Choosing a fan on airflow alone is not a selection — it is a guess. A fan has to deliver the required airflow while overcoming the resistance of the entire air system: ducts, bends, filters, dampers, coils, silencers, grilles and every other component in the path. That resistance is what the pressure figure on a fan curve has to answer.
This guide walks through calculating the pressure a fan actually has to produce, where static, velocity and total pressure differ, and how to turn the result into a duty point you can take into fan selection.
What static pressure actually is
Static pressure is the pressure in the airflow that acts in all directions and is not associated with the air moving forward. In HVAC work it is given in pascals (Pa) or inches of water gauge (in. wg), and it is the figure usually quoted for duct resistance and fan performance.
There is an important distinction hiding behind that one word, though. System static pressure, fan static pressure and fan total pressure are three different things, and treating them as interchangeable is one of the most common ways a selection goes wrong.
Static, velocity and total pressure
Air moving through a duct carries both pressure and kinetic energy. The kinetic part is expressed as velocity pressure, and the three are related simply:
Pt = Ps + Pv
where Pt is total pressure, Ps is static pressure and Pv is velocity pressure. Velocity pressure itself comes from the air density and the speed:
Pv = ρ × v² / 2
Take air at 1.20 kg/m³ moving at 8 m/s:
Pv = 1.20 × 8² / 2 = 38.4 Pa
So roughly 38 Pa of that air's total pressure is tied up in its velocity. In a low-pressure system that is not a rounding error — it is a meaningful share of the duty.
What fan static pressure means on a curve
For fan performance the term has a specific definition:
Fan static pressure = fan total pressure − fan velocity pressure
The velocity pressure here relates to the average velocity at the fan outlet. This matters because subtracting the static pressure measured before the fan from the static pressure measured after it is not necessarily the same number as the standardised fan static pressure printed on a performance curve.
In practice, system design starts the other way round: calculate the pressure losses through the air system, then use them to find the operating point the fan has to reach.
Calculating the required fan pressure
The procedure is short to state:
Required airflow + total system resistance = fan operating point
The airflow comes from the ventilation requirement. The pressure comes from the resistance of the most demanding air path through the system. What follows is a worked example of that second half.
Step 1 — the required airflow
Start with how much air the system has to move. Say:
Q = 10 000 m³/h = 2.78 m³/s
Airflow sets duct velocity, and velocity drives almost every loss that follows.
Step 2 — duct size and velocity
With a main duct of internal area A = 0.50 m²:
v = Q / A = 2.78 / 0.50 = 5.56 m/s
Most resistances rise roughly with the square of velocity, so this one choice has more influence on the final pressure than anything else on the list.
Step 3 — straight duct friction
Straight duct creates friction along its whole length. The engineering form of the Darcy-Weisbach equation is:
Δp = λ × (L / Dh) × (ρv² / 2)
where λ is the friction factor, L the length and Dh the hydraulic diameter. In day-to-day design the figure comes from a friction chart or a duct calculator rather than from the equation by hand. For 30 m of duct at a calculated 0.8 Pa/m:
Δpduct = 30 × 0.8 = 24 Pa
Work out your own duct friction → — enter airflow, duct size, length and material and the calculator returns velocity, Reynolds number, friction rate and total loss using Darcy-Weisbach with Colebrook-White.
Step 4 — fittings
Elbows, transitions, tees, branches and reducers each add resistance:
Δp = K × ρv² / 2
K is the loss coefficient of the fitting. Rather than compute every one by hand, equivalent-length or tabulated fitting data are normally used. Assume the bends and transitions in this system come to:
Δpfittings = 75 Pa → running total 99 Pa
Step 5 — the filter
Filters are often the single largest resistance in the system. A clean-filter drop of:
Δpfilter = 120 Pa → running total 219 Pa
But a filter gets more restrictive as it loads. Selecting on the clean figure means the fan falls short for most of the filter's life, so use the manufacturer's design or final pressure drop instead.
Step 6 — coils and heat exchangers
Heating coils, cooling coils and heat recovery all add air-side resistance:
Δpcoil = 80 Pa → running total 299 Pa
Step 7 — the silencer
If there is an acoustic silencer, take its resistance at the design airflow:
Δpsilencer = 45 Pa → running total 344 Pa
Step 8 — dampers, grilles and terminals
Balancing dampers, control and fire dampers, diffusers, grilles and louvers finish the path:
Δpterminals = 95 Pa → total 439 Pa
The complete example
| Component | Pressure loss |
|---|---|
| Straight duct | 24 Pa |
| Elbows and fittings | 75 Pa |
| Filter | 120 Pa |
| Heating / cooling coil | 80 Pa |
| Silencer | 45 Pa |
| Dampers and terminals | 95 Pa |
| Total | 439 Pa |
The design point is therefore 10 000 m³/h at 439 Pa, and that is what gets plotted on a fan curve. The fan you pick should sit close to it while still holding acceptable efficiency, power, sound and operating margin.
