How to Calculate Duct Sizing
What is Duct Sizing?
HVAC Duct Sizing Calculator helps contractors and tradespeople accurately size, spec, and estimate materials and systems.
Formula
Duct Area = CFM / (FPM velocity) where FPM = 700-900 for mains
Step-by-Step Guide
- 1Calculate required CFM per room, size duct for target velocity
- 2Apply the formula using your project measurements
- 3Add required safety factors or waste allowances per code
Worked Examples
Input
400 CFM, 800 FPM target
Result
Area = 400/800 = 0.5 sf → 8×9 inch duct
Common Mistakes to Avoid
- ✕Not applying code-required safety factors or derating
- ✕Forgetting to account for waste and cut losses in material estimates
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