HVAC: How Cooling Load (Tonnage) Gets Estimated From Floor Area

Before any ductwork or equipment gets sized, an HVAC take-off starts with a rough cooling load estimate — how many tons of refrigeration (TR) a space needs — derived from floor area and how that space is used. One ton of refrigeration is a fixed unit (12,000 BTU/h, or 3.517 kW), but how much floor area a single ton can cool varies a lot by occupancy type and how much heat that occupancy generates.

Floor Area Served per Ton, by Use

Field estimating rules of thumb for a hot, humid climate typically use:

Well-insulated / low internal gain — about 35 m² per ton. Storage areas, low-occupancy spaces with minimal equipment heat load.

Residential — about 28 m² per ton, roughly the 30-35 BTU/sq ft range commonly quoted for homes.

Commercial (offices, retail) — about 23 m² per ton, reflecting higher occupancy density and equipment load than residential.

High-density (restaurants, function rooms) — about 16 m² per ton — the smallest area-per-ton figure, because kitchen equipment, dense seating, and constant door traffic all add heat the system has to remove.

From Tonnage to Ductwork

Once tonnage is set, ductwork is typically estimated by surface area of the duct run — roughly 8.5 kg of galvanized sheet metal per square meter of duct surface for standard 24-26 gauge supply ductwork, plus a roughly 1:1 insulation allowance (one square meter of insulation per square meter of duct). Both scale with the airflow the tonnage requires, so an underestimated cooling load doesn't just undersize the condensing unit — it undersizes every duct run downstream of it too.

Why This Estimate Stays Rough on Purpose

A proper HVAC design uses a full room-by-room heat load calculation (occupancy, equipment, solar gain, envelope performance) — the area-per-ton figures here are a fast estimating check, not a substitute for that calculation. Where they earn their keep is early in a project, when a ballpark tonnage is needed to price the estimate long before mechanical drawings exist.

Worked Example: From Tonnage to Ductwork

A 200 m² restaurant dining area, using the roughly 16 m²-per-ton high-density figure, needs about 12.5 tons of cooling. That tonnage translates into duct sizing — using the roughly 8.5 kg of galvanized sheet metal per square meter of duct surface figure, a duct system sized for 12.5 tons of airflow will need meaningfully more duct surface area (and therefore more sheet metal and more insulation, at roughly a 1:1 ratio with duct area) than the same floor plate would need as a well-insulated storage space at 5.7 tons — the ductwork budget scales with the cooling load, not with the floor area directly, so the occupancy-type decision made at the very start of the estimate carries all the way through to the sheet-metal quantity at the end.

Common Mistakes

The most consequential mistake is applying a single occupancy-type assumption across an entire building's HVAC estimate when the building actually mixes uses — an office tower with a ground-floor restaurant, for instance, needs the restaurant floor plate priced at the high-density rate while the office floors above are priced at the commercial rate, and a blended average across both under-sizes the restaurant zone specifically. The second common mistake is treating the area-per-ton rule-of-thumb figures as a substitute for a proper room-by-room heat load calculation once the project reaches design development — these figures are explicitly a fast early-stage estimating check, not a design tool, and carrying them forward into final equipment selection instead of commissioning an actual heat load calculation risks under- or over-sizing equipment based on a shortcut that was only ever meant to produce a ballpark number for early pricing.