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Free HVAC Load Calculator
A fast Manual J-style estimator for residential cooling and heating loads. Built for techs and shop owners who need a quick sizing check before quoting a replacement — without booting up a $1,500 enterprise tool.
Quick answer
An HVAC load calculator estimates how many BTU/hr of cooling and heating a home needs based on IECC climate zone, conditioned square footage, ceiling height, insulation quality, sun exposure, windows, and occupants. As a residential rule of thumb, an average-insulated home needs roughly 20–32 BTU/hr per square foot of cooling (32 in Zone 1, 20 in Zone 5). This tool returns the answer in seconds; a full ACCA Manual J is required for new construction permits.
Inputs
When this calculator is enough
- Replacement quotes where the existing equipment is clearly oversized.
- Sanity-checking a competitor's bid before you walk a customer through it.
- Pricing conversations on the phone before you roll the truck.
- Training new techs on what drives load and how rule-of-thumb sizing fails.
When you need a full Manual J
- New construction or additions requiring a stamped load calc.
- Multi-zone or VRF system design with room-by-room loads.
- Anywhere the local AHJ asks for ACCA-compliant documentation.
How the math works
Each climate zone has a baseline cooling and heating BTU/hr per square foot, calibrated to typical Manual J residential results. We then adjust for ceiling height (volume), insulation/envelope quality, and west/south solar gain, and add fixed loads for occupants and windows. The total is rounded up to the next half-ton for equipment selection.
HVAC BTU per square foot by IECC climate zone
Baseline residential BTU/hr per square foot for an average-insulated home with 8-ft ceilings. Adjust for envelope, windows, and sun exposure.
| IECC Zone | Representative city | Cooling BTU/sqft | Heating BTU/sqft |
|---|---|---|---|
| Zone 1 | Miami, Honolulu | 32 | 10 |
| Zone 2 | Houston, Phoenix, Orlando | 28 | 15 |
| Zone 3 | Atlanta, Los Angeles, Dallas | 25 | 22 |
| Zone 4 | DC, Nashville, Albuquerque | 22 | 30 |
| Zone 5 | Chicago, Boston, Denver | 20 | 38 |
| Zone 6 | Minneapolis, Burlington | 18 | 45 |
| Zone 7 | Duluth, Fargo | 16 | 55 |
Template vs. running loads in Ratchly
| Workflow | This free calculator | Ratchly |
|---|---|---|
| Quick sizing check | Yes | Yes |
| Saved to the customer record | No | Attached to the job + quote |
| Auto-converts to a quote | No | One tap → quote → invoice |
| Tracks refrigerant + equipment history | No | EPA 608 log built in |
Run your whole shop on Ratchly
Scheduling, invoicing, refrigerant tracking, memberships, and quotes — built for 1–5 tech residential HVAC shops, no enterprise contract required.
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