Stack Draft Calculator

The Physics Behind Every Chimney Effect

Natural draft is created purely by density difference — hot flue gas inside a stack is less dense than cooler ambient air outside, and that density difference over the stack's height produces a pressure difference that pulls gas upward.

The Formula

Draft (Pa) = (g × P_atm / R_air) × Height × (1/Tambient − 1/Tgas)

Temperatures must be in Kelvin; g = 9.81 m/s2, standard atmospheric pressure = 101,325 Pa, R_air = 287 J/kg.K.

Worked Example

A 25 m stack with flue gas at 500 K and ambient air at 293 K:

Draft = 3,463.4 × 25 × (1/293 − 1/500) ≈ 49.9 Pa

How to Use This Calculator

Enter the Stack Height in meters.
Enter Ambient Temperature and Flue Gas Temperature in Kelvin.
Select Calculate to get natural draft in Pascals.

Related Calculations

Need this in mm water column for a smaller appliance? See the Chimney Draft Calculator, or check stack gas velocity in the Stack Gas Velocity Calculator.