Duct Sizing Calculator

Duct Sizing Calculator with Fittings Pressure Drop Guide

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Duct Sizing Methods & Calculator (Velocity / Equal Friction / Reverse)

Understanding Duct Sizing Methods

There are several established methods for sizing ducts in HVAC systems, each with specific applications:

Equal Friction Method

Maintains a constant friction rate per unit length through the system to help balance pressure drops.

Static Regain Method

Regains static pressure after branches by adjusting duct size, used in large/complex systems.

Velocity Reduction Method

Progressively reduces velocity from mains (≈4–7 m/s) to branches (≈1–3 m/s) to control noise.

Units
Standard
Preset
Values derived from selected standard (user must verify).

📊 Input Parameters

CFM (IP) or m³/s (SI)
FPM (IP) or m/s (SI)
Width : Height (auto if Reverse uses W&H)
ft (IP) or m (SI)
Galv. steel ≈ 0.09 mm

🔧 Fittings (loss coefficient K)

Typical K values; verify for actual geometry/radius & vanes.
K≈0.35
K≈0.20
K≈1.0
K≈2.0
K≈1.5
K≈0.25

💰 Cost (optional)

₹/m² (SI) or $/ft² (IP)
All JS runs after DOMContentLoaded (WordPress‑safe).

📏 Calculated Results

Recommended Size
—
—
Velocity
—
—
ΔP (Length + Fittings)
—
—
Cost (Segment)
—
Incl. markup
#SegmentMethodFlowSizeVelΔP (Len)ΔP (Fit)ΔP TotalCost
Note: Values derived from ASHRAE (user must verify).
📚 Methodology, Formulas & Unit Math
  • Continuity: Q = A·V, so A = Q/V
  • Circular diameter: D = 2·√(A/π)
  • Rectangular: with W, H, A=W·H, perimeter P=2(W+H), Dh=4A/P
  • Dynamic pressure: q = ½·ρ·V²
  • Fittings loss: ΔPfit = ΣK·q
  • Friction loss: Darcy–Weisbach ΔPlen = f·(L/Dh)·(ρ·V²/2); f via Swamee–Jain.
  • Equal Friction iteration: update V ← V·√(target / actual) until ΔP/L ≈ target.
Further reading:

🌀 Duct Design Calculator — What It Is, How It Works & How to Use It

Size ducts fast with three practical methods — Velocity Limit, Equal Friction, and Reverse: Given Size — while seeing velocity, pressure losses, and cost at a glance.

Quick Jump What this calculator does How it works (the physics) How to use it Full sample project Further reading

What this calculator does

Velocity Limit

Choose a velocity cap (e.g., mains 4–7 m/s, branches 1–3 m/s) and get the matching size, losses, and cost.

Equal Friction

Hold a constant Target Friction (ΔP per length) across mains for easy balancing.
Typical: 0.8 Pa/m (SI) or 0.08 in.w.g/100 ft (IP).

Reverse: Given Size

Enter a fixed size (Ø or W×H). The tool tells you the resulting velocity and pressure losses for your airflow.

KPIs
Size
Circular Ø or Rectangular W×H
Velocity
m/s or fpm
Auto‑computed from Q and area
Pressure Loss
Pa / in.w.g
Friction + fittings (ΣK)
Cost
Optional
Sheet area + fittings + markup

How it works (in plain English)

The calculator uses standard HVAC fluid‑mechanics relationships to tie flow, area, velocity, and pressure loss together. It supports both SI (m³/s, m/s, Pa) and Imperial (CFM, FPM, in.w.g) systems.

