Duct Size Calculator
Choose a calculation
Size a duct from airflow and velocity, or check the velocity through an existing duct.
Estimate the cross-sectional size of a round or rectangular duct from the airflow and target velocity you provide, or check the resulting velocity through an existing duct. Work in CFM, feet per minute and inches, or in m³/h, metres per second and millimetres. The calculator applies the continuity relationship between airflow, area and average velocity. Use the result as an early geometric check, then complete the system design with friction, static-pressure, fitting, noise and code requirements.
How to size or check a duct
-
1
Choose the calculation
Size a new round or rectangular duct from airflow and target velocity, or check velocity in an existing duct.
-
2
Enter airflow and geometry
Use CFM, fpm and inches for imperial work, or m³/h, m/s and millimetres for metric work. For a rectangular duct, provide or select the required aspect ratio or dimensions.
-
3
Review the geometric result
Read the required area, diameter or side dimensions and calculated velocity, then verify pressure loss, fittings, noise and the full system design separately.
Airflow, area and velocity
This calculator uses the continuity relationship Q = A × V. Airflow through a duct equals its clear cross-sectional area multiplied by average air velocity. Rearranging gives A = Q ÷ V for preliminary sizing and V = Q ÷ A for checking an existing duct. ASHRAE’s Duct Design chapter uses the same relationship between airflow, duct area and average velocity.
Formula legend:
- Q = volumetric airflow: CFM in imperial mode or m³/s after converting m³/h by dividing by 3,600
- A = clear internal cross-sectional area: ft² or m²
- V = average duct velocity: fpm or m/s
- D = round internal diameter, where
D = √(4A ÷ π)
For a rectangular duct with width-to-height ratio r, height = √(A ÷ r) and width = r × height. The calculator converts the resulting dimensions to inches or millimetres. When checking an existing duct, round area is πD² ÷ 4; rectangular area is width × height.
| Calculation path | Imperial preparation | Metric preparation |
|---|---|---|
| Size from airflow | A (ft²) = CFM ÷ fpm |
A (m²) = (m³/h ÷ 3,600) ÷ m/s |
| Round size | D (in) = √(4A ÷ π) × 12 |
D (mm) = √(4A ÷ π) × 1,000 |
| Existing-duct velocity | fpm = CFM ÷ area in ft² |
m/s = (m³/h ÷ 3,600) ÷ area in m² |
Worked example
For 800 CFM at a user-selected target of 800 fpm, the required area is 800 ÷ 800 = 1.00 ft². A geometrically equivalent round duct has an internal diameter of √(4 × 1 ÷ π) × 12 = 13.54 in. A 2:1 rectangular shape with the same 144 in² area is approximately 16.97 × 8.49 in. These are exact geometric results, not standard product selections; a designer may choose available dimensions and then check the actual velocity again.
What this result does not design
Velocity alone cannot complete a duct design. ASHRAE describes duct sizing methods that also account for friction rate and pressure loss, while the U.S. Department of Energy’s fan-system sourcebook explains how duct friction, length and velocity affect system resistance. This calculator does not perform ACCA Manual D, equal-friction or static-regain design; calculate static pressure; model elbows, transitions, dampers or grilles; predict regenerated noise; account for duct liner or flexible-duct compression; select a fan; or approve code compliance. Confirm actual internal dimensions and complete the design with qualified HVAC guidance and locally applicable requirements.
Frequently Asked Questions
No. It performs a preliminary geometric calculation from airflow, area and average velocity. Manual D and professional duct design also consider room loads, friction rate, equivalent lengths, fittings, available static pressure, balancing, noise and equipment performance.
The calculated diameter or side dimensions may not match an available product size. Select the actual internal dimensions of the installed duct and use the existing-duct check to recalculate velocity. Liner, seams and flexible-duct compression can also reduce clear area.
The calculator uses Livewire, so selected values are sent to this site to update results. Funnel mode may also place non-sensitive values in the page URL. Do not enter confidential project information.
No. The result does not establish acceptable friction, static pressure, airflow balance, acoustics, equipment performance or code compliance. Have the complete system checked against the applicable design method, manufacturer data and local requirements.
Related Tools
BMI Calculator
Calculate body mass index from height and weight. Shows WHO category, healthy-weight range and limitations of BMI.
One Rep Max Calculator
Estimate your one-rep max (1RM) from weight lifted and reps completed using Epley, Brzycki and Lander formulas.
Age Calculator
Calculate exact age in years, months and days from a birth date, plus total days, hours and the next birthday countdown.
Scientific Calculator
Full scientific calculator with trig, log, exponents, factorial, memory, parentheses and DEG/RAD toggle. Runs in your browser.
GPA Calculator
Calculate grade point average from letter grades and credit hours. Supports US 4.0 and 5.0 honors scales with the standard A/B/C/D letter table.
Inflation Calculator
Calculate the equivalent purchasing power of a dollar across any two years using historical CPI data from 1913 to today.