Duct / CFM sizing calculator
Sizing a duct starts with airflow: how many cubic feet per minute (CFM), or litres per second, the run has to carry. Too small and it whistles, starves the room and runs the static pressure up; too large and it wastes sheet metal and space in the framing.
This calculator turns a required airflow and a duct role — branch, trunk or return — into a buyable round diameter, a set of equivalent rectangular options for a fixed framing depth, and the resulting air velocity checked against a recommended cap. Enter several runs and it also sizes the main trunk from the summed branch airflow. Everything runs in your browser.
How to size a duct
Pick your units and a sizing method, enter each run's airflow and role, and read the round diameter, rectangular options and velocity. The friction rate, velocity caps and every curve-fit constant are editable assumptions, so you can match your own design targets rather than a fixed rule of thumb.
The default method is equal-friction: every duct is sized to the same pressure loss per unit length, the standard residential trade approach. The alternate method sizes straight from a target velocity instead.
How to size a duct with this calculator
- Choose units and a sizing methodPick metric or imperial and equal-friction (the default) or the velocity method.
- Add your runsEnter each run's required airflow and its role — branch, trunk or return. Add a row per room for a whole-home schedule.
- Tune the assumptionsSet the friction rate, velocity caps, aspect-ratio cap and a preferred rectangular height to match your own design targets and framing depth.
- Read the sizesRead each run's round diameter, equivalent rectangular options and velocity with its guard tier; the rail sizes the main trunk from the summed branch airflow.
A couple of worked sizes at the default 0.08 in.wc/100 ft friction rate. Your own numbers will differ once you set the friction rate, material and caps to your design.
Single branch — 400 CFM
Branch- Given
- Required airflow 400 CFM
- Friction rate 0.10 in.wc/100 ft, sheet metal
- Formula
D = (0.109136 × 400^1.9 ÷ 0.10)^(1 ÷ 5.02)- Steps
- Raw diameter ≈ 9.83 in
- Snap up to the nearest buyable size → 10 in round
- Velocity: 576 × 400 ÷ (π × 10²) ≈ 733 fpm (comfortable for a branch)
- Result
- 10 in round ≈ 733 fpm — or a 16 × 8 in rectangle at the same equivalent diameter
Whole home — trunk from branches
Trunk- Given
- Living room 400 CFM, bedroom 200 CFM, kitchen 150 CFM
- Default friction rate and trunk velocity cap
- Formula
trunk airflow = Σ branch airflow- Steps
- Sum the branches: 400 + 200 + 150 = 750 CFM
- Size the trunk at that total using the trunk role cap
- The rail shows the trunk round diameter, a rectangular option and its velocity
- Result
- A 750 CFM trunk sized from the summed branch airflow
The equal-friction round size
The round diameter comes from the Huebscher equal-friction relation, which ties airflow and a design friction rate to a duct diameter. The default friction rate is 0.08 in.wc per 100 ft, a common residential design value. The raw diameter is then snapped up to the nearest buyable size on a manufacturer ladder — never down, because undersizing is the worse failure.
Flex duct is rougher than smooth sheet metal, so when the material is set to flex the calculator upsizes the raw diameter before snapping. That multiplier is a labelled estimate you can adjust.
Every size here comes from a handful of transparent, editable formulas — no black box.
Equal-friction round size
D(in) = (0.109136 × CFM^1.9 ÷ fr)^(1 ÷ 5.02)- The Huebscher equal-friction relation: fr is the design friction rate in in.wc per 100 ft.
- The raw diameter is then snapped up to the nearest buyable round size.
Velocity cross-check
V(fpm) = 576 × CFM ÷ (π × D²)- An exact identity from airflow = area × velocity, with D in inches.
- Rearranged, D = √(576 × CFM ÷ (π × V)) is the velocity sizing method.
ASHRAE equivalent rectangle
De(in) = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25- a and b are the rectangle's sides in inches; De is the round duct that carries the same air.
- For each standard height the width is solved so De matches the round target, then rounded up.
Airflow from tonnage
total airflow = capacity × airflow per capacity- The residential rule of thumb is about 400 CFM per ton of cooling.
- A 3-ton system is roughly 1,200 CFM — enough to size the trunk.
