Duct Size Calculator
Size the run — CFM ÷ velocity gives the area, the area gives the round, and the 0.1-inch friction rule sanity-checks it.
Duct Size Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Velocity method — no equal-friction chart lookup
- Round equivalents only; rectangular named, not sized
In short: 400 CFM — the per-ton line — at a 700 FPM design velocity needs 0.571429 ft² of duct: a 10.235694-inch round, which is the friction chart's 10-inch almost exactly. At the 0.1 in. w.c./100 ft rule a 10-inch carries 350–400 CFM, so the velocity math and the chart agree from two directions. Flex duct takes one to two inches of that back.
Formula
area = CFM ÷ FPM; diameter = √(4 × area ÷ π) × 12 in
Duct sizing is one division and one square root: the CFM the branch carries divided by the velocity it should move at is the cross-section in square feet, and the equivalent round falls out of the circle's area formula. The two house rules frame the answer: 400 CFM per ton of cooling sets the flow, and a friction rate of 0.1 inches of water column per 100 feet (0.08 for flex and long runs) keeps the blower's bill sane. Residential velocities run 400–900 FPM — above that the duct whistles, below it the duct dusts.
Worked Example
- Enter the branch CFM — 400 per ton is the residential rule.
- Pick the design velocity; 700 FPM is the quiet default.
- Read the round, then upsize one to two sizes for flex.
Defaults: 10.235694 inches — the chart's 10. Drive to 1,200 CFM at 800 FPM (the 3-ton trunk) and the round reads 16.583719 inches — trunk sizes are not branch sizes.
Strengths & Limits Of This Model
Where this engine is strong
- The 400/ton and 0.1-in. rules ride inside the answer
- Flex upsize named — the classic field error, pre-empted
Where it stops
- Chart-exact sizes need Manual D, not a formula
Practical Use Cases
Branch runs
the 4-to-8-inch feeders
Trunk sizing
the 1,200-CFM question
Flex checks
what the upsize costs
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Duct Size Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What size duct do I need for 400 CFM?
At a 700 FPM design velocity: 0.571 ft², a 10.2-inch round — call it the chart's 10-inch, which carries 350–400 CFM at the 0.1 in. w.c./100 ft friction rule. The velocity method and the friction chart meet at the same nominal size.
What is the 400 CFM per ton rule?
Residential cooling moves about 400 CFM per ton (12,000 BTU/h) of capacity — a 3-ton system targets 1,200 CFM total, a 5-ton 2,000. High-efficiency variable-speed coils run nearer 350. It sets blower speeds and trunk sizes before Manual D draws the map.
What does the friction rate mean?
Pressure lost per 100 feet of duct: the design baseline is 0.1 inches of water column per 100 feet for rigid metal, 0.08 for flex or long fitting-heavy runs. The rate is what the friction chart's whole table is drawn at.
How much does flex duct cost me?
One to two duct sizes, per every chart's advice — flex's corrugated wall runs roughly double the friction of smooth metal. A 10-inch metal branch becomes a 12-inch flex branch for the same CFM.
Why cap the velocity at 900 FPM?
Noise first — regenerative noise rises steeply with velocity — then friction, which grows with its square. Residential supply runs sit at 400–900 FPM; main trunks nearer 700–900.
Round or rectangular?
Round is the efficient shape — the least wall per area. Rectangular equivalents exist for tight spaces (an 8 × 10 carries a 10-round's air), and the equivalent-diameter tables keep the friction match.
Is the duct size the whole design?
No — it is one branch of Manual D. Total effective length, fitting equivalents and the blower's curve decide whether the design friction rate is honest. This page is the sizing step, not the commissioning step.
What velocity for a return duct?
Returns run quieter and slower — about 600–750 FPM where supplies take up to 900. Undersized returns are the classic complaints call: the system starves before the supply ever whistles.