HVAC Tools
Find the right AC and heat size for your home, by square feet and climate zone.
Simplified Manual J. Enter your space and climate zone. Get BTU/hour required, tonnage, and a recommended unit size range with an over/undersized flag. (Load sizing means figuring out how large a system your space needs.)
Updated May 2026
Last reviewed May 2026
Sensible default. Modify inputs below to match your situation.
Cold-climate heat pumps rated to -15 F are the 2026 standard for Zone 6. Equipment lead times in ND run 4 to 12 weeks. Order before demo starts. Your local HVAC distributor is the fastest path; national brands include Carrier, Lennox, Trane, and Mitsubishi. No affiliate deal yet. We are working on it.
This is a simplified Manual J calculation, suitable for field estimates and preliminary equipment selection. It is not a substitute for a full ACCA Manual J load calculation for permitted installations.
| Zone | Description | Cooling BTU/sq ft | Example States |
|---|---|---|---|
| 1 | Hot, humid | 27.5 | S. Florida, Hawaii |
| 2 | Hot | 25 | TX, AZ, LA (south) |
| 3 | Warm | 21 | GA, NC, CA (coastal) |
| 4 | Mixed | 19 | KS, MO, OR (coastal) |
| 5 | Cool | 17 | IL, CO, WA |
| 6 | Cold | 15 | ND, MN, WI, MT |
| 7 | Very Cold | 12.5 | N. MN, N. WY, N. ME |
| 8 | Sub-Arctic | 10 | Interior Alaska |
Cooling BTU/sq ft rates reviewed by the FigureNerd HVAC-R team, 2026-05-16: warmer, more humid zones carry a larger cooling load per square foot than cold zones, even though cold zones carry the larger heating load. This calculator estimates cooling (AC) sizing. Heating loads run much higher in cold climates; see the Boiler Sizing Calculator for a heating-side estimate.
If you enter an existing system size, the calculator compares it against the calculated load. Here is how to interpret the flags:
"The biggest mistake on ND installs is undersizing the heating side because the contractor used a cooling-climate rule of thumb. Zone 6 needs 40 percent more capacity than you think." -- r/HVAC
"Manual J is the law in most jurisdictions for new construction. Simplified field estimates are fine for rough budgeting but always get a full J for the actual equipment purchase." -- r/HVACAdvice
"Oversized AC is the most common residential problem I see. Short-cycling is death to compressors and your humidity levels." -- r/HVAC
Manual J is the Air Conditioning Contractors of America (ACCA) standard method for calculating residential heating and cooling loads. It is the industry and code-required method for sizing HVAC equipment in new construction and major retrofits. A proper Manual J accounts for your specific climate zone, insulation levels, window count and orientation, ceiling height, infiltration rate, and internal heat gains from people and appliances. This calculator uses a simplified version of Manual J suitable for quick field estimates. For permit-required installations, a full Manual J by a licensed HVAC contractor or engineer is required.
BTU stands for British Thermal Unit, the amount of energy needed to raise one pound of water by one degree Fahrenheit. In HVAC, BTU/hour (also written BTUh) is the rate at which a system adds or removes heat. A system that produces 36,000 BTU/hour of cooling is a 3-ton unit (12,000 BTU/hour per ton). Heating systems are also rated in BTU/hour output. Your space's load calculation tells you how many BTU/hour are needed to maintain comfort during the design day: the hottest day for cooling, the coldest day for heating.
The rough rule of thumb is 20 to 25 BTU per square foot for cooling in a temperate US climate. For heating in a cold climate like North Dakota (IECC Zone 6), the load is significantly higher: roughly 40 to 60 BTU/hour per square foot of floor area depending on ceiling height and insulation. This calculator estimates cooling load using square footage times a cooling BTU/sq ft rate for your IECC climate zone, plus a bonus for ceilings above 8 ft, plus 400 BTU per window and 1,000 BTU per exterior door.
The IECC (International Energy Conservation Code) divides the US into 8 climate zones based on heating degree days and cooling degree days. Zone 1 is the hottest (Miami area), Zone 7 is sub-Arctic (interior Alaska). North Dakota is Zone 6, one of the most demanding heating zones in the continental US. Cold climate multipliers significantly increase the required BTU/hour load compared to temperate zones. The correct zone multiplier is one of the most commonly missed steps in field HVAC estimates.
Neither. An undersized system runs constantly without reaching setpoint on design days and shortens equipment life. An oversized system short-cycles: it reaches setpoint quickly, shuts off, then restarts. Short-cycling reduces dehumidification in cooling mode (the coil does not stay cold long enough to condense moisture), creates hot and cold spots, increases wear on the compressor from frequent starts, and wastes energy. The goal is a system that runs for 80 to 90 percent of the design hour. Oversizing by 15 percent or less is a common safety margin; oversizing by 30 percent or more is a problem.
Tonnage is simply BTU/hour divided by 12,000. A 2,000 sq ft home in a moderate climate might need 36,000 BTU/hour cooling load, which is a 3-ton unit. That same home in a cold climate like North Dakota needs a significantly larger heating system. Residential systems are typically available in half-ton increments: 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 5.0 tons. Always round up to the next available standard size, then verify with a full Manual J before purchasing equipment.
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