How to use the duct size calculator
Ducts that are too small whistle and starve rooms; ducts that are oversized waste space and can drop velocity so low that dirt settles. This calculator gives ACCA Manual D–style starting sizes from airflow and either a target velocity or a friction rate, then shows both round diameter and a rectangular equivalent at your aspect ratio. Treat the results as layout estimates before a full Manual D run.
- Choose units — cfm / inches / fpm, or L/s / mm / m/s.
- Enter the design airflow for that trunk or branch (from Manual J / room loads, not just tons × 400).
- Pick velocity or friction mode. Velocity mode uses role defaults (trunk / branch / return) unless you choose custom. Friction mode targets a residential equal-friction rate such as 0.08–0.10 in.wg per 100 ft.
- Set rectangular aspect ratio if the duct must fit a shallow bay, then read round OD and width × height.
Duct sizing formulas
Area (ft²) = cfm ÷ velocity (fpm)
Round diameter (in) = 12 × √(4 × Area / π) ≈ 13.54 × √(cfm / velocity)
Rectangular: choose aspect r = W/H, then H = √(Area_free / r), W = r × H (inside clear dimensions)
Equivalent round rule of thumb: D_eq ≈ 1.30 × (W × H)^0.625 / (W + H)^0.25
Friction estimate (Darcy-style approximation for smooth metal): ΔP/100 ft scales roughly with velocity² / D5 — chart values used here are calibrated estimates
Default velocities: supply trunk 800 fpm (4.1 m/s), supply branch 650 fpm (3.3 m/s), return 600 fpm (3.0 m/s). Friction defaults to 0.10 in.wg/100 ft (0.82 Pa/m). Rectangular sizes keep a practical inside dimension rounded to common 1 in / 25 mm increments.
| Role | Typical velocity | Metric |
|---|---|---|
| Supply trunk | 700–900 fpm | 3.6–4.6 m/s |
| Supply branch | 600–700 fpm | 3.0–3.6 m/s |
| Return | 500–700 fpm | 2.5–3.6 m/s |
| Flex duct | Often ≤ 600 fpm | ≤ 3.0 m/s |
Worked examples
A 400 cfm supply trunk
At 800 fpm, area = 400 ÷ 800 = 0.5 ft². Round diameter ≈ 1.128 × √(400/800) = 9.6 in, so use a 10 in round trunk. At 2:1 aspect, a rectangular equivalent near 14 × 7 in keeps similar area after allowing for aspect penalty. Friction at 0.10 in/100 ft lands in a similar 9–10 in round band on a ductulator.
A 120 cfm bedroom branch
At 650 fpm branch velocity, diameter ≈ 1.128 × √(120/650) ≈ 5.5 in → a 6 in round run or a 10 × 4 in rectangular boot feed. That is a common residential bedroom size when the Manual J load is around 3,000 BTU/h.
Metric living-room run
190 L/s (≈ 400 cfm) on a supply trunk at 4.1 m/s needs about 240 mm round duct. A 2:1 rectangular chase might land near 350 × 175 mm inside. Check noise if you push much past 5 m/s in occupied rooms.
Who this calculator helps
- Architects reserving shaft and joist space before HVAC drawings arrive.
- HVAC designers and technicians sketching equal-friction starting sizes.
- Energy auditors and renovators checking whether existing trunks can carry a heat-pump airflow.
- Self-builders coordinating duct chases with framing and lighting.
Practical tips
- Derive room CFM from loads with the BTU calculator and the heat loss calculator, and fresh-air CFM with the ventilation calculator.
- Keep rectangular aspect ratios under 4:1; wide flat ducts need a larger equivalent area for the same friction.
- Flex duct needs larger diameters than hard pipe for the same CFM — size from the flex manufacturer's friction chart.
- Seal and insulate ducts in unconditioned spaces; leakage shows up as rooms that never hit setpoint.
- Leave static-pressure margin for the filter, coil and long fitting strings — duct friction is only part of ESP.
Coordinating grille locations with furniture? Test the layout on a photo with Room Redesign.
