Calculator 09 / HVAC

HVAC Calculator

Get a quick BTU load estimate for a room, or a recommended round duct diameter for a target airflow.

Both tools give rule-of-thumb estimates for planning purposes. Proper equipment sizing (Manual J) and duct design (Manual D) require a full room-by-room load calculation by an HVAC professional — use these numbers as a starting point, not a final design.

Room length (ft)

Room width (ft)

Ceiling height (ft)

Insulation quality

Usual occupants

Windows (sun-exposed)

How this calculator works

BTU load: Base BTU = Floor area (ft²) × 25 Height adjustment = × (Ceiling height ÷ 8) Insulation adjustment = × (1 ± 10%) Occupant adjustment = + 400 × max(0, Occupants − 2) Window adjustment = + 1,000 × Sun-exposed windows Duct size: Duct area (ft²) = CFM ÷ Design velocity (FPM) Diameter (in) = √(Duct area × 144 × 4 ÷ π) → rounded up to the nearest standard round duct size

BTU load uses a common quick-estimate formula (25 BTU per sq ft baseline, adjusted for ceiling height, insulation, occupants, and sun exposure) — useful for a ballpark before calling in a contractor, not a substitute for a full Manual J calculation that accounts for windows, orientation, climate zone, and construction details room by room. Duct size uses a target air velocity to keep noise and static pressure reasonable, then rounds up to a size you can actually buy.

Why does duct velocity change by duct type?

Supply trunk lines can run faster since noise is less noticeable further from the room; supply branches and returns run slower to keep register and grille noise down.

Is a bigger BTU number always better?

No — an oversized AC unit cools too fast without properly removing humidity, leaving a room cold and clammy. Right-sizing (not over-sizing) is the goal, which is exactly why a real Manual J calculation matters for final equipment selection.

Estimates only. Confirm final equipment and duct sizing with a licensed HVAC contractor.