1. 800 CFM through 12 in round
Airflow 800 CFM; 12 in round; area 0.7854 ft²; velocity 1,018.59 FPM.
Air CFM & Ductwork Velocity Calculator
Enter a positive value.
Enter a positive value.
Application guidance: Custom only. Enter your project criteria. No universal velocity range or application presets are supplied.
Dimensions are clear internal flow dimensions. Unit changes preserve physical quantities. Software bounds: flow 1e−12–1e9 m³/s, dimensions 1e−12–1e6 m, area 1e−12–1e12 m², velocity 1e−12–1e12 m/s, aspect ratio 1e−12–1e6. These are software limits, not HVAC standards.
AIR VELOCITY
1,019 FPM
Average air velocity from entered airflow and area.
- Airflow
- 800 CFM
- Duct
- 12 in round
- Area
- 0.7854 ft²
- Average air velocity
- 1,018.6 FPM
No selected target
HVAC note: This calculator estimates duct area and average air velocity. It does not determine friction loss, available static pressure, fitting losses, equipment performance, room airflow requirements, acoustics, or code compliance.
Duct cross-section schematic
Nearby supported round sizes
Local nominal comparison list only; no universal availability claim. Nominal diameters are modeled as internal diameters. Verify actual clear ID and lining. Rectangular results remain calculated dimensions; no rectangular stock matrix is inferred.
All supported round options within your selected target: none.
| Duct size | Area | Velocity | Target status |
|---|---|---|---|
| 9 in | 0.4418 ft² | 1,810.8 FPM | No selected target |
| 10 in | 0.5454 ft² | 1,466.8 FPM | No selected target |
| 12 in | 0.7854 ft² | 1,018.6 FPM | No selected target |
| 14 in | 1.069 ft² | 748.4 FPM | No selected target |
| 16 in | 1.396 ft² | 573 FPM | No selected target |
Show calculation
- Q = A × v; v = Q / A
- A = πD² / 4 = π × 0.3048² / 4 = 0.072965877 m²
- v = 0.3775579546 m³/s / 0.072965877 m² = 5.1744455098 m/s (1,018.6 FPM)
- Full precision is used for calculation, raw target comparisons and supported-size selection.
Equal-area round diameter = √(4A / π). It describes the same area and average velocity at the same airflow; it does not establish equal friction loss.
Airflow and unit sources
Duct results
Air CFM & Ductwork Velocity Calculator Mode: Find Velocity; shape: Round; target mode: Max velocity target Airflow: 800 CFM Duct: 12 in round Area: 0.7854 ft² Average air velocity: 1,018.6 FPM Target: no minimum; no maximum Status: No selected target Equal-area round diameter: 12 in. Equal area does not imply equal friction loss. Q = A × v; v = Q / A A = πD² / 4 = π × 0.3048² / 4 = 0.072965877 m² v = 0.3775579546 m³/s / 0.072965877 m² = 5.1744455098 m/s (1,018.6 FPM) Full precision is used for calculation, raw target comparisons and supported-size selection. Nearby supported nominal round sizes (local list, no availability claim) 9 in; 0.4418 ft²; 1,810.8 FPM; No selected target 10 in; 0.5454 ft²; 1,466.8 FPM; No selected target 12 in; 0.7854 ft²; 1,018.6 FPM; No selected target 14 in; 1.069 ft²; 748.4 FPM; No selected target 16 in; 1.396 ft²; 573 FPM; No selected target All supported options within target: none Planning estimate only. Dimensions represent clear internal flow dimensions. Nominal supported diameters are modeled as IDs; verify actual clear ID, lining and availability. Custom targets only; no application presets or universal velocity range. Schematic only; no pressure, friction, turbulence, noise or installation model. HVAC note: This calculator estimates duct area and average air velocity. It does not determine friction loss, available static pressure, fitting losses, equipment performance, room airflow requirements, acoustics, or code compliance. NASA Glenn: continuity and swept volume: https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/mass-flow-rate/ NIST SP 811: SI conversion factors: https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors ASHRAE Handbook: duct design and average duct velocity: https://handbook.ashrae.org/Handbooks/F25/IP/F25_Ch21/F25_Ch21_ip.aspx
How to use
Calculate average duct velocity from airflow and round or rectangular area, or find duct dimensions for an editable target velocity. Compare nearby supported round sizes.
