Reproduce the free cross-sectional area and ideal circle diameter for one completed, same-section airflow and average-velocity record without selecting a duct.
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A quick decision brief for this specific tool
Inputs that matter
One named and revisioned completed duct cross-section arithmetic record; exact duct section or checkpoint, completed airflow source and row, completed average-velocity source and row, measurement or design method, responsible reviewer, positive airflow in CFM or m³/s, positive average velocity in FPM or m/s, completed measured/design/published source basis, explicit same-section/same-condition coherence, and cross-section-arithmetic-only decision boundary
Output to expect
Normalized airflow in m³/s and average velocity in m/s; required free cross-sectional area in m², ft² and in²; ideal-circle diameter in mm and in; complete section/source/method/reviewer evidence; deliberate page-session retention, copy/Markdown/print/share actions; and stable Project outputs that keep airflow/load derivation, duct-system design/sizing, standard size/shape/material selection, friction/effective-length/fitting calculation, blower/static/leakage/noise/balancing and code/installation/operation approval false
How it works
Validate one coherent completed same-section source record; convert CFM with exactly 0.00047194745m³/s per CFM and FPM with exactly 0.00508m/s per FPM; reproduce free area as volumetric airflow divided by average velocity; derive only the mathematical ideal-circle diameter as √(4A/π); retain exact source values and never round the diameter to a product size or create an equal-area rectangular recommendation
The exact section, condition, source revisions and rows, measurement/design method, airflow, average velocity, basis and reviewer must travel together. Another section, station, operating point, design condition, time, measurement, inferred value or source revision needs another responsible record.
Cross-section arithmetic is not duct-system design. Keep airflow/load derivation, equipment and blower operating point, geometry, standard size, shape, material, fittings/effective length, pressure/friction/dynamic loss, leakage, noise, balancing, fire/smoke, cost, code, installation, commissioning and operation with the applicable Manual D/ASHRAE workflow and responsible professionals.
Choose your path
Built around the job you need to finish
Reproduce the free cross-sectional area and ideal circle diameter for one completed source-owned, same-section airflow and average-velocity record without presenting that arithmetic as duct-system design, standard-size or shape selection, friction analysis, equipment approval or installation advice.
Commissioning or test-data reviewer checking one section point
Reproduce a completed section's area while keeping its measured airflow, average velocity, exact station, condition, method and responsibility together.
Name one completed record and section, retain both source rows and the method, enter each source unit, affirm same-section/same-condition coherence and reconcile the arithmetic.
Receives exact normalized values, free area and ideal-circle geometry without the Tool claiming measurement quality, system performance, leakage, noise or balancing acceptance.
Designer or record historian reproducing one worksheet row
Audit Q = vA across CFM/FPM and SI units without silently rounding the result to a catalog size or equal-area rectangle.
Identify the source-owned design checkpoint, enter the completed airflow and average velocity, inspect all unit-normalized outputs, retain a snapshot, edit the live record and verify stale-result removal.
Can reproduce and retain the arithmetic while material, shape, standard size, friction rate, effective length, fittings and blower operating point remain with the owning design method.
Phone technician testing a familiar duct-size promise
Avoid receiving a false installation recommendation from two numbers that cannot represent the complete system design.
Load the static example, trigger mixed-section and system-design refusals, recover with a coherent arithmetic-only record, reconcile, retain, edit and Reset.
Unsupported requests fail at their owning field with no stale result/action, and the Tool never recommends a nominal round size, rectangle, material, velocity target or installation decision.
Authoritative checks for this tool
Outputs and checklists are planning aids. Review the linked current authorities and the records, terms, instructions, and requirements that apply to your exact situation before a consequential decision.
Name the exact section, revision, airflow source, average-velocity source, measurement or design method, source basis and responsible reviewer. Both numeric values must represent the same section and the same operating or design condition.
one section + one condition + source-owned Q and v
Normalize airflow and average velocity
CFM is converted with 1 CFM = 0.00047194745 m³/s and FPM with 1 FPM = 0.00508 m/s. Source values and units remain visible so the arithmetic can be traced.
QSI = QCFM × 0.00047194745; vSI = vFPM × 0.00508
Reproduce area and ideal-circle geometry
Divide volumetric airflow by average velocity for free cross-sectional area, then report the diameter of an ideal circle with that area. The diameter is geometry evidence, not a nominal or standard duct selection.
A = Q ÷ v; dideal = √(4A ÷ π)
Updated: August 2026
Example Scenarios
Retain one measured airflow and average-velocity point, its test method and exact station. Reproduce area without treating the result as proof of measurement quality, system performance, leakage, noise or balancing.
Use the row's completed airflow and target average velocity to verify its free-area arithmetic. Keep material, shape, standard size, friction rate, effective length, fittings and blower operating point in the owning design record.
The Tool refuses that decision. A responsible duct-design workflow must reconcile equipment and blower data, geometry, materials, fittings, pressure losses, leakage, noise, balancing, code and installation constraints.
Common Mistakes to Avoid
✕
Rounding the ideal circle diameter to a product size
✓
Keep the exact ideal-circle geometry visible and route selection to the responsible duct-design record. This Tool owns no product series, nominal dimension or acceptance rule.
✕
Calling equal-area rectangles equivalent ducts
✓
Equal area does not prove equal friction or performance. Do not create a rectangular recommendation without the applicable shape/friction method and full design inputs.
✕
Mixing airflow and velocity from different sections or conditions
✓
Fail closed. Reconcile only one coherent source checkpoint, or return to the responsible measurement/design workflow to establish a valid pair.
FAQ
No. It is the diameter of a mathematical circle with the reproduced free area. Nominal dimensions, liners, connections, available products, shape, material, friction and installation can all change a real design.
Equal area alone does not establish equal friction or equivalent performance across shapes. Shape selection and equivalency belong to the responsible Manual D or other applicable duct-design method.
No. It accepts one completed source-owned average velocity. It does not derive or approve a velocity criterion for supply, return, branch, commercial, residential, rigid or flexible duct.
Keep the exact section and condition, source revision and rows, method, basis and responsible reviewer. Mixed sections, operating points or inferred values are refused.
Airflow and equipment ownership, blower operating point, geometry, material, shape, fittings and effective length, pressure/friction losses, leakage, noise, balancing, fire/smoke, code, installation and commissioning remain outside this arithmetic record.
About Duct Cross-Section Arithmetic
Reconcile one completed source-owned airflow and average-velocity checkpoint for one exact duct section. The Tool normalizes CFM or m³/s and FPM or m/s, reproduces free area with A = Q ÷ v, and reports the ideal circle diameter for that area. It does not derive airflow, recommend a standard round size, create a rectangular equivalent, calculate friction or design, select or approve a duct system.
