01
Loan Calculator
Works out the monthly payment, total interest and full repayment schedule for any loan from…
Duct Size Calculator
Sizes round and rectangular HVAC duct from airflow and target velocity, giving the friction rate and equal-friction rectangular equivalents.
Calculators
Nothing uploaded
No sign-up
Free, unlimited
Leave at 0 to use the figure for the application above.
Optional — gives the total friction loss over the run.
Enter your figures and press Calculate.
In short
Duct Size Calculator is a free online tool on inputtesthub.com that sizes round and rectangular HVAC duct from airflow and target velocity, giving the friction rate and equal-friction rectangular equivalents. It needs no account, no download and no payment, and there is no daily usage limit. Everything is processed inside your own browser, so the data you paste is never uploaded to a server.
How to use the Duct Size Calculator
- Enter the airflow in CFM — the Room CFM Calculator works one out if you do not have it.
- Pick the application, or set your own target velocity.
- Read the round size, then pick a rectangular equivalent if the duct has to fit a joist bay.
About the Duct Size Calculator
The Duct Size Calculator sizes round and rectangular HVAC duct from airflow and target velocity, giving the friction rate and equal-friction rectangular equivalents. It runs as plain JavaScript inside the page, which means there is no upload step, no queue and no size limit imposed by a server — the only limit is your own device.
What people use it for
- Sizing a branch to a bedroom without reaching for a duct wheel.
- Converting a round size into a rectangular duct that fits a joist bay.
- Checking whether an existing duct is undersized for the airflow it carries.
Details
| Sizing method | Equal friction — the method duct calculators and ASHRAE tables use |
|---|---|
| Residential branch | 600–900 fpm; noise rises steeply above about 900 |
| Return air | Sized slower than supply, typically 600 fpm |
| Rectangular equivalence | Matched on friction, not on area — the two differ by around 15% |
| Aspect ratio | Keep under 4:1; past that friction and fitting cost climb fast |
The maths behind it
This tool uses duct area = airflow ÷ velocity, and friction per 100 ft = 0.109136 × CFM^1.9 ÷ diameter^5.02 for galvanised duct at standard air density. Rectangular equivalents use the Huebscher relation De = 1.30 × (ab)^0.625 ÷ (a+b)^0.25. Every intermediate value is shown on screen, so the result can be checked by hand.
Velocity and friction are two views of the same decision, and the tool shows both. Velocity is what you hear; friction is what the fan has to overcome. A duct that satisfies the velocity target but has a high friction rate is usually a long run that needs sizing up, and the total loss over the run makes that visible if you enter a length.
Where this tool stops
Straight galvanised duct at standard air density and sea level. Fittings, elbows, takeoffs and dampers add equivalent length that this does not model, and at altitude or high temperature the air is thinner and the figures shift.
Frequently asked questions
Why is 8 × 12 in equivalent to 10.7 in round and not 12.4 in?
Because equivalence is about friction, not area. A rectangular duct with the same cross-section as a round one has more wall for the same volume of air, and that extra wetted perimeter drags. Sizing rectangular duct by matching area under-sizes it every time — the standard relation, De = 1.30 × (ab)^0.625 ÷ (a+b)^0.25, is what this tool uses.
What friction rate should I design to?
0.08 to 0.10 inches of water per 100 feet is the usual residential target, and this tool reports what your chosen size actually gives. Designing to a friction rate rather than a velocity is the more rigorous approach; velocity is easier to reason about and lands in the same place for a single branch.
Does the duct material matter?
Yes. The friction equation here is for galvanised sheet metal. Flexible duct is far rougher — a fair working figure is that flex needs to be one size larger for the same airflow, and much more than that if it is left compressed or snaking.
Is the Duct Size Calculator free?
It is free, with no account, no trial and no daily cap. Every calculators tool on inputtesthub.com works the same way: open the page and use it.
Is what I paste into the Duct Size Calculator stored?
No. The Duct Size Calculator runs entirely in your browser using JavaScript. Your input never reaches inputtesthub.com or any third party — open your browser's Network tab while you use it and you will see no request go out.
What is the equation behind the Duct Size Calculator?
It uses duct area = airflow ÷ velocity, and friction per 100 ft = 0.109136 × CFM^1.9 ÷ diameter^5.02 for galvanised duct at standard air density. Rectangular equivalents use the Huebscher relation De = 1.30 × (ab)^0.625 ÷ (a+b)^0.25. The full working is printed under the result, so you can follow every step rather than trusting a black box.
Who is the Duct Size Calculator for?
The most common reasons people reach for it: sizing a branch to a bedroom without reaching for a duct wheel. Converting a round size into a rectangular duct that fits a joist bay. Checking whether an existing duct is undersized for the airflow it carries. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
Is there anything the Duct Size Calculator cannot do?
Straight galvanised duct at standard air density and sea level. Fittings, elbows, takeoffs and dampers add equivalent length that this does not model, and at altitude or high temperature the air is thinner and the figures shift. Knowing where a tool stops being trustworthy matters more than the cases where it works.
What figures does the Duct Size Calculator work from?
Sizing method is Equal friction — the method duct calculators and ASHRAE tables use. Residential branch is 600–900 fpm; noise rises steeply above about 900. Return air is Sized slower than supply, typically 600 fpm. These are printed on the page itself, so you do not have to take them on trust.
How precise is the result?
The calculation runs in double precision and the result is shown to ten significant figures, so converting a value and converting it back returns the original. What is displayed is rounded for readability; what is copied is not.
Related calculators tools
All 38202
EMI Calculator
Calculates your Equated Monthly Instalment with a full principal-versus-interest breakdown and…
03
Mortgage Calculator
Estimates a full monthly housing payment including principal, interest, property tax…
04
Car Loan Calculator
Calculates a car finance payment from the price, deposit, trade-in, interest rate and term…
05
Personal Loan Calculator
Works out instalments and total cost for an unsecured personal loan, including any processing…
06
Amortization Schedule Calculator
Produces a complete month-by-month table showing how each payment splits between interest and…
07
Compound Interest Calculator
Shows how savings grow with compound interest, including regular contributions and any…
08
Simple Interest Calculator
Calculates simple interest and the final amount from principal, rate and time, without…
09
SIP Calculator
Projects the maturity value of a monthly Systematic Investment Plan, showing invested amount…
10
Lumpsum Investment Calculator
Calculates what a one-off investment grows to at a given annual return over any number of years.