HVAC Load Calculator — BTU Sizing for Heating & Cooling
Estimate a fast rule-of-thumb load for a room or addition. Adjust the envelope, glazing, occupants, and sun exposure to get a more realistic starting point before final Manual J sizing.
Cooling load starts with a 120 BTU/sq ft rule-of-thumb base and then adjusts for height, insulation, climate, windows, people, and solar gain.
Hot-climate cooling rule of thumb
| Condition | Cooling target |
|---|---|
| Tight envelope / shaded | 100 BTU/sq ft |
| Average residential room | 120 BTU/sq ft |
| Sunny room / poor envelope | 150 BTU/sq ft |
What moves the load most?
- Ceilings over 8 ft raise the air volume that must be conditioned.
- Poor insulation and single-pane glass push both cooling and heating demand up fast.
- Full sun and extra occupants add latent and sensible cooling load in summer.
- Cold climates increase the heating-degree-day factor and move furnace size up.
Frequently Asked Questions
How accurate is this HVAC load calculator?
It is a rule-of-thumb screening tool. Use it for a quick equipment range, then confirm the final size with a Manual J or engineered load calculation.
How do I convert BTU to AC tons?
Divide cooling BTU by 12,000. A 24,000 BTU result is about 2 tons of cooling capacity.
Why does the furnace size round up?
Furnaces come in standard cabinet sizes. The calculator rounds to the next common size so you can compare actual equipment options.
Keep the project moving
Use the free calculator for fast takeoffs, then plug the result into a reusable field template.
Need templates? Browse our professional template store — contracts, checklists, and planners from $17.