Duct sizing and geometry
Combine inside duct dimensions, segment length, and quantity to calculate cross-sectional area and internal volume.
Calculate the internal air space in round, rectangular or flat-oval straight duct from clear inside dimensions and length, including a quantity of identical segments.
Internal duct volume is clear cross-sectional area carried along a straight constant-section length. Clear dimensions exclude space occupied by liner or other reductions in the airway. The flat-oval shape is a central rectangle closed by two semicircular ends, so it has a different area from an ellipse with the same overall width and height. Equal major and minor dimensions reduce that shape to a circle.
Quantity scales identical straight segments only. Combining runs of different sizes under one average dimension can misstate their volume, and bends or transitions need their own geometry. Internal air volume is a static space measurement. It does not establish airflow, material surface area, purge time or mass without additional operating and physical information.
Vtotal = Aclear × L × n. Dimensions in inches are divided by 12 first. Round A = π(D/2)²; rectangle A = W × H; flat oval A = (W − H)H + π(H/2)², with W ≥ H.
Aclear = clear inside area (ft²); L = segment length (ft); n = number of identical segments; Vtotal = total internal volume (ft³). D/W/H are converted from inches to feet before area calculation.
The per-segment volume belongs to one entered size and length; total volume multiplies that by the identical-segment count. Keep the result in cubic feet distinct from CFM and from sheet-metal or insulation quantities.
Inputs: One rectangular straight segment, 24 × 12 in clear inside, 10 ft long.
Calculation: Convert dimensions to 2 × 1 ft. Area = 2 ft²; internal volume = 2 × 10 = 20 ft³.
Result: 20 ft³ of internal air space. This reproduces the directly observed peer example; it is not a CFM result.
Inputs: One round straight segment, 18 in clear inside diameter, 8 ft long.
Calculation: Radius = 18 / 24 = 0.75 ft. Area = π × 0.75² = 1.767145868 ft²; volume = area × 8.
Result: 14.137 ft³ internal volume, agreeing with the peer's 14.14 ft³ displayed rounding.
Inputs: Constructed case: two identical flat-oval straight segments, each 24 × 12 in clear major/minor dimensions and 10 ft long.
Calculation: In feet, the central rectangle has area (2 − 1) × 1 = 1 ft²; the two semicircular ends total π × 0.5² ft². Area = 1.785398163 ft²; multiply by 10 ft and then quantity 2.
Result: 17.854 ft³ per segment and 35.708 ft³ total. Equal major/minor dimensions reduce to round geometry.
Use the measured clear inside opening after the liner. The calculator makes no wall or liner deduction, so an outside dimension overstates the available airway volume.
The area-times-length model assumes the same clear section throughout. Calculate changing sections separately rather than assigning the whole transition the larger or smaller end area.