Static, velocity, total: the three pressures on your fan curve

Two vendors quote a fan at 100 mmWG. One means static pressure; the other means total pressure. Those are not two prices for the same fan — they are two different fans. Three definitions settle it permanently, and one line on your RFQ keeps you from ever comparing them by accident.

The three pressures

• Static pressure (SP) — the pressure the air exerts in every direction. It is the part that overcomes filter, hood and duct resistance, and the part that bursts a duct. It does not depend on the air moving.
• Velocity pressure (VP) — the kinetic part, carried by the moving airstream: VP = ½ · ρ · v². It exists only while the air is in motion, and it climbs with the square of velocity.
• Total pressure (TP) — the sum: TP = SP + VP. A fan adds total-pressure energy to the air; the duct system then trades that energy between static and velocity every time the duct area changes.

Fan static vs fan total — the trap on the datasheet

Fan static pressure (FSP) excludes the velocity pressure at the fan outlet. Fan total pressure (FTP) includes it. Industrial practice rates fans in static pressure — but a quotation only means something when it says which one it states. At real outlet velocities that gap is nowhere near rounding error. Velocity pressure at standard air density (ρ ≈ 1.2 kg/m³) alone looks like this:

Outlet velocity	Velocity pressure (Pa)	≈ mmWG
10 m/s	60	~6
15 m/s	135	~14
20 m/s	240	~24
25 m/s	375	~38

On that 100 mmWG duty, quoting on FTP instead of FSP quietly moves the fan by 6 to 38 mmWG — set only by how fast the outlet runs. That is enough to land the wrong wheel, the wrong motor, or a fan that reads fine on paper and falls short of its duty on site.

Why draft direction makes this worse

The distinction bites hardest on draft fans. A forced draft (FD) fan pushes ambient air into a furnace or process against positive static; an induced draft (ID) fan pulls hot flue gas out under negative static. The ID fan handles gas that is hotter and far less dense, so both its static and its velocity-pressure numbers shift with temperature and altitude. State the pressure basis and the gas condition together, or the two never reconcile.

How the number is actually measured

A water-filled manometer across the right tappings reads SP, TP, or both — and where the tappings sit changes what the number means. The standard setups differ for a fan with a ducted inlet and outlet, a free inlet with a ducted outlet, and a ducted inlet with a free outlet. That is why a serious test certificate names its method. A credible certificate is run to the IS 4894 / ISO 5801 / AMCA 210 method, with the record stating the exact setup used — no fan carries an AMCA product stamp, so it is the method, not a badge, that you verify. None of this is new: fan builders were publishing static and dynamic water-gauge readings against tip speed as far back as 1916.

What to write on your RFQ

• State the required static pressure at the fan outlet — and say that it is static.
• State the gas, temperature and altitude; density changes every pressure number on the page.
• If your consultant's schedule gives total pressure, pass it on as total pressure — never relabel it.
• Ask every bidder to confirm the basis (FSP or FTP) inside the quotation itself.

Get those four lines right and every quote you receive is finally comparable on the same axis. It is the same discipline behind a clean duty point — see specifying the duty point — and it heads off one of the classic errors in buying an industrial fan. When a buyer sends us a total-pressure schedule, we convert it to fan static and show the conversion, so the basis is never left to guesswork.

Further reading: Specifying the duty point · Seven mistakes buying an industrial fan · Choosing the fan wheel.

Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.