Duct Sizing Calculator

The Duct Sizing Calculator is a free online tool that simplifies the process of determining the correct circular duct diameter for HVAC systems. Whether you are a professional engineer, a contractor, or a student, this calculator provides an instant circular duct size based on your required airflow and acceptable pressure loss per meter of duct length. It uses standard metric units and produces results that are easy to interpret and apply directly to real-world ductwork design.

What the Duct Sizing Calculator Does and When to Use It

This calculator solves a core HVAC design problem: given a specific airflow rate and a chosen pressure drop per meter, what size round duct is needed? It automatically computes the required airflow velocity and the minimum circular duct diameter, then rounds that diameter up to the nearest standard nominal size (100 mm, 125 mm, 160 mm, 200 mm, 250 mm, or 315 mm). This ensures the result is practical and ready for procurement and installation.

You should use this calculator whenever you need to size a new duct run, check an existing system, or compare different duct sizing scenarios. Common applications include designing supply air ducts for residential or light commercial HVAC systems, sizing exhaust ducts for kitchen or bathroom ventilation, and optimizing duct layouts to balance air velocity with acceptable noise levels. The tool is especially helpful in the early design phase, allowing you to quickly test what happens if you change the target pressure loss.

The Formula Explained Variable by Variable

The calculator uses a standard fluid dynamics approach to find the duct diameter. The core calculations are performed in metric units (meters, seconds, Pascals, and kilograms per cubic meter). Here is a breakdown of each variable in the formula:

• Q (caudal_m3h): The airflow rate, entered in cubic meters per hour. This is the volume of air that must pass through the duct each hour. The calculator converts this to cubic meters per second (m³/s) by dividing by 3600.
• dP (perdida_carga_pa_m): The desired pressure loss per meter of duct, entered in Pascals per meter (Pa/m). A typical value for low-velocity duct systems is between 0.5 and 2 Pa/m. The default is 1.5 Pa/m.
• rho (ρ): Air density, fixed at 1.2 kg/m³ at standard conditions. This constant is used in the velocity calculation.
• v (velocidad_ms): The calculated air velocity in meters per second (m/s). It is derived from the pressure loss equation. The formula used is: v = 2 * sqrt( (dP * 2 / ρ) * 0.5 ), which simplifies to v = sqrt( (4 * dP) / ρ ). This ensures the velocity delivers the specified friction loss.
• A_m2: The required cross-sectional area of the duct in square meters (m²). It is found by dividing the airflow rate in m³/s by the velocity (A = Q / v).
• D_mm: The calculated theoretical circular duct diameter in millimeters (mm), before rounding to a standard size. It is derived from the area using the formula for the diameter of a circle: D = √(4 * A / π).
• D_norm: The final, practical duct diameter. The calculator rounds the theoretical diameter up to the nearest standard nominal size: 100, 125, 160, 200, 250, or 315 mm. This ensures the duct is always slightly larger than the theoretical minimum, providing a small safety margin.

The calculator outputs three values: the velocity (m/s), the nominal diameter (mm), and the exact required area (m²).

Worked Examples with Concrete Numbers

Example 1: Sizing a Main Supply Duct

Inputs: Airflow (Q) = 800 m³/h, Pressure Loss (dP) = 1.0 Pa/m

Step 1: Convert airflow to m³/s: 800 / 3600 = 0.2222 m³/s.

Step 2: Calculate velocity: v = √( (4 * 1.0) / 1.2 ) = √(3.333) = 1.83 m/s.

Step 3: Calculate required area: A = 0.2222 / 1.83 = 0.1214 m².

Step 4: Calculate theoretical diameter: D = √(4 * 0.1214 / π) = √(0.1545) = 0.393 m = 393 mm.

Step 5: Round up to the nearest standard size: 393 mm rounds up to 400 mm? The calculator only uses nominal sizes up to 315 mm. Since 393 mm exceeds 315 mm, the calculator will output 315 mm (the largest size it handles). For this example, the correct nominal diameter within the tool's range is 315 mm. The velocity output will be 1.83 m/s, and the area 0.1214 m². If a larger duct is required, you would need to use multiple parallel ducts or a rectangular duct.

Result for this calculator: Velocity = 1.83 m/s, Diameter = 315 mm, Area = 0.1214 m².

Example 2: Sizing a Branch Duct

Inputs: Airflow (Q) = 200 m³/h, Pressure Loss (dP) = 1.5 Pa/m (default)

Step 1: Convert airflow: 200 / 3600 = 0.0556 m³/s.

Step 2: Calculate velocity: v = √( (4 * 1.5) / 1.2 ) = √(5.0) = 2.24 m/s.

Step 3: Calculate required area: A = 0.0556 / 2.24 = 0.0248 m².

Step 4: Calculate theoretical diameter: D = √(4 * 0.0248 / π) = √(0.0316) = 0.178 m = 178 mm.

Step 5: Round up: 178 mm rounds up to 200 mm (the next size above 160 mm).

Result: Velocity = 2.24 m/s, Diameter = 200 mm, Area = 0.0248 m².

Common Mistakes When Using a Duct Sizing Calculator

Avoiding these errors will help you get accurate results and a better working system.

• Ignoring the pressure loss per meter: Entering a value that is too low (e.g., 0.1 Pa/m) will result in an excessively large duct diameter, while a value that is too high (e.g., 5 Pa/m) will give a very small duct with high velocity and noise. Always use realistic friction loss targets based on system type. For standard ductwork, 0.8 to 2.0 Pa/m is common.
• Forgetting to convert units: The calculator expects airflow in cubic meters per hour. If you have data in cubic feet per minute (CFM), multiply CFM by 1.699 to get m³/h. Likewise, pressure loss is in Pa/m; if you have inches of water gauge per 100 feet, convert carefully (1 in. w.g./100 ft ≈ 8.35 Pa/m).
• Assuming the output diameter is exact: The calculator returns a nominal diameter, not a precise calculated diameter. This is intentional and correct. Do not try to order a custom 178 mm duct; use the 200 mm size the calculator suggests. Undersizing by even a few millimeters can increase system pressure drop and reduce airflow.
• Applying circular duct results to rectangular ducts without conversion: This calculator is designed for round spiral or rigid ducts. If you need a rectangular or flat-oval duct, you cannot directly use the diameter value. You must use the calculated area (A_m2) and select a rectangular size with an equivalent cross-sectional area and acceptable aspect ratio (typically no higher than 4:1).

Frequently Asked Questions

Why does the calculator round up to the nearest standard size?

Rounding up is standard practice in HVAC design because standard duct sizes (100, 125, 160, 200, 250, 315 mm) are readily available from manufacturers. Using a slightly larger duct than theoretically required reduces system friction, which can compensate for minor installation imperfections and provides a small margin of safety. It also simplifies ordering and construction.

What if my required diameter is larger than 315 mm?

If the theoretical diameter exceeds 315 mm, the calculator will output 315 mm as the largest available size. For larger airflows, you have two options: either design the system using multiple parallel round ducts of 315 mm or smaller, or switch to a rectangular or oval duct that provides the necessary cross-sectional area. The area output (A_m2) can be used to size the rectangular duct.

Can I use this calculator for exhaust or return air ducts?

Yes, the same principles apply to exhaust and return air ducts. However, be aware that return air ducts often operate at slightly lower velocities to reduce noise, and exhaust ducts carrying moist or greasy air may require higher velocities to prevent condensation or grease buildup. Adjust the desired pressure loss per meter accordingly. For general ventilation, 1.5 Pa/m is a good starting point for supply and 1.0 Pa/m for return.