Introduction
This duct size calculator helps you pick the right duct for your HVAC system. Type in your airflow (CFM), and the tool tells you what size round or rectangular duct you need.
Duct size matters a lot. A duct that is too small makes air move too fast. That means loud noise, weak airflow, and a fan that works too hard. A duct that is too big wastes money and space. The right size keeps air moving quietly and evenly to every room.
The calculator runs two checks: air speed (velocity) and friction loss. It picks the bigger of the two answers, then rounds up to the next real duct size you can buy. You can choose metal or flex duct, supply, return, or exhaust air, and switch between imperial and metric units.
You also get a side-by-side look at round versus rectangular duct, a chart that shows what happens if you go up or down a size, and a full step-by-step solution so you can see how each number was found. Use it to plan a new system, fix weak airflow, or double-check a duct design before you build.
How to use our Duct Size Calculator
Enter your airflow and a few duct details, and the calculator shows the duct size you need, the air speed inside it, the friction loss, and a round versus rectangular comparison.
Unit System: Pick Imperial or Metric. This sets the units for all input boxes at once. Your numbers convert on their own.
Airflow Rate: Type the air volume this duct run carries. Choose CFM, m³/h, L/s, or m³/s. This is the only input you must fill in.
Maximum Air Velocity: Set the fastest air speed you allow in the duct. Lower speeds mean quieter ducts. Higher speeds mean smaller ducts.
Max. Static Loss / 100 ft: Set the friction loss you allow per 100 feet of duct. Most homes use 0.08 to 0.10 inWg/100 ft.
Duct Material: Choose Metal or Flex. Flex duct is rougher, so the calculator adds a 1.5× roughness factor and a bigger diameter.
Duct Shape: Choose Round or Rectangular. The one you pick becomes the main answer, but both shapes are always shown side by side.
Application: Choose Supply, Return, or Exhaust. Each one has its own target air speed range used to check your result.
Max Aspect Ratio: This shows only for rectangular duct. It limits how flat the duct can get, like 2:1. Flatter ducts are less efficient and louder.
Display Units for Results: Choose the units for duct size, velocity, and friction loss in the results. Change them any time without redoing your inputs.
Click Calculate to see your duct size, velocity gauge, warnings, sensitivity table, chart, step-by-step math, and a size reference chart. Click Reset to start over, or Fill with Sample Data to try an example.
What Duct Sizing Means
Ducts are the tubes that carry air from your furnace, air handler, or fan to each room. Duct sizing is picking the right width of that tube for the amount of air it must carry. If a duct is too small, the air moves too fast. That makes noise, cuts airflow, and makes the blower work harder. If a duct is too big, it costs more, wastes space, and can let air drop too slowly out of the vents.
The Three Numbers That Set Duct Size
- Airflow (CFM): Cubic feet of air per minute. This is how much air the duct must move. More air needs a bigger duct. Use the CFM Calculator to work this out room by room.
- Air velocity (FPM): Feet per minute. This is how fast the air travels. Fast air is loud. Slow air is quiet but needs a larger duct.
- Friction loss: The pressure the air loses as it rubs on the duct walls. It is measured per 100 feet of duct. Most homes are designed around 0.08 to 0.10 inches of water per 100 feet.
Duct size is set by whichever limit is stricter, speed or friction. The duct must pass both tests, then get rounded up to the next size you can actually buy.
Normal Air Speed by Duct Job
| Duct Type | Common Velocity | Why |
|---|---|---|
| Supply | 600–900 FPM | Pushes air out to rooms with good throw and low noise |
| Return | 400–700 FPM | Slower air keeps returns and grilles quiet |
| Exhaust | 400–600 FPM | Steady pull without whistling at the grille |
Round vs. Rectangular Duct
Round duct moves air best. It has the least surface rubbing on the air, so it loses the least pressure and is often cheaper to install. Rectangular duct is used when the space is tight, like inside a wall or above a low ceiling. To compare the two, we use the equivalent diameter, the size of round duct that flows the same as a given rectangle.
Rectangular duct also has an aspect ratio, which is width divided by height. A 20 x 10 duct is 2:1. Try to stay at or below 3:1. Flat, wide ducts above 4:1 lose more pressure, use more metal, and get noisy.
Metal vs. Flex Duct
Smooth sheet metal has low friction. Flexible duct has ribbed walls that drag on the air, so it loses about 1.5 times more pressure than metal at the same size. Because of that, flex runs usually need to be one size larger. Flex must also be pulled tight and fully stretched. A sagging or squeezed flex duct can lose a large part of its airflow.
Tips for Better Duct Design
- Always round up to the next standard size, never down.
- Keep runs short and straight. Every elbow adds pressure loss.
- Size the return duct as carefully as the supply. Starved returns are a common cause of weak airflow.
- Seal every joint with mastic or foil tape, and insulate ducts in attics, crawl spaces, and garages.
- For very high airflow, split the run into two ducts instead of using one huge one.
Good duct sizing gives you even room temperatures, quiet vents, lower power bills, and longer equipment life. For a full job, pair these numbers with a room-by-room load calculation using the BTU Calculator before you buy material.