waste energy. ACCA Manual S therefore caps a single-stage AC at 115 percent of the load; two-stage and variable-speed (inverter) systems can run at part load, so they hold a wider size window and dehumidify better. Tonnage is about capacity, not efficiency — SEER2 is the separate efficiency rating that affects your electric bill but not the size you need. This calculator estimates the load and applies the Manual S window, but it is a screening tool: get a full ACCA Manual J and Manual S, especially in humid climates or if your home has a lot of west-facing glass.
Frequently Asked Questions
What size air conditioner do I need?
Match the tonnage to your home's cooling load, not its square footage. A typical well-insulated 2,000-square-foot home often needs only about 2 tons, while a leaky older home in a hot-humid climate might need 3. The old rule of thumb of one ton per 400 to 600 square feet almost always oversizes — real load calculations average closer to 1,400 square feet per ton. Enter your home's details here to get a load-based tonnage under the ACCA Manual S sizing rule, then confirm it with a full Manual J.
What size AC do I need for a 2,000 square foot house?
It depends far more on climate, insulation, and windows than on the 2,000 square feet. A tight, well-insulated 2,000-square-foot home in a mild climate may need only 1.5 to 2 tons; the same size home that's leaky, in a hot-humid climate, with lots of west glass could need 3. The '1 ton per 500 square feet' rule would suggest 4 tons, which is usually one to two sizes too big. This calculator estimates the actual cooling load so you don't oversize and end up with a cold, clammy house.
How many tons of AC per square foot?
There's no single number, which is exactly the problem with square-foot rules. The traditional 400 to 600 square feet per ton ignores insulation, windows, air leakage, and climate — the things that actually determine the load. Modern, well-built homes commonly run 1,000 to 1,800 square feet per ton, so a contractor using 500 is often installing an AC two to three times larger than needed. That's why this calculator works from the envelope and climate instead of multiplying square footage by a fixed factor.
Is it bad to oversize an air conditioner?
Yes, and it's the most common AC mistake. An oversized unit cools the air to the thermostat setpoint quickly and shuts off before it has run long enough to remove humidity, so on a humid day the house feels cold and clammy — which invites mold and dust mites. It also short-cycles, turning on and off in rapid bursts that stress the compressor, wear out parts, and waste energy. ACCA Manual S caps a single-stage AC at 115% of the load for exactly this reason. A right-sized unit that runs longer, steadier cycles is more comfortable and lasts longer.
What's the difference between sensible and latent cooling?
Sensible cooling lowers the air temperature; latent cooling removes moisture (humidity). Your total cooling load is the sum of both, and the ratio between them — the sensible heat ratio — depends on climate. In a dry climate like Phoenix, the latent load is nearly zero and almost all the work is sensible. In a humid climate like Houston, latent can be 20 to 30 percent of the load, so the AC must be sized and selected to actually dehumidify. This calculator reports both so you can see whether moisture removal is a priority where you live.
Does a higher SEER AC change the size I need?
No. SEER2 is an efficiency rating — how much cooling you get per unit of electricity — while tonnage is capacity, the amount of heat the unit can remove. A 2-ton unit removes 24,000 BTU/hour whether it's 14 SEER2 or 20 SEER2; the higher-SEER model just does it using less electricity, lowering your bill. Size the tonnage to your load first, then choose the efficiency level that fits your budget and payback. Buying a higher-SEER unit is never a reason to change the tonnage.
Why do windows matter so much for AC sizing?
Sunlight streaming through glass is often 25 to 40 percent of the entire cooling load, and unlike a wall, a window lets that solar heat in almost instantly. Orientation is everything: west and east glass catch the low afternoon and morning sun and drive the peak, while north glass only sees diffuse light. That's why this calculator asks for glass area by direction rather than just a total — a house with lots of unshaded west windows can need noticeably more cooling than the same house facing north. Low-SHGC glazing or exterior shading on the west side has the biggest payoff.