Built to spec
Round, rectangular, and spiral duct built to SMACNA standards.
Free tool · No signup
HVAC Duct Size Calculator Size by CFM, Friction Rate, or Velocity
Size round or rectangular duct against a target friction rate or a target velocity. Enter your CFM and the tool returns duct size, air velocity, friction loss, and total effective length including the equivalent length of your bends.
- Size by
- Friction rate
or velocity
- Shapes
- Round &
rectangular
- Standard sizes
- 4" through
36" round
- Cost
- Free,
no signup
Duct2Go CFM chart tool
Metal duct calculator
Enter the airflow you need and the run you're working with. Leave the size blank to have the tool pick the nearest standard duct.
Calculation parameters
Results
Enter your parameters and select Calculate to see results.
Reference
Standard sizes & target ranges
| Duct role | Velocity (FPM) | Friction rate (in wc / 100 ft) |
|---|---|---|
| Supply duct | 600–1200 | 0.05–0.15 |
| Return duct | 400–800 | 0.05–0.15 |
| Main trunk | 900–1400 | 0.05–0.15 |
| Low-pressure system | — | 0.05–0.08 |
| High-pressure system | — | 0.10–0.15 |
Standard round duct sizes we stock and fabricate: 4", 5", 6", 7", 8", 9", 10", 12", 14", 16", 18", 20", 22", 24", 26", 28", 30", 32", 34", and 36".
Why it matters
Wrong duct size costs the same as the wrong unit
Ducts that are too large or too small produce identical symptoms: uneven room temperatures, noisy systems, and rising energy bills.
Undersized or oversized runs wear out HVAC components early, cause poor airflow and uncomfortable rooms, generate callbacks and warranty claims, and raise long-term energy costs. For a contractor that means unhappy clients and thinner margins on jobs you've already priced.
Sizing method
How to calculate duct size
The calculator handles steps four and five. These are the numbers you need before you get there.
Measure square footage
Start with each individual room, not the building total. A 10×10 room is 100 sq ft. For irregular shapes, divide the space into sections and add them up.
Determine airflow in CFM
Run a Manual J load calculation to find the BTUs each room needs, then: Room CFM = (room load ÷ whole-house load) × equipment CFM. As a rule of thumb 1 CFM covers 1–1.25 sq ft of floor, though sunny rooms and large window walls can need closer to 2.
Select the equipment
Divide the building's BTU load by 12,000 for required tonnage — 24,000 BTU ÷ 12,000 = 2 tons. Multiply tonnage by 400 CFM to get system airflow.
Calculate friction loss and static pressure
Friction rate = (available static pressure × 100) ÷ total effective length. This is the pressure drop your system can absorb, and it drives both duct size and layout.
Account for the material
Sheet metal offers the least airflow resistance of any duct material, well ahead of flex or fiberglass duct board. Galvanized fabrication gives you the durability and airflow consistency both residential and commercial installs depend on.
The fabricator
Sized right, then built right
Accurate sizing is half the job. The other half is a shop that fabricates to the number you calculated.
Fast turnaround
Short lead times on standard work, so duct isn't what holds the schedule up.
Nationwide delivery
Reliable palletized freight for contractors and builders across the U.S.
Made in the USA
High-quality sheet metal, cut and fabricated on our own floor in Chandler.
Next step
Got your sizes? We'll build them
Take the numbers from the calculator straight to a quote, or configure the run yourself and check out online.
- Shop
- 3385 N Nevada St
Chandler, AZ 85225 - Hours
- Mon–Fri, 8:00–5:00
- Phone
- (480) 452-8069
- contact@duct2go.com
- Freight
- Palletized LTL, all 50 states