💨 HVAC

Duct Pressure Drop Calculator – Air Duct Friction Loss (Pa/m)

Free duct pressure drop calculator: air duct friction loss (Pa/m), velocity pressure and total static for HVAC ductwork (ASHRAE/SMACNA). No sign-up.

📐 Standard: ASHRAE Handbook / ISHRAE
✅ Free to use
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Duct Pressure Drop Calculator Calculator
Reference: ASHRAE Handbook / ISHRAE
💨 HVAC
Free duct pressure drop calculator: air duct friction loss (Pa/m), velocity pressure and total static for HVAC ductwork (ASHRAE/SMACNA). No sign-up.
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ℹ️ About This Calculator

Accurate duct pressure drop calculations ensure fans are correctly sized and air is distributed evenly throughout the HVAC system. The equal friction method sizes ducts to maintain a constant friction rate (typically 0.8–1.2 Pa/m) through all branches, balancing the system. This calculator covers both rectangular and circular ducts using the Darcy-Weisbach equation with Colebrook-White friction factors.

ASHRAE Fundamentals Handbook (Chapter 21) and ISHRAE guidelines govern duct sizing in India. Equal friction method is most common for comfort conditioning; static regain method for high-velocity systems; constant velocity for exhaust/return ducts. Maximum duct velocity limits: living areas < 5 m/s (noise); offices < 8 m/s; plant rooms < 12 m/s. Sheet metal ducts: Gauge 26 (0.5 mm) for up to 750 mm width; Gauge 24 (0.6 mm) for 750–1500 mm; Gauge 22 (0.7 mm) for over 1500 mm.

📐 Duct Pressure Drop (Darcy-Weisbach)

ASHRAE Handbook / ISHRAE

Friction Pressure Drop:
  ΔP_f = f × (L/D_h) × (ρV²/2)
  f = friction factor (from Moody chart / Colebrook-White)
  D_h = hydraulic diameter = 4A/P
  For rectangular duct: D_h = (4 × a × b) / (2(a+b))
  ρ_air = 1.2 kg/m³ at 20°C, 1.15 at 25°C

Velocity Pressure:
  P_v = ρV²/2 (Pa)
  Recommended duct velocity: main ducts 5–10 m/s; branches 3–6 m/s

Dynamic Loss (fittings):
  ΔP_d = ζ × P_v
  ζ (loss coefficient): elbow 0.2–0.8; tee branch 0.5–1.5; diffuser 0.3

🧮 Worked Example

Example: A 600×300 mm rectangular main duct carries 1.0 m³/s of supply air over a 25 m run at ρ = 1.2 kg/m³. Velocity V = 1.0/(0.6×0.3) = 5.56 m/s, and hydraulic diameter D_h = (4×0.6×0.3)/(2×(0.6+0.3)) = 0.4 m. Using a typical friction factor f ≈ 0.02 from the Colebrook-White chart: ΔP_f = 0.02 × (25/0.4) × (1.2×5.56²/2) ≈ 23.2 Pa, close to the recommended 0.8–1.0 Pa/m friction rate for the 25 m length. Adding two 90° elbows (ζ = 0.3 each) contributes dynamic loss ΔP_d = 2 × 0.3 × 18.5 ≈ 11.1 Pa, giving a total section loss of about 34 Pa for fan sizing.

📊 Duct Velocity & Friction Reference (ASHRAE)

The equal-friction method sizes ductwork to a constant pressure loss per metre - typically 0.8-1.0 Pa/m for low-velocity comfort systems. Staying within the recommended velocity band for each application controls both noise and fan energy.

Recommended Air Velocities

ApplicationMain duct (m/s)Branch duct (m/s)
Residences3.5 – 5.03.0
Offices, schools, hospitals5.0 – 6.53.0 – 4.5
General commercial6.0 – 7.54.0 – 5.0
Industrial6.0 – 9.04.0 – 6.0

Design Notes

Keep the duct aspect ratio below about 4:1 - flatter ducts have more surface area, higher friction and greater heat gain. Add fitting losses (bends, transitions, dampers) as equivalent length or loss coefficients; on a typical layout these can equal or exceed the straight-duct loss, so they must not be ignored when selecting the fan.

Indicative velocities for low-velocity systems; noise-critical spaces (studios, wards) require the lower end of each range.

❓ Frequently Asked Questions

What friction rate should I use for duct sizing? +
For standard comfort cooling/heating in offices and commercial buildings: 0.8–1.0 Pa/m friction rate. For quiet applications (bedrooms, boardrooms): 0.5–0.8 Pa/m to keep velocities low and noise down. For industrial ventilation and large plant rooms: 1.0–1.5 Pa/m. Higher friction rates give smaller ducts (cheaper) but increase system pressure and fan energy consumption.
How do I convert between rectangular and circular duct sizes? +
Use the equal friction diameter (hydraulic equivalent diameter): D_eq = 1.30 × (a×b)^0.625 / (a+b)^0.25, where a and b are the rectangular dimensions (mm). This gives the circular duct diameter that has the same friction loss per unit length at the same air volume. The actual circular duct will be slightly different from the hydraulic diameter formula used for pressure loss.
What is the maximum velocity in air ducts to prevent noise? +
Velocity limits for acceptable noise (NC 35): main supply ducts – 7 m/s max; branch ducts – 5 m/s; duct to diffuser – 3 m/s; diffuser face velocity – 0.5–1.5 m/s. For bedrooms and quiet spaces (NC 25): reduce all velocities by 20–30%. Higher velocities generate duct noise (rushing sound) and diffuser noise. Fan speed must be reduced, not duct size increased, to fix a noisy system.
Do I need to consider static regain in duct design? +
Static regain analysis is important for long high-velocity systems (supply trunks over 50 m) or where the equal friction method results in very unbalanced system pressures. In the static regain method, velocity is progressively reduced at each branch, converting velocity pressure to static pressure to partially offset friction losses. Result: more uniform pressure at all outlets without balancing dampers. For short systems, equal friction with volume control dampers is simpler.
What duct leakage should I allow in the calculation? +
ASHRAE and SMACNA classify duct leakage: Class A (maximum 0.5% fan flow) for clean rooms and pressurised systems; Class B (2%) for typical commercial supply ducts; Class C (4%) for return ducts and low-pressure systems. Add the expected leakage to the fan flow calculation. For unsealed galvanised ducts in Indian buildings, assume 5–10% leakage if leakage testing is not specified.
What friction rate should I use for duct design? +
For low-velocity comfort systems the equal-friction method typically targets 0.8-1.0 Pa/m (about 0.1 in.wg per 100 ft). Lower rates give quieter, larger ducts; higher rates save space but increase fan energy and noise. Keep the rate constant along each duct run.
What is the maximum air velocity in a duct? +
It depends on the space. Residences keep main ducts to about 3.5-5 m/s for quietness; offices and hospitals 5-6.5 m/s; general commercial up to 7.5 m/s; industrial up to 9 m/s. Branch ducts near occupied spaces run slower to limit noise at the outlets.
Equal friction vs static regain - which method should I use? +
Equal friction is simple and suits most low-velocity comfort systems, sizing every run to the same Pa/m. Static regain is used for large high-velocity systems to balance pressure at each branch. For typical Indian commercial HVAC, equal friction as used by this calculator is the standard approach.

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