Round option: — in | Rectangular option: — | Duct area: — sq in
Result Summary
| Airflow | — |
| Recommended size | — |
| Equivalent diameter | — |
| Actual velocity | — |
| Static pressure loss | — |
Common Round Duct Sizes (at ~700 FPM)
| Round size | Approx. CFM | Typical use |
|---|---|---|
| 4 in | ~40 | Small branch / single register |
| 5 in | ~60 | Branch run |
| 6 in | ~100 | Branch to one room |
| 7 in | ~140 | Larger room branch |
| 8 in | ~180 | Two-register branch |
| 10 in | ~300 | Small trunk |
| 12 in | ~450 | Trunk |
| 14 in | ~600 | Main trunk |
| 16 in | ~800 | Large main trunk |
CFM per Room (rule of thumb)
| Room | Typical CFM |
|---|---|
| Bedroom | 75 – 100 |
| Bathroom | 50 |
| Kitchen | 100 – 150 |
| Living room | 150 – 250 |
| Master suite | 150 – 200 |
Roughly 400 CFM per ton of cooling, or about 1 CFM per 30 BTU/hr.
Friction Rate Reference
| System | Typical friction rate |
|---|---|
| Residential | 0.08 – 0.10 in wc/100 ft |
| Light commercial | 0.10 – 0.15 in wc/100 ft |
| Low-velocity / quiet | 0.06 in wc/100 ft |
What Size Duct Do I Need?
Duct size depends on the airflow it must carry, measured in cubic feet per minute, and the speed you want that air to travel. Too small and the air moves fast and noisy with high pressure loss; too large wastes material and space. Start with the CFM the room or run needs, pick a target velocity for the duct role, and the cross-section follows: area equals CFM divided by velocity. The calculator above turns that into an equivalent round diameter, a standard round size, and a matching rectangular size, then estimates the static pressure lost over the run.
Duct CFM Chart
As a rule of thumb, a forced-air system moves about 400 CFM per ton of cooling, or roughly 1 CFM for every 30 BTU per hour of cooling load. From there, each room gets a share: bedrooms often need 75 to 100 CFM, a living room 150 to 250, and a bathroom around 50. The common-size chart above shows how much air each round duct carries at a typical residential velocity, so you can match branch and trunk sizes to the CFM they serve. Use the BTU-to-CFM helper to convert a room load straight into airflow.
Round vs Rectangular Duct Size
Round and rectangular ducts carry the same airflow when they share the same equivalent diameter, the round size that produces the same friction. Round duct is the most efficient shape and uses the least material for a given airflow, but rectangular duct fits flat in joist bays and walls. The trade-off is aspect ratio: a tall, narrow rectangular duct has more surface area and friction than a square one, so keep the ratio under about four to one. The calculator gives both a round size and a rectangular size for your chosen height so you can pick what fits.
Frequently Asked Questions
What velocity should I target? About 600 to 900 FPM for residential supply, lower for returns; commercial systems run higher.
How do I convert BTU to CFM? Divide cooling BTU per hour by about 30, or use 400 CFM per ton.
Is round or rectangular better? Round is more efficient and quieter; rectangular fits tight spaces. Match the equivalent diameter either way.
What is static pressure loss? The pressure the blower must overcome from friction along the duct and its fittings, measured in inches of water column.