The raw diameter is snapped up to the nearest size on these manufacturer ladders — never down, because undersizing is the worse failure.
| System | Available diameters |
|---|---|
| Imperial (in) | 4 · 5 · 6 · 7 · 8 · 9 · 10 · 12 · 14 · 16 · 18 · 20 · 22 · 24 |
| Metric (mm) | 100 · 125 · 160 · 200 · 250 · 315 · 400 · 500 · 630 · 800 · 1,000 |
| Friction rate (in.wc/100 ft) | ≈ Pa/m | Typical use |
|---|---|---|
| 0.06 | 0.49 | Longer runs or lower available static — larger, quieter ducts. |
| 0.08 | 0.65 | The common residential design default. |
| 0.10 | 0.82 | Shorter runs or higher available static — smaller, faster ducts. |
What the live diagram shows
The tool draws the same figure for every run and the trunk: the round duct beside its equivalent rectangle at one shared scale, and a velocity-versus-cap gauge coloured by the guard tier.
Same airflow, three friction rates
400 CFM sized at three design friction rates. A higher friction rate allows a smaller, faster duct; a lower one calls for a larger, quieter duct — the same airflow, three diameters.
Round and rectangular carry the same air
A round diameter beside its equivalent rectangle, drawn to one scale so the eye reads the same cross-section either way — shown at two airflows.
Equivalent rectangular ducts
When a round duct won't fit the framing, the calculator offers rectangular ducts that move the same air. It uses the ASHRAE equivalent-diameter relation and, for each standard height, solves the width that matches the round target, rounding the width up to the nearest increment so the option is never undersized. Options whose aspect ratio exceeds the cap are dropped, because very flat ducts are inefficient and hard to seal.
Set a preferred height to flag the row that matches your joist or soffit depth.
The velocity cross-check
Whatever size you land on, the air still has to move through it at a sensible speed. The calculator computes the velocity at the snapped diameter and compares it to the role's cap — branches quieter than trunks, returns in between — then flags it green when comfortable, amber near the cap and red when over. High velocity means noise and pressure loss; very low velocity can mean an oversized, wasteful duct.
The velocity at the snapped size is checked against the role's cap. Branches run quietest, trunks fastest, returns in between.
| Role | Cap (fpm) | Cap (m/s) |
|---|---|---|
| Branch | 600 | 3.05 |
| Trunk | 900 | 4.57 |
| Return | 700 | 3.56 |
Airflow from tonnage
When you don't have a room-by-room airflow, you can estimate a total from the cooling capacity. The standard rule of thumb is about 400 CFM per ton of cooling. Enter the capacity and the airflow-per-capacity figure and the helper gives a total airflow you can drop straight onto a run to size the trunk.
Metric and imperial
Switch units at the top and every field, result and label follows. Imperial reads airflow in CFM, diameters in inches, friction in in.wc per 100 ft and velocity in feet per minute; metric reads litres per second, millimetres, pascals per metre and metres per second. Your entries convert once on the switch, so nothing drifts as you keep editing.
Frequently asked questions
What size duct do I need for a given CFM?
Enter the airflow and role and the calculator returns a buyable round diameter sized to the design friction rate, plus equivalent rectangular options. As a rough guide at 0.08 in.wc per 100 ft, a 100 CFM branch is about a 6 in round and a 400 CFM run about a 10 in round — but the exact size depends on your friction rate and material, which are editable here.
Should I size by friction rate or velocity?
Equal-friction is the standard residential method and the default: every duct shares the same pressure loss per unit length. The velocity method sizes straight from a target speed and is handy when noise is the binding constraint. Either way the calculator always cross-checks the resulting velocity against the role cap.
How do I convert a round duct to rectangular?
The calculator uses the ASHRAE equivalent-diameter relation: for each standard height it finds the width that carries the same air as the round duct, rounds it up to a buyable increment and drops options that are too flat. Pick the row whose height matches your framing depth.
Is this a Manual-D design?
No. This is a rule-of-thumb sizing estimate, not a full static-pressure ductulator or an ACCA Manual-D duct design, which accounts for total effective length, fittings, registers and the equipment's available static pressure. Use this to sanity-check a size, then verify with an HVAC professional before fabrication.