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Diffuser Neck Size Calculator

Find the round or square neck a ceiling diffuser needs for its airflow at the neck velocity you choose, or check how fast air moves through a neck you already have.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Example value. Take the neck velocity from the diffuser maker’s performance table for the noise level (NC) and throw you need — they differ between models.

Neck shape
Check an existing neck?

Result

Round neck size (next standard)
9in
Neck velocity at that size
453fpm

200 CFM at up to 500 fpm needs a 9-inch round neck.

  • Throw, spread and noise depend on the diffuser model; check the maker’s performance data at your airflow and neck size.
Exact diameter
8.56in
Neck area needed
57.6in²
Show the working
  1. Neck area = CFM ÷ velocity
    200 ÷ 500 = 0.4 ft² = 57.6 in²
  2. Diameter = √(4 × area ÷ π)
    8.56 in
  3. Velocity at the chosen size
    200 ÷ 0.4418 ft² = 453 fpm

How it works

Neck area = airflow ÷ neck velocity. The calculator turns the area into a diameter (round) or side (square), rounds up to the next standard round size or whole inch, and shows the velocity at that size.

Choosing the neck velocity

Faster air through the neck means more noise and longer throw. Diffuser makers publish performance tables listing, for each neck size, the airflow, throw and noise rating (NC). Pick the velocity from the table for the noise level your room needs; the form’s value is only an example.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Airflow through the diffuser
200 CFM
Maximum neck velocity
500 fpm
Neck shape
Round neck
Check an existing neck?
No

Result

Round neck size (next standard)
9 in
Neck velocity at that size
453 fpm
Exact diameter
8.56 in
Neck area needed
57.6 in²

200 CFM at up to 500 fpm needs a 9-inch round neck.

Assumptions

  • Neck area = airflow ÷ chosen neck velocity. Round necks are rounded up to the same standard round sizes as the duct size calculator; square necks to whole inches.

Limitations

  • The velocity limit, throw and noise rating are product data from the diffuser maker; this tool does not supply them.

Frequently asked questions

What size diffuser neck for 200 CFM?

At a 500 fpm neck velocity, 200 CFM needs 57.6 square inches — about 8.6 inches round, so a 9-inch neck (or the next size your supplier stocks).

Is neck velocity the same as face velocity?

No. Neck velocity is in the connection collar; face velocity is across the diffuser face. Makers list one or the other — use the one their table gives.

Does the calculator predict throw or noise?

No. Throw and noise depend on the diffuser’s design and come from the maker’s data.

Sources

  1. Manual D calculation checklist (Table 3-1 velocity limits) — ACCA (hosted by AHRI). Friction rate = available static pressure × 100 / total effective length; residential velocity limits (rigid trunk 700 recommended / 900 max; flex 600 / 700).
  2. NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.

Spotted an error? Report a problem with this calculator — see our corrections policy.