What Is the Duct Sizing Calculator?
The Duct Sizing Calculator is a free online tool designed for contractors and DIY builders who need quick, accurate calculations in the construction and building space. By entering your airflow input method, airflow, room size, you get instant results including round duct diameter, actual velocity, rectangular equivalent. No formulas to memorize, no spreadsheets to build — just enter your numbers and get the answer in seconds. Whether you're a beginner or experienced professional, this calculator saves you time and eliminates guesswork.
Why This Calculation Matters
Getting round duct diameter right can make the difference between success and costly mistakes. In construction and building, small errors compound quickly. Manual calculations are error-prone and time-consuming, especially under pressure. This calculator applies proven formulas used by contractors and DIY builders worldwide, giving you confidence that your numbers are correct. Use it to plan your projects with precision and avoid common pitfalls that trip up beginners.
When Should You Use This Calculator?
This tool is most useful when you know your airflow input method and need to find the right round duct diameter. It's also great for quick estimates before committing to a decision, and to double-check manual calculations or professional quotes, and when comparing different scenarios side by side. Bookmark this page and come back whenever you need a fast, reliable answer — the calculator is always free and requires no signup.
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 to Use This Calculator
- Enter Your Airflow Input Method: Start by entering your airflow input method — this is the primary input for the calculation.
- Fill In Additional Details: Complete the remaining fields: airflow, room size, duct shape, maximum air velocity. Each value refines the calculation for greater accuracy.
- Click Calculate: Hit the Calculate button to run the numbers. Results appear instantly below.
- Review Your Results: Check your round duct diameter, actual velocity, rectangular equivalent. Use these figures to inform your next decision or compare against alternative scenarios.
How It Works
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.
Tips & Considerations
- Double-check your airflow input method before calculating — even small input errors can significantly change your results.
- Run the calculator with different values to compare scenarios and find the optimal approach for your situation.
- Pay attention to both round duct diameter and actual velocity — they work together to give you the full picture.
- Bookmark this page for quick access next time you need to plan your projects.
- If you're unsure about your maximum air velocity, start with a conservative estimate and adjust from there.
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.