Duct Size Calculator

Q: What does inWg per 100 ft mean?

inWg means inches of water gauge . It is a way to measure small amounts of air pressure. "Per 100 ft" means the pressure lost over 100 feet of straight duct. So 0.08 inWg/100 ft means the air loses 0.08 inches of pressure every 100 feet.

Q: What friction rate should I use?

Most homes use 0.08 to 0.10 inWg/100 ft . Use 0.08 for long duct runs or systems with many turns. Use 0.10 for short, simple runs. Commercial jobs often go higher. If you are not sure, leave it at 0.08.

Q: Does this calculator include elbows and fittings?

No. It sizes straight duct only. Elbows, tees, boots, and dampers add extra pressure loss. Each elbow can act like 10 to 25 extra feet of duct. Add that length to your run when you plan total pressure, or keep the friction rate low to leave room for fittings.

Q: Does duct length change the size I need?

Not the size from this tool. Friction loss is shown per 100 feet , so the answer is the same for a 20 foot run or a 60 foot run. Length matters when you add up total system pressure. Longer runs with more elbows should use a lower friction rate, like 0.06 to 0.08.

Duct Calculator

Updated Aug 27, 2026 By Jehan Wadia

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 does the math for you using 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. If you do not know your airflow yet, start with the CFM Calculator to find the air volume each room needs.

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. Pair it with the AC Tonnage Calculator and the Heat Loss Calculator to size the equipment feeding these ducts.

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. For one-off swaps outside this tool, the Unit Converter Calculator and the Inches to Mm Calculator come in handy.

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. A general Flow Rate Calculator can help if you measured volume over time instead.

Maximum Air Velocity: Set the fastest air speed you allow in the duct. Lower speeds mean quieter ducts. Higher speeds mean smaller ducts. See the Velocity Calculator if you want to work the speed out from area and flow by hand.

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. The Friction Loss Calculator covers the same idea for piping runs.

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. The Ratio Calculator is useful if you want to test other width-to-height pairs.

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. The Circle Area Calculator and the Area of a Rectangle Calculator let you check the cross-section areas yourself.

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. The Aspect Ratio Calculator handles the quick width-to-height math.

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. If you are ordering sheet metal, the Sheet Metal Weight Calculator helps you price the material.

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. Measure total run length with the Linear Feet Calculator.
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. The Insulation Calculator shows how much wrap you need.
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. If you are sizing a ductless system instead, the Mini Split Sizing Calculator is the better starting point, and the Electricity Cost Calculator shows what running the system will cost each month.

Formulas used

Friction loss per 100 ft (Darcy-Weisbach approximation)

\Delta p = \frac{0.109136 \cdot K \cdot Q^{1.9}}{D^{5.02}}

Huebscher equivalent round diameter for rectangular duct

D_e = \frac{1.30\,(a b)^{0.625}}{(a+b)^{0.25}}

Diameter required by velocity limit

D_V = \sqrt{\frac{576\,Q}{\pi V_{design}}}

Diameter required by friction limit

D_{\Delta p} = \left(\frac{0.109136 \cdot K \cdot Q^{1.9}}{\Delta p_{max}}\right)^{1/5.02}

Governing required diameter (with 10% flex bump)

D_{req} = \max\left(D_V,\; D_{\Delta p}\right),\quad D_{adj} = D_{req} \times 1.10 \;(\text{flex})

Actual velocity in round duct

V = \frac{576\,Q}{\pi D^2}

Actual velocity in rectangular duct

V = \frac{144\,Q}{a\,b}

Ideal rectangular height from aspect ratio R

b_{ideal} = \frac{D_e\,(R+1)^{0.25}}{1.30\,R^{0.625}}

Frequently asked questions

What size duct do I need for 800 CFM?

With the default settings (metal, supply air, 800 FPM max, 0.08 inWg/100 ft), 800 CFM needs a 14 inch round duct. In rectangular, that is about 16 x 10 inches. If you use flex duct instead, you need a 16 inch round. Change any input and the tool re-figures the size right away.

How much air can each round duct size carry?

Here are rough numbers for smooth metal round duct at 0.08 inWg/100 ft:

Duct Size	About CFM
6 in	100
8 in	200
10 in	370
12 in	600
14 in	900
16 in	1,250

Flex duct carries less air at the same size. Use the calculator for your own numbers.

Why does the calculator round up to the next size?

The math almost never lands on a size you can buy. A 13.5 inch answer is not sold in stores. Rounding up to 14 inches keeps the air speed and friction under your limits. Rounding down would push both over the limit and make the duct loud and weak.

