# Air Ducts - Major Friction Head Loss, Online Calculator

## Head loss or major loss in air ducts - equations and online calculator for rectangular and circular ducts - Imperial and SI units.

The major loss, or friction loss, in a circular duct in galvanized steel with turbulent flow can for imperial units be expressed

Δh_{ }= 0.109136 q^{1.9}/ d_{e}^{5.02 }(1)

where

Δh= friction or head loss (inches water gauge/100 ft of duct)

d_{e}= equivalent duct diameter (inches)

q= air volume flow - (cfm - cubic feet per minute)

For rectangular ducts the equivalent diameter must be calculated.

### Air Ducts Friction Loss Calculator - Imperial Units

The head loss calculator below is based on formula *(1)*:

Head loss and air flow velocity for some common duct sizes and air flow volumes can be taken from the table below:

Head Loss (inches water gauge per 100 feet duct) | |||||||
---|---|---|---|---|---|---|---|

Air velocity (ft/min) | |||||||

Air Volume (cfm) | Duct Size (inches) | ||||||

4 | 5 | 6 | 8 | 10 | 12 | 16 | |

100 | 0.65 | 0.21 | 0.09 | 0.02 | 0.01 | ||

1146 | 733 | 509 | 286 | 183 | |||

200 | 0.8 | 0.32 | 0.08 | 0.02 | 0.01 | ||

1467 | 1019 | 573 | 367 | 255 | |||

400 | 1.19 | 0.28 | 0.09 | 0.04 | 0.01 | ||

2037 | 1146 | 733 | 509 | 286 | |||

800 | 0.34 | 0.14 | 0.03 | ||||

1467 | 1019 | 573 | |||||

1600 | 0.12 | ||||||

1146 |

Air velocity should not exceed certain limits to avoid unacceptable noise generation.

### Air Ducts Friction Loss Calculator - SI Units

This pressure loss calculator is based on formula *(1)* with input and output values modified for SI units.

### Air Ducts - Velocity and Friction Loss - Excel Template

This Excel template can be used to calculate velocity and friction loss in air ducts.

### Air Ducts - Friction Loss Diagram

The chart below can be used to estimate friction loss in air ducts.

The default values are for air flow *400 cfm (680 m ^{3}/h)*, duct size

*8 in (200 mm)*and

*friction loss*

*0.28 inH2O/100ft (2.3 Pa/m)*.