Size spiral, rectangular and flexible ductwork in seconds. Enter your airflow, choose a sizing method and the calculator returns a standard duct size, air velocity, friction rate, total pressure drop and heat loss along the run. Built for UK installers and engineers, aligned with CIBSE Guide C and ASHRAE Fundamentals.

How to size a duct

Duct sizing balances three things: air velocity, pressure loss and physical space. Run the duct too small and velocity climbs, bringing noise, regenerated sound at fittings and higher fan energy. Run it too large and you pay for metal, insulation and ceiling void you did not need.

There are two standard approaches, and this calculator supports both:

Target velocity method. You choose an air velocity appropriate to the application and the calculator finds the duct area needed to carry your airflow at that speed. Typical UK guidance puts main residential ducts at 3 to 5 m/s, commercial mains at 5 to 8 m/s and industrial or high velocity systems at 8 to 12 m/s, with branches sized slower in each case. The calculator flags your result against these bands automatically.

Equal friction method. You choose a friction rate, typically 0.7 to 1.0 Pa per metre for comfort systems, and the calculator solves for the duct size that produces it. This is the classic method for laying out full distribution systems because every section loses pressure at a similar rate, which keeps the index run predictable.

If you already have a duct size in mind, a third mode lets you fix the dimensions and check what velocity, friction and capacity it gives you.

What the calculator works out

For any spiral, rectangular or flexible duct run the tool calculates:

Duct size. Rounded up to real stock sizes: standard UK and EU spiral diameters from 63 mm to 1250 mm, and rectangular duct on a 50 mm grid to BS EN 1505 conventions.

Air velocity and friction rate. Pressure loss is calculated properly with the Darcy Weisbach equation and the Colebrook White friction factor rather than a simplified chart lookup, so the answer responds correctly to duct roughness, air temperature and Reynolds number.

Total run pressure drop. Straight friction plus bend and fitting losses for your count of 90 and 45 degree bends, plus any additional allowance you enter. This is the figure you compare against available fan pressure.

Heat loss or gain and air temperature change. Enter the duct air temperature and the ambient around it, choose an insulation build up, and the calculator estimates watts lost per metre and the air temperature arriving at the end of the run. Useful for cold supply duct in warm voids, or heated air travelling through unheated spaces, with insulation thickness selection informed by BS 5422.

Flexible duct penalties. Flexible duct is dramatically worse than rigid duct and the calculator does not hide it. Fully extended flexi has a wall roughness around 3 mm against 0.09 mm for galvanised steel, and compression makes it worse still: lightly compressed flexi loses roughly twice the pressure of extended flexi, and heavily compressed runs can lose four times as much. If your design only works with the flexi drawn perfectly straight, you will see that warning here rather than on site.

Methods and standards

Calculations align with recognised HVAC references: Darcy Weisbach with Colebrook White friction (CIBSE Guide C; ASHRAE Handbook, Fundamentals), circular equivalent diameter for rectangular duct (ASHRAE; cf. BS EN 1505), standard sizes to BS EN 1506 and BS EN 1505 with fabrication to BESA DW/144, velocity guidance drawn from CIBSE Guide B, and steady state heat loss consistent with BS EN ISO 12241.

Results are for guidance and preliminary sizing only. They are estimates based on standard assumptions and do not constitute an engineering design. A full HVAC design must be carried out and verified by a competent engineer before procurement or installation.

Who this tool is for

Installers sizing a run on site or checking whether an existing duct can carry a new airflow. Consulting engineers doing quick feasibility checks before committing a scheme to full calculation. Architects and designers confirming whether a duct will actually fit the void or bulkhead they have allowed.

Frequently asked questions

What size duct do I need for 100 L/s? At a typical residential main duct velocity of 4 m/s, 100 L/s needs about 180 mm diameter, so a standard 200 mm spiral duct is the usual choice. At commercial velocities of 6 m/s a 160 mm duct would carry it. Enter your exact airflow and application in the calculator above for a sized answer with pressure drop included.

What is a good air velocity for ductwork? For residential systems, keep main ducts between 3 and 5 m/s and branches between 2 and 3.5 m/s. Commercial systems typically run mains at 5 to 8 m/s. Higher velocities save space but increase noise and fan energy, which is why bedrooms and living spaces are sized towards the lower end.

What is the equal friction method? The equal friction method sizes every duct section to lose pressure at the same rate, commonly 0.7 to 1.0 Pa per metre. It produces balanced systems with predictable total pressure loss, making fan selection straightforward. It is the most widely used sizing method for comfort ventilation.

How much worse is flexible duct than rigid duct? Significantly. Fully extended flexible duct has around 30 times the surface roughness of galvanised spiral duct, and compressing it makes things far worse: a moderately compressed flexi run can lose four times the pressure of the same run fully extended. Use flexi only for short final connections and keep it pulled taut.

How is rectangular duct sized compared with round duct? Rectangular duct is converted to a circular equivalent diameter, the round duct size with the same pressure loss at the same airflow, using the ASHRAE equivalent diameter formula. Keep the aspect ratio below 4 to 1 where possible: flatter ducts cost more, lose more pressure and are harder to fabricate.

Does duct insulation affect the airflow calculation? External wrap does not change the airflow but does change the overall size you must fit in the void, which the calculator reports as the outside dimension. Internal acoustic lining is different: it reduces the clear bore and roughens the airway, so lined rectangular duct needs to be ordered larger. The calculator handles both cases.

Related tools

Need the duct fabricated? Every result includes a one-click quote request to our fabrication team. You may also find these useful:

• [Part F Ventilation Calculator]: work out the airflow you need before you size the duct
• [Cooling and Heating Load Estimator]: size the conditioning load the ductwork serves