What does inWg per 100 ft mean?

inWg means inches of water gauge. It is a way to measure small amounts of air pressure. "Per 100 ft" means the pressure lost over 100 feet of straight duct. So 0.08 inWg/100 ft means the air loses 0.08 inches of pressure every 100 feet.

What friction rate should I use?

Most homes use 0.08 to 0.10 inWg/100 ft. Use 0.08 for long duct runs or systems with many turns. Use 0.10 for short, simple runs. Commercial jobs often go higher. If you are not sure, leave it at 0.08.

Does this calculator include elbows and fittings?

No. It sizes straight duct only. Elbows, tees, boots, and dampers add extra pressure loss. Each elbow can act like 10 to 25 extra feet of duct. Add that length to your run when you plan total pressure, or keep the friction rate low to leave room for fittings.

Does duct length change the size I need?

Not the size from this tool. Friction loss is shown per 100 feet, so the answer is the same for a 20 foot run or a 60 foot run. Length matters when you add up total system pressure. Longer runs with more elbows should use a lower friction rate, like 0.06 to 0.08.

Is this the same as a Manual D duct design?

No. Manual D is a full room-by-room design that covers every branch, fitting, and register in the house. This tool sizes one duct run at a time using the same velocity and friction rules. It is great for checking a size, fixing a weak run, or planning. For permits or a new build, get a full Manual D.

What happens if my duct is too small?

Air moves too fast. You get:

Loud rushing or whistling at vents
Less airflow to far rooms
Higher static pressure on the blower
Higher power bills and shorter equipment life

In bad cases the furnace can overheat or the AC coil can freeze.

What happens if my duct is too big?

Air moves too slowly. It costs more in material and space, and the air may not throw far enough into the room from the register. Very slow supply air can also feel drafty and cold. One size up is usually fine. Two or three sizes up is waste.

Why did my duct size get bigger when I picked Return?

Return ducts use a slower target air speed than supply ducts. Slower air needs more room, so the duct grows. Returns are kept slow because grilles sit near people and fast air at a return grille is very loud.

What does the velocity gauge show?

It shows where your actual air speed lands. The blue zone is below target, the green zone is the good range for your job type, and the red zone is too fast. The black marker is your duct. Aim for green. Blue is usually safe but bigger than needed. Red means step up a size.

My result says Below Target. Is that bad?

Usually no. It happens when rounding up to the next real duct size drops the speed under the range. The duct still works and will be quiet. Just check that you typed the right airflow. If the speed is very low, you may be paying for more duct than you need.

What is equivalent diameter?

It is the size of a round duct that flows the same as your rectangular duct. A 16 x 10 rectangle acts like about a 13.7 inch round duct, not a 13 inch one. The tool uses it so friction loss for both shapes can be compared fairly.

Why does the rectangular duct have more area than the round one?

Corners do not move much air. A rectangle has more wall surface for the same area, so it rubs the air more. To flow the same amount, a rectangle needs a bigger cross-section than a circle. The flatter it gets, the more extra area you need.

What does the sensitivity table tell me?

It shows what happens two sizes below and two sizes above your answer. You can see the speed and friction change with each step. Use it to decide if going one size up is worth it for quieter air, or if one size down still passes your limits.

Should I use this for the main trunk or a branch?

Both, one at a time. Enter the total airflow the trunk carries to size the trunk. Then enter the airflow of a single branch to size that branch. Each duct run is sized for the air it actually carries, not for the whole system.

Can I use this for a bathroom fan or dryer vent?

You can use it for a bathroom fan by picking Exhaust and entering the fan CFM. Do not use it for dryer vents. Dryers need a 4 inch rigid metal duct by code, no matter what the math says.

How do I find my CFM if I do not know it?

A quick rule is 400 CFM per ton of cooling. A 3 ton system moves about 1,200 CFM total. For one room, take the room's share of the load. For real numbers, do a room-by-room load calculation first, then size the ducts here.

What if I cannot buy the size the tool gives me?

Go to the next size up that you can buy. Never go down. If space will not allow it, switch to a rectangular shape with the same equivalent diameter, or split the airflow into two smaller ducts running side by side.

Can I enter metric units?

Yes. Click Metric at the top and every box switches to m³/h, m/s, and Pa/100 m. Your typed numbers convert on their own. You can also set the result units on their own, so you can type in CFM and read the answer in millimetres.

Why does flex duct need a larger size than metal?

The ribbed inner liner drags on the air. The tool adds a 1.5x roughness factor and a 10% larger diameter to make up for it. For 800 CFM that means 16 inch flex instead of 14 inch metal. Flex must also be pulled tight, or it flows even less.