How to use this calculator
- Enter airflow. Use the design airflow through that duct section, not the whole-system flow unless you are sizing the main trunk.
- Choose sizing method. Use target velocity for a quick area-based size, or target friction rate for equal-friction duct layout.
- Set rectangular aspect ratio. Use W/H for the rectangular option. Lower aspect ratios are usually easier to seal and have lower friction for the same area.
- Check velocity and pressure drop. Compare the round and rectangular results, then add fitting losses and fan/static-pressure allowances outside this straight-duct screen.
How it works
In velocity mode, the required free area is:
A = Q / v
where Q is airflow and v is the target duct velocity.
A round duct then uses D = √(4A/π). For the rectangular option, the
entered aspect ratio sets W/H, so H = √(A/(W/H)) and
W = (W/H)·H.
In friction-rate mode, the calculator iterates the round diameter and the
rectangular height until Darcy-Weisbach gives the target straight-duct loss:
Δp/L = f · (ρv²/2) / D_h
with hydraulic diameter D_h = 4A/P. Turbulent flow uses the
Swamee-Jain friction approximation; laminar flow uses f = 64/Re.
If the airflow came from a load estimate, cross-check it with the HVAC airflow and BTU load calculator. After you choose a duct size, carry the straight-duct drop into the static pressure calculator and size outlets with the grille size calculator.
Worked example
Verified against the live calculator
For 900 m³/h of supply air at a target velocity of 5 m/s, the calculator gives a round duct near 252 mm. With a 2:1 rectangular aspect ratio, the same free area is about 316 × 158 mm. The straight round duct friction is about 1.2 Pa/m, before fittings and terminals.
Frequently asked questions
How do you size HVAC ductwork?
Two common first-pass methods are velocity sizing and equal-friction sizing. Velocity sizing chooses area from A = Q/v. Equal-friction sizing increases or decreases duct size until the pressure loss per length matches a target friction rate, such as about 0.8 Pa/m or 0.10 in. w.g. per 100 ft.
What is a typical duct velocity?
Comfort HVAC ducts often screen around 3-7 m/s (about 600-1400 fpm), with lower velocities for quiet branches and returns and higher velocities only when noise and fan static pressure are acceptable. The right value depends on building type, duct location and acoustic limits.
What friction rate should I use for duct sizing?
A common starting point for low-pressure comfort systems is about 0.08-0.12 in. w.g. per 100 ft, which is roughly 0.65-1.0 Pa/m. Higher friction rates make ducts smaller but raise fan static pressure and noise risk.
Does this include elbows, fittings and diffusers?
No. This page sizes straight duct from airflow and estimates straight-duct pressure drop. Add fitting losses, transitions, dampers, coils, filters, diffusers, leakage and balancing allowances separately before selecting a fan. Use the HVAC static pressure calculator to add those losses into one pressure budget.
Method & assumptions
- Steady, fully developed, single-phase air flow in a straight duct section.
- Pressure drop uses Darcy-Weisbach, hydraulic diameter and a fixed room-temperature air viscosity of 1.81×10⁻⁵ Pa·s.
- Rectangular equivalent round diameter is reported for layout comparison; pressure loss itself uses hydraulic diameter.
- Default roughness is a galvanized-sheet-metal screening value. Flexible duct, lined duct, dirty duct and poor installation can lose more pressure.
- Final HVAC design should follow the applicable ASHRAE/SMACNA method and include fittings, balancing, leakage, acoustics, code requirements, grille/register selection and fan selection.