Show formulas & unit math
  • Continuity: Q = A·V → A = Q/V
  • Circular diameter: D = 2·√(A/π)
  • Rectangular: A=W·H, perimeter P=2(W+H), hydraulic diameter Dh=4A/P
  • Reynolds number: Re = V·Dh/ν
  • Friction factor: Swamee–Jain explicit formula using roughness ε
  • Dynamic pressure: q = ½·ρ·V²
  • Friction loss (length): ΔPlen = f·(L/Dh)·q
  • Fittings loss: ΔPfit = ΣK·q
  • Equal Friction iteration: adjust V so (ΔP/L) ≈ target (e.g., 0.8 Pa/m)
Pro tip: choose a standard (ASHRAE / SMACNA / CIBSE) in the toolbar; presets auto‑fill typical velocities and Equal‑Friction targets. User must verify

How to use the calculator

  1. Set Units (SI or Imperial) and Standard (ASHRAE/SMACNA/CIBSE).
  2. Pick a Preset (e.g., Equal Friction or a velocity band for comfort mains/branches).
  3. Fill in Input Parameters: Segment name, Method, Airflow, Length, Roughness (galv. steel ≈ 0.09 mm), Shape, Aspect Ratio (for rectangles).
  4. Add Fittings counts (elbows/tees/grilles/dampers/reducers).
  5. Optionally set Cost rates (sheet area + fittings + markup).
  6. Click Calculate → review KPIs and Results table.
  7. Use Add Segment to build a multi‑segment run, then Generate HTML Report for a printable summary.

Full sample project — end‑to‑end

Goal: Size a main supply duct for an office zone using Equal Friction.

Inputs (SI)

  • Units: SI
  • Standard: ASHRAE (typical defaults)
  • Method: Equal Friction
  • Target friction: 0.8 Pa/m
  • Airflow Q: 1.20 m³/s (≈ 2545 CFM)
  • Shape: Circular
  • Length L: 20 m
  • Roughness ε: 0.09 mm (galvanized steel)
  • Fittings: 2× 90° elbows, 1× grille, 1× damper

Optional costing

  • Currency: INR
  • Duct sheet rate: ₹ 1200 per m²
  • Fittings rate: ₹ 600 each
  • Markup: 10 %
Show calculation steps
  1. Iterative sizing to match 0.8 Pa/m
    Converged values (from the calculator):
    • Diameter D ≈ 0.482 m (≈ 482 mm)
    • Velocity V ≈ 6.57 m/s
  2. Continuity check → A = Q/V = 1.20/6.57 ≈ 0.1827 m²
    D = 2√(A/π) = 2√(0.1827/π) ≈ 0.482 m ✅
  3. Dynamic pressure → q = ½·ρ·V² = 0.5×1.2×6.57² ≈ 25.8 Pa
  4. Friction loss (length) → 0.907 Pa/m × 20 m ≈ 18.1 Pa (≈ target 0.8 Pa/m with acceptable tolerance)
  5. Fittings loss → ΣK:
    • 2× elbows (K≈0.35 each) → 0.70
    • 1× grille (K≈2.0) → 2.00
    • 1× damper (K≈1.5) → 1.50
    • ΣK ≈ 4.20
    ΔPfit = ΣK·q ≈ 4.2 × 25.8 ≈ 108.5 Pa
  6. Total pressure loss → ΔP ≈ 18.1 + 108.5 ≈ 126.6 Pa
  7. Costing
    • Sheet area (circular): π·D·L ≈ π×0.482×20 ≈ 30.31 m²
    • Duct cost: 30.31 × ₹ 1200 ≈ ₹ 36,374
    • Fittings: 4 × ₹ 600 = ₹ 2,400
    • Subtotal: ₹ 38,774 → +10 % markup → ≈ ₹ 42,651

KPIs you should see (approx.)

  • Recommended size: Ø 482 mm
  • Velocity: 6.57 m/s
  • ΔP (Len + Fit): ≈ 127 Pa (friction line ~0.91 Pa/m)
  • Cost (Segment): ≈ ₹ 42.7k (with the example rates)

What if it must be quieter?

Switch Method → Velocity Limit and set V = 5 m/s:

  • A = Q/V = 1.20/5 = 0.24 m²
  • D = 2√(A/π) ≈ 0.552 m → Ø 552 mm
  • Lower V → lower noise & fittings loss, but larger ducts (more sheet area).

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