Choose Find Velocity to enter airflow and clear internal duct dimensions. Choose Find Duct Size to enter airflow and a Custom max velocity, min velocity or target range. Select round or rectangular, then the units. Rectangular reverse sizing also needs a planning aspect ratio, fixed width or fixed height. Read the result after all inputs; open Show calculation for the arithmetic. Copy Duct Results and Print / Save PDF include the assumptions and sources. Reset restores 800 CFM, 12 in round, Find Velocity, US and empty optional targets.
Airflow and duct velocity
Q = A × v relates volumetric airflow Q to cross-sectional area A and average velocity v. CFM means cubic feet per minute; FPM means feet per minute. Dividing CFM by square feet gives FPM. Airflow is the user’s supplied quantity at the duct conditions; no room airflow requirement or density correction is inferred. NASA’s swept-volume derivation gives volume = area × velocity × time; dividing by time gives this relationship.
Round duct area
A = πD² / 4 for a round duct. D is clear internal diameter, normalized to meters. For dimensions in inches, square inches / 144 gives square feet. Lining, wall thickness and nominal labeling can change the available internal area; use the actual clear flow dimension.
Rectangular duct area
A = W × H for a rectangular duct, using clear internal width and height in consistent units. Normalize each dimension before multiplying. Different width/height combinations can produce the same area.
How to calculate velocity
v = Q / A. FPM = CFM / areaFt2. The engine converts airflow to m³/s, calculates area in m², and divides to obtain m/s. Results then convert to the selected output units. This is a cross-section average, not the velocity at every point in the duct.
How to calculate required duct size
A = Q / targetVelocity. For round sizing, D = √(4A / π). For rectangular sizing, one area does not uniquely determine both width and height. With planning aspect ratio r = W / H, W = √(A × r) and H = √(A / r). With fixed width, H = A / W. With fixed height, W = A / H. The default 2:1 ratio is editable planning input, not a recommendation. A max target gives a minimum area; a min target gives a maximum area. A range uses the max for the lower area boundary and independently checks supported options against both bounds.
Velocity targets
Targets come from your project criteria. BELOW SELECTED TARGET means raw velocity is below your minimum; ABOVE SELECTED TARGET means it exceeds your maximum; WITHIN SELECTED TARGET includes equality and respects the bounds you actually supplied. Optional min-only or max-only targets compare only that bound. No hidden slack or display rounding is used. A min greater than max is invalid. For round sizing, all supported options within a two-sided range are listed; if none match, the page says so. Max-only sizing identifies the smallest supported diameter meeting the maximum. Min-only sizing shows the calculated maximum area/diameter and does not identify a minimum size.
Application guidance
Custom only in this version. No Main trunk, Branch, Supply run or Return range is supplied. General application ranges depend on system criteria and need appropriate sourcing; this page leaves every target editable and avoids universal good, bad, safe or unsafe velocity judgments. Targets do not certify equipment performance or code compliance.
Round vs rectangular ducts
Round and rectangular shapes with the same clear area give the same average velocity at the same airflow. Their perimeter and shape differ, so equal area alone cannot establish equal pressure loss or acoustic behavior. Supported nominal round diameters are a deliberately local comparison list, not a catalog or universal market availability. No rectangular stock matrix is supplied.
Equal-area round diameter
Equal-area round diameter = √(4A / π). This is the diameter of a circle with the same area as the entered rectangle. The label is explicitly Equal-area round diameter. It is not an equal-friction equivalent diameter. Equal-friction methods use additional empirical relationships and are outside this version.
US and metric units
Airflow units: CFM, L/s, m³/h and m³/s. Dimension units: in, mm, cm and m. Velocity units: FPM, ft/s and m/s. US defaults to CFM/in/FPM and Metric to L/s/mm/m/s. Switching units converts existing physical quantities rather than reinterpreting them. Raw normalized values are retained across switches until edited. Exact 1 ft = 0.3048 m and 1 in = 0.0254 m give 1 CFM = 0.3048³ / 60 m³/s = 0.0004719474432 m³/s. L/s = 0.001 m³/s and m³/h = 1/3600 m³/s. NIST provides the unit-conversion basis.
