Stack Effect Calculator

About the Stack Effect Calculator

The Stack Effect Calculator estimates airflow caused by thermal buoyancy in buildings, shafts, and chimneys. Using opening area, stack height, and the temperature difference between indoor and outdoor air, it applies a standard stack effect equation to determine volumetric flow rate. This tool is useful for HVAC engineers, building physicists, fire safety engineers, and architecture professionals assessing natural ventilation or smoke movement.

What You Can Calculate

• Stack-Induced Airflow (Q): Volumetric flow rate caused by temperature-driven density differences between indoor and outdoor air.
• Effect of Stack Height (h): Understand how taller shafts, chimneys, or building cores increase buoyancy-driven airflow.
• Impact of Temperature Difference (T_in − T_out): See how larger temperature differences amplify natural draft and pressure differences.
• Opening Area (A): Evaluate how changing inlet or outlet area affects stack flow capacity.

Formula Used in the Calculator

The calculator uses a common stack effect relationship for volumetric flow rate:

• Stack Effect Flow: \( Q = C \times A \times \sqrt{ 2 \times g \times h \times \left(\dfrac{T_{in} - T_{out}}{T_{in}}\right) } \)
• Where:
• Q = volumetric flow rate (m³/s)
• C = discharge coefficient (dimensionless)
• A = opening area (m²)
• g = gravitational acceleration (9.81 m/s²)
• h = vertical height between openings (m)
• T_in = indoor absolute temperature (K)
• T_out = outdoor absolute temperature (K)

All temperature inputs in °C or °F are internally converted to Kelvin, and lengths/areas entered in feet or centimetres are converted to SI units before applying the formula.

How to Use the Calculator

1. Enter a discharge coefficient (C) based on opening type (typical values range from about 0.6 to 0.7).
2. Specify the opening area and select the appropriate unit (m², cm², or ft²).
3. Enter the stack height between inlet and outlet and choose the unit (m, ft, or cm).
4. Enter the indoor temperature and outdoor temperature in °C, °F, or K.
5. The calculator will automatically compute the stack-induced airflow rate in m³/s and common ventilation units.

Applications of Stack Effect Calculation

Stack effect is a key phenomenon in natural ventilation design, chimney and flue design, stairwells and lift shafts, and fire and smoke control in tall buildings. By estimating buoyancy-driven airflow, engineers can evaluate ventilation effectiveness, potential uncontrolled air leakage, pressure differences across doors, and the risk of smoke spread during fire scenarios.

This calculator provides a fast, first-order estimate that supports conceptual design, system comparison, and educational analysis of stack-driven airflow in building and HVAC applications.