~10" duct · 733 fpm actual
Recommended Round Duct
10"
or 10" × 8" rectangular
Actual Velocity
733
fpm
Friction Loss
0.09
in. w.c. / 100 ft
How we calculated this
= 0.50 ft²
→ 10″ standard
Sized for straight galvanized duct. Flex duct needs one size up, and long runs with many fittings need a full friction-rate calculation.
Reference
Round Duct CFM Capacity
Airflow each standard round duct size carries at a quiet branch velocity (600 fpm) and a typical trunk velocity (800 fpm).
| Duct Diameter | CFM @ 600 fpm | CFM @ 800 fpm |
|---|---|---|
| 4" | 50 | 70 |
| 5" | 80 | 110 |
| 6" | 120 | 155 |
| 7" | 160 | 215 |
| 8" | 210 | 280 |
| 9" | 265 | 355 |
| 10" | 325 | 435 |
| 12" | 470 | 630 |
| 14" | 640 | 855 |
Learn
Understanding Duct Sizing
How duct sizing works
A duct is sized so the air it carries moves at a sensible speed. Divide the airflow (CFM) by the target velocity (feet per minute) and you get the cross-sectional area the duct needs: Area = CFM / velocity. From there it's geometry. Round duct gets a diameter, rectangular duct gets a width and height, and you round up to the next size that's actually manufactured.
Velocity targets by application
Velocity is a noise and efficiency trade-off, not a physics constant. Supply trunks run 700–900 fpm because they're usually buried in framing where some air noise is fine. Branch runs feeding individual rooms drop to 500–700 fpm. Return ducts run slowest, 400–600 fpm, because they sit closest to living spaces and any whoosh is audible at the grille. Pick the application, and the right velocity follows.
Round vs. rectangular
Round duct is the most efficient shape: the most area for the least metal and the least friction. Rectangular duct exists to fit where round won't, inside joist bays, wall chases, and tight ceilings. The catch is that a rectangle and a circle with equal area do not carry equal air, because the rectangle has more wall surface creating drag. The equivalent diameter formula (De = 1.30 × (a×b)^0.625 / (a+b)^0.25) corrects for this, and it's what this calculator uses to convert. The flatter the rectangle, the bigger it has to be.
Friction loss and static pressure
Every foot of duct resists airflow, and that resistance is friction loss, measured in inches of water column per 100 ft. Residential designs typically target around 0.08–0.10 in. w.c. per 100 ft. If the number for your run comes out well above that, the duct is too small for the airflow even if the velocity looks acceptable. Fittings, takeoffs, and elbows add resistance on top, which is why a full design uses equivalent lengths for every fitting.
Flex duct is not sheet metal
The friction numbers here assume straight galvanized duct. Flexible duct, even pulled tight, has roughly 50% more friction, and sagging or compressed flex can double or triple it. The practical rule: if a run will be flex, go up one duct size from the sheet-metal recommendation and pull it taut with proper supports.
FAQ
Common Questions About Duct Sizing
What size duct do I need for 400 CFM?
At a typical 800 fpm trunk velocity, 400 CFM works out to a 10" round duct (the exact requirement is 9.6", so you round up to the next standard size). At a quieter 600 fpm target it needs 12". In rectangular duct with an 8" height, the equivalent is about 10" × 8".
How do you calculate duct size from CFM?
Divide CFM by your target velocity (feet per minute) to get the required cross-sectional area in square feet: Area = CFM / velocity. For round duct, diameter = √(4 × Area / π) × 12 inches, rounded up to the next standard size. A 400 CFM run at 800 fpm needs 0.5 ft², which is a 10" round duct. This calculator does both steps, plus the rectangular conversion, as you type.
What is a good air velocity for residential ductwork?
For homes: supply trunks 700–900 fpm, branch runs 500–700 fpm, and return ducts 400–600 fpm. Faster air means smaller (cheaper) ducts but more noise and static pressure, so returns run slowest because they're closest to living spaces and the blower. Commercial systems tolerate 1,000–1,500 fpm where background noise is higher.
How do round and rectangular duct sizes compare?
Through the equivalent diameter formula: De = 1.30 × (a×b)^0.625 / (a+b)^0.25. It finds the rectangle with the same friction loss and airflow as a given round duct. A 10" × 8" rectangle is equivalent to roughly a 9.8" round duct. The flatter the rectangle, the more perimeter (and friction) it has for the same airflow, so a 20" × 4" duct carries much less air than its area suggests. Round duct is always the most efficient shape; rectangular exists to fit joist bays and chases.
What happens if my ducts are undersized?
Velocity and static pressure climb. The blower works harder and moves less air than design, rooms at the end of long runs starve for airflow, and the registers get noisy. Undersized return ducts are the most common culprit in homes with high static pressure. If a run is borderline between two sizes, go up: the larger duct costs a little more once, while the pressure penalty costs comfort and blower life forever.
Related Tools
CFM Calculator
Work out how much airflow a room needs before sizing the duct.
↳ 12×10 bedroom · 5 ACH → 80 CFM
HVAC Load Calculator
Size the whole system the ductwork has to serve.
↳ 2,000 sq ft · moderate → 3–3.5 tons
BTU Calculator
Match cooling capacity to the room at the end of the run.
↳ 250 sq ft · hot climate → ~9,000 BTU