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Duct Sizing Calculator

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Input Parameters

Unit System

Typical: Supply 700-900 FPM, Return 500-700 FPM

Typical: 0.08-0.15 in WG per 100 ft

Design Guidelines

  • • Supply ducts: 700-900 FPM
  • • Return ducts: 500-700 FPM
  • • Main trunks: 1000-1200 FPM
  • • Friction rate: 0.08-0.15 in WG/100 ft

Sizing Results

Recommended Duct Sizes

15
Circular "
20
Rectangular × 9.9"

Flow Characteristics

Design Velocity800 FPM
Reynolds Number107,139
Friction Factor0.0189

Pressure Loss Analysis

Friction Loss0.1 in WG
Actual Length100 ft
Equivalent Length150 ft

Performance Check

✓ Velocity within recommended range
✓ Friction rate acceptable

Standard Sizes

Circular:Round up to nearest standard size
Rectangular:Use standard sheet metal dimensions
Standard circular: 4", 5", 6", 7", 8", 10", 12", 14", 16", 18", 20", 22", 24"

Duct Size Analysis

Detailed Duct Analysis

Flow Characteristics

Flow Type:Turbulent
Velocity Head:0.040 in WG
Dynamic Pressure:0.048 in WG

Sizing Recommendations

• Recommended: 16" diameter

Cost Considerations

Material Cost Index:33.0 sq ft
Energy Cost Factor:0.02 HP
• Annual energy: $3

Alternative Duct Sizes

DiameterVelocityPressure LossNoise LevelRecommendation
13"1085 FPM0.116 in WGMediumUndersized
15"815 FPM0.068 in WGMediumGood
17"634 FPM0.043 in WGLowGood
19"508 FPM0.029 in WGLowOversized

Duct Sizing Calculation Steps

1

Calculate Required Cross-sectional Area

Formula:
A=QVA = \frac{Q}{V}
Calculation:
A=1000800A = \frac{1000}{800}
Result:
1.2500 ft²1.2500 \text{ ft²}
2

Calculate Circular Duct Diameter

Formula:
D=4Aπ×12D = \sqrt{\frac{4A}{\pi}} \times 12
Calculation:
D=4×1.2500πD = \sqrt{\frac{4 \times 1.2500}{\pi}}
Result:
15.1 inches15.1 \text{ inches}
3

Calculate Rectangular Duct Dimensions (equal-friction, 2:1 aspect ratio)

Formula:
Deq=1.30 (WH)0.625(W+H)0.25⇒H=DK, W=2HD_{eq} = \frac{1.30\,(WH)^{0.625}}{(W+H)^{0.25}} \Rightarrow H = \frac{D}{K},\ W = 2H
Calculation:
K=1.30×20.62530.25=1.523, H=15.11.523K = \frac{1.30 \times 2^{0.625}}{3^{0.25}} = 1.523,\ H = \frac{15.1}{1.523}
Result:
19.9×9.9 inches19.9 × 9.9 \text{ inches}
4

Calculate Pressure Loss

Formula:
ΔP=fr×L100\Delta P = f_r \times \frac{L}{100}
Calculation:
ΔP=0.1×100100\Delta P = 0.1 \times \frac{100}{100}
Result:
0.100 in WG0.100 \text{ in WG}
5

Calculate Reynolds Number

Formula:
Re=V×DνRe = \frac{V \times D}{\nu}
Calculation:
Re=13.3×1.2621.57×10−4Re = \frac{13.3 \times 1.262}{1.57 \times 10^{-4}}
Result:
107139 107139 \text{ }
6

Calculate Friction Factor (Altshul-Tsal)

Formula:
f′=0.11(εD+68Re)0.25; f=f′ if f′≥0.018, else 0.85f′+0.0028f' = 0.11\left(\frac{\varepsilon}{D} + \frac{68}{Re}\right)^{0.25};\ f = f'\ \text{if}\ f' \ge 0.018,\ \text{else}\ 0.85f' + 0.0028
Calculation:
f′=0.11(0.00031.262+68107139)0.25f' = 0.11\left(\frac{0.0003}{1.262} + \frac{68}{107139}\right)^{0.25}
Result:
0.0189 0.0189 \text{ }
7

Calculate Equivalent Length (including fittings)

Formula:
Leq=Lactual×1.5L_{eq} = L_{actual} \times 1.5
Calculation:
Leq=100×1.5L_{eq} = 100 \times 1.5
Result:
150 ft150 \text{ ft}