New-construction sizing means a full block load — envelope, glass, orientation, occupancy, the works. On a replacement, you've got something better than a calculation: history. There's a unit up there that's been conditioning that space, and the people inside can tell you whether it kept up. Use that.
Step 1: Start with what's there
Read the nameplate on the existing unit — nominal tonnage, heating input, voltage/phase. Then ask the single most valuable question on the whole job: "Did this unit keep up?"
- Kept up fine, comfortable year-round? The existing size is your baseline. Match it.
- Struggled in July, ran constantly, never satisfied? It may be undersized — or, just as often, it was the right size but failing. Don't automatically upsize; a tired compressor and a dirty coil mimic undersizing.
- Short-cycled, cold and clammy, humidity complaints? Classic oversizing. The old unit was too big. Resist matching it — you'll repeat the problem.
History tells you whether to match, trim, or bump the size. That judgment is worth more than any rule of thumb — but pair it with a sanity-check calculation so you're not betting the job on one comment from a facility manager.
Step 2: Sanity-check with the rule of thumb
This is where a fast load estimate earns its keep. Take the square footage the unit serves and divide by a per-ton factor for the building type, then nudge for climate:
- General office: ~350 sq ft/ton
- Retail / showroom: ~300 sq ft/ton
- Restaurant / commercial kitchen: ~175 sq ft/ton (huge internal and ventilation load)
- School / assembly: ~275 sq ft/ton
- Warehouse / storage: ~600+ sq ft/ton
That's exactly what the tonnage estimator does — it runs the math and adjusts for climate. If the rule-of-thumb number lands close to the existing unit and the building was comfortable, you've got strong agreement and you can move fast with confidence. If they disagree badly, stop — that's your signal to dig deeper.
Step 3: Size the heat separately
Cooling tonnage and heating capacity are two different calculations. For heating, estimate the load from square footage and a climate-driven BTU/hr-per-square-foot factor, then convert to either gas input (MBH, accounting for ~80% AFUE) or electric resistance (kW). The heat sizing tool does this and recommends a fuel. In DE/FL/MD/PA/VA, gas almost always wins on operating cost where there's gas service.
The three places rules of thumb will burn you
Rules of thumb are fast, and fast is good. But they assume an "average" building, and these three situations are not average:
1. Glass and orientation
A space with a wall of west-facing glass has a cooling load a rule of thumb will badly underestimate. Afternoon solar gain is brutal. If you see a lot of glass, especially facing south or west, kick the tonnage up or do a real load calc.
2. Internal loads — people and equipment
A call center packed with bodies and computers, a commercial kitchen, a server closet, a busy gym — these throw off heat that a square-footage rule never sees. The restaurant factor is low for a reason. When the internal load is high, the square footage lies.
3. Ventilation requirements
Current code may require more outside air than the old unit brought in. Conditioning that extra fresh air is real load. If the energy code pushes you to an economizer and higher ventilation rates (see the permits guide), the unit has to handle it.
Match history, sanity-check with the rule of thumb, and do a real load calc the moment the two disagree.
The ten-minute method, start to finish
- Read the existing nameplate — tonnage, heat, voltage.
- Ask if it kept up. Note short-cycling or humidity complaints.
- Run the tonnage tool and the heat tool on the served square footage.
- Compare. Agreement + comfortable building = size with confidence.
- Disagreement, big glass, high internal load, or new ventilation requirements = stop and do a full Manual N / block load.
That's it. Fast where it can be fast, careful where it has to be careful. The tools give you the number; your read of the building tells you whether to trust it.
About the author: Gregory Frazier is a heavy commercial HVAC estimator working Maryland, Pennsylvania, Delaware, Virginia and Florida for 25+ years. Read his story →