Is the Duct Sizing & Velocity Checker free to use?

Yes. The Duct Sizing & Velocity Checker is a free browser-based planning calculator from I Love MEP.

Can the Duct Sizing & Velocity Checker be used for final engineering design?

No. The result is for planning, tender review, and coordination support. Final design must be checked by a qualified engineer against project specifications, authority requirements, and applicable standards.

Does the PDF report replace approved design calculations?

No. The PDF report records inputs, assumptions, warnings, and planning results. It should be reviewed before any construction, procurement, or authority submission decision.

Duct Sizing & Velocity Checker

Free duct size calculator by CFM for HVAC planning. Check rectangular duct velocity, equivalent round duct size, and airflow limits before final pressure-loss design.

Last updated: July 27, 2026

What is a duct sizing calculator?

A duct sizing calculator checks whether a rectangular duct size is reasonable for a given airflow. It converts CFM into metric airflow, calculates duct area, estimates velocity, and compares the result with a target velocity for comfort HVAC planning. Engineers use it to catch undersized ducts, likely noise issues, oversized ductwork, and coordination problems before detailed pressure loss calculations are finished. It is useful for AHU branches, FCU connections, risers, corridor ducts, and tender drawing reviews. The calculator also gives an equivalent round duct diameter for quick comparison. Final duct design still needs friction loss, fittings, dampers, leakage class, acoustic criteria, available static pressure, fire and smoke damper coordination, insulation, access, and approved project standards. For best results, keep the entered units consistent, load the sample first, then adjust one assumption at a time so the result can be reviewed and explained.

Inputs

Forms are empty on load. Use sample data to preview the workflow or enter your own project values.

Learn more

Formula logic, use cases, assumptions, and limits for this tool.

How this tool helps

Use this to check whether a selected duct is likely to be noisy, oversized, undersized, or acceptable for planning. Final duct selection must consider pressure loss, fittings, acoustic criteria, and approved project standards.

Assumptions and limits

This tool provides a planning-level result. It does not replace detailed engineering design, approved software, manufacturer data, authority requirements, or project specifications.

Typical use cases

Early estimation and tender support.
BOQ sanity checking and technical review.
Site discussion before final engineering verification.
Comparison of alternatives using consistent assumptions.

Result interpretation

Read the recommendation, warning, and confidence sections together. A result without warnings is still not an approval; it only means the entered values did not trigger built-in planning alerts.

Input definitions and typical values

These notes explain what each input means and which values usually produce a more reliable planning result.

Code / standard profile

Selects the planning profile used for conservative defaults. Use the project authority or client standard where known; otherwise use the closest regional profile. Options: GCC project planning, ASHRAE planning, CIBSE planning.

Airflow CFM

Air volume flow rate. Comfort HVAC rooms often use hundreds to thousands of CFM depending on load, air changes, and occupancy.

Duct width mm

Measured width of the room, duct, tray, or equipment zone. Use internal clear dimensions when calculating area, velocity, or capacity.

Duct height mm

Clear internal height or equipment height. Typical occupied rooms are 2.7-3.2 m; warehouses, atria, and plant rooms can be much higher.

Target maximum velocity m/s

Target velocity limit. Comfort ducts often target about 5-7 m/s in mains; water services often stay near 1-2.5 m/s depending system.

Planning friction allowance Pa/m

Planning pressure-loss allowance. Use project duct class or hydraulic design target where known.

Practical verification checklist

Confirm the selected standard profile is suitable for the project location and authority.
Replace sample values with project drawings, schedules, equipment data, or measured site values.
Review warnings, methodology, and PDF report before using the result outside early planning.
Verify final decisions against official standards, manufacturer data, consultant specifications, and local approvals.

Standards references for verification

For final design checks, verify calculator assumptions against the project documents and official standards bodies such as ASHRAE, NFPA, IEC, IET BS 7671, ICC, and CIBSE.

Calculation methodology

Velocity = airflow / duct area. Equivalent round diameter uses a rectangular duct approximation for planning comparison.

Inputs are treated as planning assumptions. The calculator shows warnings when entered values fall outside practical thresholds or selected standard-profile limits.

View full methodology notes

Version history

v1.3.6-prelaunch: Added methodology, standard profile support, version notes, and direct PDF report download.
v1.3.5: Calculator page added with sample data, warnings, and print-ready report layout.

Stage 2 roadmap: optional saved projects, dashboards, and shared calculation history without forcing login for free use.

Trust and verification

Every result includes assumptions, warning logic, and a downloadable report. Final engineering decisions still need qualified review, local authority approval, manufacturer data, and project specifications.

Worked example

Use the sample data button to load a realistic scenario, then download the PDF report to review inputs, results, methodology, warnings, and disclaimers in one place.