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HVAC Load & AC Size Calculator

Estimate whole-house cooling tonnage and heating BTU from your ZIP code and a few known details. Get a right-sized range — not an oversized guess — with the UA math shown.

Location

5-digit US ZIP. Unlisted ZIPs: switch to 'By climate zone'.

ft²

Heated/cooled floor area — exclude an unconditioned garage or attic.

Sets the envelope U-factors and window SHGC.

Window area

Glass as a share of floor area.

Sun exposure

Trees, awnings and neighboring buildings cut solar gain.

Air tightness

Older homes leak more air (higher infiltration load).

Advanced — design temps & setpoints
°F

ASHRAE 1% outdoor DB.

°F

ASHRAE 99% outdoor DB.

°F
°F
Recommended AC size
Pick a climate zone
—

ZIP not in the built-in table — pick your climate zone manually.

How the load estimate works

This is a UA block load: it estimates the house's envelope from its floor area, stories, and ceiling height, then builds a heat-loss coefficient (UA) from per-surface U-factors and air infiltration. Load is UA times the design temperature difference — the same physics a full load calc uses, run once for the whole house instead of room by room.

  • Heating = (envelope UA + infiltration UA) × (indoor − 99% outdoor design temp), with no solar or internal credit — the conservative case.
  • Cooling sensible adds envelope conduction, an attic sol-air adder on the ceiling, solar gain through glass, internal gains from people and appliances, and infiltration.
  • Cooling latent comes from the zone's design humidity and the occupants — the moisture the system has to remove, which dominates in hot-humid zones.

Tonnage is total cooling divided by 12,000 BTU/hr, reported as a ±15% range because a single- point number implies a precision a block load doesn't have. Climate zones follow the IECC 2021 map; outdoor design temperatures are representative values compiled from public NOAA station data, and the per-vintage U-factors are documented estimator assumptions, not reproduced code tables. Override the design temps in Advanced for a specific city.

Frequently asked

What size AC do I need for a 2,000 sq ft house?
There is no single answer — it depends on climate, insulation, glass, and air-tightness, which is exactly why this tool asks for them. As a range, a 2,000 ft² 1980s-era house lands near 2–3 tons in a cool climate like Chicago and 3–3.5 tons in a hot-humid climate like Houston. The old "1 ton per 500–600 sq ft" rule usually oversizes; size to the calculated load instead.
How does my ZIP code set the AC size?
Your ZIP maps to an IECC climate zone (1 very hot through 8 subarctic, plus a moisture letter A/B/C). Each zone carries representative outdoor design temperatures — the 1% summer dry-bulb for cooling and the 99% winter dry-bulb for heating. The bigger the gap between that outdoor design temperature and your indoor setpoint, the bigger the load. Unlisted ZIPs fall back to a manual zone picker.
Why is a bigger AC not better?
An oversized system cools the air to the thermostat setpoint before it runs long enough to pull humidity out, so it short-cycles: the house feels cold and clammy, comfort drops, and the compressor wears from frequent starts. Right-sizing to the load gives longer, steadier runtimes and better dehumidification.
How many BTU do I need to heat my house?
The heating side uses the same UA heat-loss coefficient times the temperature difference between your indoor setpoint and the 99% winter design temperature, with no solar or internal-gain credit (the conservative case). For the 2,000 ft² example in zone 5A that is roughly 55,000–65,000 BTU/hr of output — then match a furnace or heat pump to that output, not to its input rating.
Does this replace a professional load calculation for a permit?
No — use it to plan and to sanity-check a contractor's quote. This is a simplified whole-house block-load estimate, not an ACCA-approved Manual J® calculation, which is the room-by-room analysis a permit and final equipment selection require.
How accurate is a block-load estimate?
A block load treats the house as one zone, so it is good for whole-house sizing and quote-checking but not for duct design or room airflow. It is more nuanced than a flat BTU-per-square-foot rule because it accounts for climate, envelope, glass, and infiltration — but it uses representative design temperatures and preset U-factors, so verify inputs and override the design temps for a specific city.

Related tools

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Size the trunks and branches to deliver the CFM this load needs.

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Check airflow (350–400 CFM/ton) matches the tonnage you sized.

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Convert between airflow, capacity, and temperature split.

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Superheat & Subcooling

Charge the installed system to the manufacturer target.

Related guides

Sources & references

For permits and final equipment selection, a room-by-room ACCA-approved load calculation is required — this estimator is for planning and sanity-checking quotes.

Estimates for planning purposes only. Verify all results against the code edition adopted in your jurisdiction and with your Authority Having Jurisdiction (AHJ). This tool is not a substitute for a licensed HVAC contractor. See our methodology, sources, and code editions.