Duct Sizing Calculator
Round Duct Size by CFM and Velocity
Standard round duct diameter needed for given CFM.
| CFM | 600 FPM | 900 FPM | 1200 FPM |
|---|---|---|---|
| 100 | 6" | 4" | 4" |
| 200 | 8" | 6" | 6" |
| 400 | 12" | 10" | 8" |
| 600 | 14" | 12" | 10" |
| 800 | 16" | 12" | 12" |
| 1,000 | 18" | 14" | 12" |
| 1,200 | 20" | 16" | 14" |
| 1,600 | 22" | 18" | 16" |
How We Calculate This
This duct sizing calculator uses established formulas and industry-standard data to provide accurate estimates.
- Enter your specific values into the calculator fields above
- Our algorithm applies the relevant formulas using your inputs
- Results are calculated instantly in your browser — nothing is sent to a server
- Review the detailed breakdown to understand how each factor affects your result
These calculations are estimates based on standard formulas. For critical decisions, always consult a qualified professional.
How to Convert Oven Recipes to Air Fryer
Duct sizing balances airflow delivery against noise and friction. Undersized ducts create noise and starve rooms of air; oversized ducts waste material and space.
The basic rule:
- Calculate required cross-sectional area: Area (sq ft) = CFM ÷ Velocity (FPM). For a round duct: Diameter = √(4 × Area / π)
- Residential trunk ducts typically use 700-900 FPM velocity. Branch ducts to bedrooms should stay under 600 FPM to minimize noise
- Rectangular ducts are sized to match the equivalent round duct area, using the formula: equivalent diameter = 1.3 × (a×b)^0.625 / (a+b)^0.25
Standard round duct sizes increase in 2-inch increments (6, 8, 10, 12 inches, etc.). This calculator rounds up to the next standard size to ensure adequate airflow.
When Would You Use This Calculator?
This duct sizing calculator is designed for anyone who needs quick, reliable estimates without complex spreadsheets or professional consultations.
- When you need a quick estimate before committing to a purchase or project
- When comparing different options or scenarios side by side
- When planning a budget and need to understand potential costs
- When you want to verify a quote or estimate you've received from a professional
- When teaching or learning about the concepts behind these calculations
Frequently Asked Questions
How do I calculate CFM for a room?
The simplest method is 1 CFM per square foot of floor area. A 300 sq ft room needs about 300 CFM. More precisely, divide your AC tonnage by the number of rooms: a 3-ton system (1,200 CFM total) serving 8 rooms averages 150 CFM per room. Adjust up for kitchens and sunny rooms.
What size duct do I need for 400 CFM?
At 900 FPM velocity (residential trunk), 400 CFM needs about 0.44 sq ft of duct area, which is a 9-inch round duct (round up to 10-inch standard). At 600 FPM (low noise), you'd need a 12-inch round duct. Lower velocity means larger duct but less noise.
What is the recommended air velocity for residential ducts?
Main trunks: 700-900 FPM. Branch ducts: 600-700 FPM. Near bedrooms: under 600 FPM for quiet operation. Flex duct should be sized for even lower velocity due to higher friction. Commercial systems can run 1,200-1,500 FPM.
How do I convert round duct size to rectangular?
Match the cross-sectional area. A 10-inch round duct (78.5 sq in) can be replaced with an 8" × 12" rectangular duct (96 sq in) or a 10" × 10" duct (100 sq in). Rectangular ducts should maintain an aspect ratio under 4:1 for efficient airflow.
Does flex duct need to be sized larger than rigid duct?
Yes. Flex duct has significantly higher friction loss than smooth rigid duct, especially when not pulled taut. Size flex duct one size larger (e.g., use 8-inch flex where 6-inch rigid would work) and keep runs as short and straight as possible.
How many CFM per ton of air conditioning?
The standard rule is 400 CFM per ton of cooling. A 3-ton AC system needs 1,200 CFM total airflow through the duct system. The supply and return duct trunks must each be sized for this total CFM.