HVAC and Plumbing Calculators
These calculators help with air and water distribution in buildings: sizing round and rectangular ducts for a given airflow using velocity or friction rate, working out the air changes per hour a room gets from its ventilation or the airflow needed for a target, and choosing a water pipe diameter that keeps velocity within recommended limits.
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All HVAC & Plumbing (3)
Which tool do I need?
Size a supply duct for 400 CFM
Enter the airflow and a target velocity or friction rate to get the round diameter and equivalent rectangular sizes.
Check a room's ventilation rate
Enter the room size and fan airflow to get the air changes per hour, or the airflow needed for a target rate.
Choose a water supply pipe size
Enter the flow and a maximum velocity to get the minimum diameter and the nearest standard size.
About HVAC & Plumbing
For room heating and cooling loads, use the BTU Calculator in Construction & Home; for pressure losses in long pipe runs, use the Pipe Pressure Drop Calculator in Fluid Mechanics. Recommended velocities and air change rates are typical guidance values, clearly labelled, and local codes and ASHRAE standards take precedence.
Results are for checking, learning and preliminary design. Anything that carries load, current, pressure or people must be confirmed by a qualified engineer or licensed tradesperson against the code that governs your project.
Guides
- Duct Sizing: The Velocity and Equal-Friction Methods ExplainedHow duct friction is calculated from Darcy-Weisbach and Colebrook, what friction rate and velocity to design for, and how the Huebscher formula converts round to rectangular.
- Air Changes per Hour: What ACH Means and How Much Ventilation You NeedACH from airflow and volume, the CDC 5 ACH clean-air target, ASHRAE 62.2 for homes, ASHRAE 170 for hospitals, mixing and decay, and why ACH50 is a different number.
- Water Pipe Sizing by Velocity and Hazen-Williams Friction LossNominal size versus inside diameter for copper, PEX and Schedule 40, velocity limits for cold and hot water, the Hazen-Williams formula and C values, and a full sizing example.
Frequently asked questions
How many air changes per hour does a room need?
It depends on the use. Homes are often around 0.35 ACH or more for whole-house ventilation, while kitchens, bathrooms, labs and hospitals need far higher rates set by codes and ASHRAE standards.
Why is a rectangular duct not equivalent to a round duct of the same area?
A rectangle has more perimeter for its area, so more friction. The equivalent diameter formula finds the round duct with the same friction and airflow.
What velocity should water flow at in a pipe?
Many designers keep domestic water below about 1.5 to 2.5 m/s, roughly 5 to 8 ft/s, to limit noise, erosion and water hammer. Check your plumbing code and pipe manufacturer.
Do these tools replace a Manual D duct design?
No. They size individual ducts and pipes from airflow or flow. A full system design also balances branch pressures, fittings and the equipment fan curve, which ACCA Manual D and similar methods cover.