HVAC Load Calculator

1. Enter total square footage. Include all conditioned living space. Don't include unfinished basements, garages, or unconditioned attics.

2. Select your climate zone. Zones 1-2 are hot (southern US), zones 4-5 are moderate (mid-Atlantic, Midwest), zones 6-7 are cold (northern US). The zone affects both cooling and heating load.

3. Rate insulation quality. "Poor" means pre-1980 construction with minimal upgrades. "Excellent" means spray foam walls, triple-pane windows, and verified air sealing.

4. Count windows and doors. Each window and exterior door is a heat transfer point. Window type matters — single-pane transfers 2.4× more heat than triple-pane Low-E.

5. Set sun exposure and occupants. South and west-facing homes in zones 1-3 take heavy solar load. Each occupant adds ~600 BTU of body heat.

This calculator uses a simplified Manual J approach with a 20 BTU/sq ft baseline and seven adjustment factors.

The climate zone adjusts cooling demand (hot zones multiply up, cold zones multiply down) and applies a separate heating factor. Cold zones like Minneapolis need significantly more heating BTU than cooling BTU.

The ±15% range accounts for variables this calculator doesn't capture — duct losses, building orientation, thermal mass, and infiltration rate. Use the low end for tight, well-built homes and the high end for older construction.

Sales calls and estimates. Run a quick load calc in front of the customer to show you're sizing the system properly — not just guessing or matching the old unit's tonnage.

Sanity-checking existing systems. Customer complains their system runs all day? Check if it was properly sized. A 2-ton unit on a home that needs 4 tons was never going to keep up.

Pre-qualifying leads. Before driving to the job site, ask the homeowner for square footage and approximate home details. Get a ballpark tonnage so you know what to quote before you arrive.