Follow the critical path, not every branch
This is the mistake that produces wildly oversized fans. In a branched system the fan does not have to overcome the sum of the losses in every branch. It has to overcome the critical path — the route from the fan to the terminal with the highest total resistance. Calculate that path. The other branches are then balanced to take their share of the airflow at the pressure the fan already provides.
Should you add a safety margin?
Adding a blanket 20% or 30% is not good practice. An oversized fan brings higher energy consumption, more noise, unnecessary throttling, a bigger motor and a bigger invoice.
What the calculation should carry is the uncertainty that genuinely exists: filter loading, duct routing that is not final, components not yet specified, and possible system-effect losses near the fan inlet or outlet. Name the uncertainty and price it. Do not sprinkle a percentage over everything.
Static or total pressure for the selection?
Here the definitions start to matter commercially. For a fan with a duct on its outlet, fan total pressure is normally the right parameter for matching fan to system. For an unducted outlet — plenum fans, free discharge — fan static pressure is usually the one, because the outlet velocity pressure cannot be recovered by downstream ductwork.
So check how the manufacturer rated the curve before comparing anything. A total-pressure requirement read against a static-pressure curve is not a comparison; it is two different numbers sharing a unit.
Where the fan actually ends up
A fan does not choose its airflow. The operating point is the intersection of the fan curve and the system resistance curve, and the system curve rises roughly with the square of airflow:
Δp₂ = Δp₁ × (Q₂ / Q₁)²
Take a system needing 400 Pa at 10 000 m³/h and ask for 20% more air:
Δp₂ = 400 × (12 000 / 10 000)² = 400 × 1.44 ≈ 576 Pa
Twenty per cent more airflow, forty-four per cent more pressure. This is why a small change in the airflow assumption moves the fan duty — and the motor — much further than people expect.
What happens when you change fan speed
For the same fan running at a different speed, the affinity laws give a good approximation:
- Airflow scales with speed: Q₂ / Q₁ = n₂ / n₁
- Pressure scales with the square: p₂ / p₁ = (n₂ / n₁)²
- Power scales with the cube: P₂ / P₁ = (n₂ / n₁)³
The cube is why dropping speed with a VFD or an EC motor saves so much energy — and why reaching for extra pressure by speeding a fan up gets expensive very quickly.
The mistakes that come up most often
- Selecting on airflow alone and ignoring system resistance
- Adding the losses of every branch instead of following the critical path
- Leaving out filters, coils, silencers or terminal devices
- Using clean-filter resistance for a system that will run with a loaded one
- Mixing a static-pressure curve with a total-pressure requirement
- Ignoring outlet velocity pressure
- Forgetting system-effect losses right at the fan
- Taking nominal catalog pressure instead of the actual operating point
- Adding an arbitrary oversized margin
- Ignoring air density at unusual temperature or altitude
No fan compensates efficiently for a system that was calculated wrongly.
Air density is part of the answer
Published fan data are referenced to a stated air density. Density changes with temperature, altitude, barometric pressure and — to a smaller degree — humidity. A fan handling hot air or working at altitude will not follow its standard-air pressure and power relationship, so correct the performance for the real conditions before you commit.
From pressure to a chosen fan
Calculating the pressure is only the first half. Two fans can both deliver 10 000 m³/h at 440 Pa and behave completely differently in service. Before the final choice, compare efficiency, absorbed power, motor loading, FEI, SFP, sound power, available speed control, dimensions and where the duty point sits in the operating range.
The best fan is not the first one whose curve crosses the required point. It is the one that crosses it in a sensible part of its range.
In short
- Determine the required airflow.
- Define the duct system and the resulting velocity.
- Calculate straight-duct friction.
- Add fitting losses.
- Add filters, coils, silencers, dampers and terminals.
- Follow the most resistant path, not the sum of all paths.
- Account for system effects and real operating conditions.
- Decide whether the fan is selected on static or total pressure.
- Plot the duty point on the fan curve.
- Check efficiency, power, acoustics and operating range before choosing.
The principle is simple enough: the required fan pressure is the resistance the fan has to overcome at the required airflow. Getting it right depends on knowing which pressure definition you are holding, and how the whole air system behaves around it.