Why velocity alone does not size a duct system
ASHRAE’s duct-design chapter treats average duct velocity alongside system pressure losses, fittings, balancing and other design constraints. Area and velocity alone do not establish a complete duct system. This page performs no friction-rate, static-pressure, fitting-loss, fan/blower selection, equipment sizing, leakage, balancing, Manual D, Manual J, Manual S, noise prediction or room airflow adequacy calculation.
Worked examples
2. 800 CFM through 14 × 8 in rectangular
Airflow 800 CFM; 14 in × 8 in rectangular; area 0.7778 ft²; velocity 1,028.57 FPM.
3. 1000 CFM at a selected maximum of 800 FPM: round
Airflow 1,000 CFM; 15.14 in round; area 1.25 ft²; velocity 800 FPM.
Calculated minimum ID 15.14 in. Next supported round size 16 in; velocity 716.2 FPM. Selection uses the raw diameter, never rounded display.
4. 1000 CFM at 800 FPM: rectangular, planning aspect ratio 2:1
Airflow 1,000 CFM; 18.97 in × 9.49 in rectangular; area 1.25 ft²; velocity 800 FPM.
5. 400 L/s through 500 mm round
Airflow 400 L/s; 500 mm round; area 0.1963 m²; velocity 2.04 m/s.
6. 800 CFM, 12 in round; user-selected 600–900 FPM target
Airflow 800 CFM; 12 in round; area 0.7854 ft²; velocity 1,018.59 FPM.
ABOVE SELECTED TARGET. This is an example project target only, not a universal requirement.
Assumptions and limitations
Use clear internal dimensions and airflow applicable to the same duct section. The model is geometrical with a cross-section average; it does not infer a compressible-flow operating solution. Full precision is used internally; displayed dimensions normally use two decimals, velocity a whole number or one decimal in US, and area four significant digits. Software bounds are broad implementation limits, not HVAC standards: flow 1e−12–1e9 m³/s, dimensions 1e−12–1e6 m, area 1e−12–1e12 m², velocity 1e−12–1e12 m/s and aspect ratio 1e−12–1e6, input strings up to 64 characters. Zero/negative, malformed, nonfinite, overflowing or invalid active target values clear all results and disable copy/print. Dot and comma decimals use the shared NumericInput. Calculations stay in page memory without numeric storage, URL persistence or network calculation.
HVAC note: This calculator estimates duct area and average air velocity. It does not determine friction loss, available static pressure, fitting losses, equipment performance, room airflow requirements, acoustics, or code compliance.
Supported nominal round sizes in inches: 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 36, 40, 42, 48. Next supported size is the smallest listed diameter at least as large as the raw calculated minimum ID. Recalculate its velocity and check all target bounds. When the calculated diameter exceeds 48 in, no supported next size is available.
FAQ
How do I calculate duct velocity from CFM?
Divide CFM by duct area in square feet: FPM = CFM / areaFt2.
How do I calculate the area of a round duct?
Use A = πD² / 4 with clear internal diameter in consistent units.
How do I calculate the area of a rectangular duct?
Multiply clear internal width by height: A = W × H.
How do I calculate duct size from CFM and target velocity?
Find A = Q / v, then D = √(4A / π) for round ducts. Rectangular ducts also need an aspect ratio, fixed width or fixed height.
What is FPM in ductwork?
FPM is feet per minute, a linear air velocity unit. CFM measures volumetric airflow.
How do round and rectangular ducts compare?
At equal area and airflow, their average velocity is equal. This does not imply equal friction loss.
What does the selected velocity target mean?
It is your editable project criterion. Raw velocity is compared only against the bounds supplied; it is not a universal HVAC requirement.
Is equal-area round diameter the same as equal-friction equivalent diameter?
No. Equal-area round diameter preserves cross-sectional area. Equal-friction sizing uses a different empirical method and is not calculated here.
Does this calculator calculate duct pressure loss?
No. It estimates area and average velocity without friction, fitting or static-pressure calculations.
Does this calculator determine the correct or code-compliant duct size for an HVAC system?
No. It calculates geometry for the selected velocity. Complete system design needs additional project, equipment and compliance assessment.
Sources
Primary references reviewed October 5, 2026. NASA supports the swept-volume relationship; NIST supports conversion factors; ASHRAE supports duct terminology and the distinction between average velocity and broader system design. Geometry formulas are elementary circle and rectangle areas shown explicitly above. No application range is imported, no proprietary table is reproduced, and these sources do not endorse BuzCalculator.