By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed July 16, 2026
Static pressure is the pressure a fluid exerts on the walls that contain it, and it exists whether the fluid moves or not. Dynamic pressure is the extra pressure carried by motion: q = ½ρv². Total pressure, also called stagnation pressure, is their sum: p0 = ps + ½ρv².
The physics takes one line. The confusion starts with instruments, because each of the three pressures is read by a different port. A wall-mounted gauge reads static pressure. A tube facing into the flow reads total pressure. The difference between those two ports is the dynamic pressure, and that difference is what every pitot-type flow meter lives on.
This guide gives the three definitions with a comparison table, the Bernoulli identity that connects them, a port-by-port map of the pitot-static tube, and a worked duct traverse you can repeat with your own numbers.
Contents
- The three pressures
- Static pressure
- Dynamic pressure
- Total pressure and Bernoulli
- Pitot-static tube ports
- Duct traverse worked example
- Static pressure in HVAC
- Static pressure on DP datasheets
- Instrument selection
- FAQ
The three pressures
| Quantity | Symbol | What it is | What reads it |
|---|---|---|---|
| Static pressure | ps | Pressure on a surface that does not face the flow; present at rest and in motion | Wall tap; gauge or absolute transmitter; the side holes of a pitot-static probe |
| Dynamic pressure | q | Kinetic energy of the moving fluid expressed as a pressure, ½ρv² | No port reads it directly; it is the DP between the impact and static ports |
| Total pressure | p0 (or pt) | Static plus dynamic; the pressure where the flow is brought to rest | Forward-facing impact port of a pitot tube |
Know any two of the three and the identity p0 = ps + q gives you the third.
Static pressure
Static pressure is what a fluid presses against a surface that is not facing the flow: the pipe wall, the duct wall, the diaphragm of a transmitter on a wall tap. The name misleads people. It does not require the fluid to be static; a pipe running at 3 m/s still has a static pressure, and that is the number a wall-mounted gauge reports.
For a column of fluid at rest, static pressure follows the hydrostatic law p = ρgh, which is why a 10 m water column reads about 98 kPa at the bottom. In a pressurized system it is whatever the pump, compressor, or regulator maintains. Either way it acts equally in all directions at a point, so the orientation of the tap does not change the reading as long as the opening sits flush with the wall, with its axis perpendicular to the flow.
Static pressure carries a reference point like any other pressure reading: it can be stated as gauge or absolute, which is a separate choice covered in our absolute vs gauge pressure guide. The atmosphere itself is a static pressure, read against a vacuum, which is the subject of our barometric pressure guide. Units follow the industry you are in: kPa and bar in process plants, psi in North America, inches of water column in HVAC. The full conversion tables are in the pressure units guide.
Dynamic pressure
Dynamic pressure is the kinetic energy per unit volume of the moving fluid, written as a pressure:
q = ½ ρ v²
with ρ the fluid density in kg/m³ and v the velocity in m/s, giving q in Pa. It is zero in a still fluid and grows with the square of velocity, so doubling the flow velocity quadruples the dynamic head.
Two worked numbers show the scale, and why gas work and liquid work need different instruments:
- Air at 1.20 kg/m³ moving 10 m/s: q = 0.5 × 1.20 × 10² = 60 Pa. That is 0.06 percent of atmospheric pressure.
- Water at 998 kg/m³ moving 2 m/s: q = 0.5 × 998 × 2² = 1,996 Pa, close to 2 kPa.
The air number is tiny. Measuring it takes a DP cell spanned in tens of pascals, which is why duct instruments are so sensitive to installation effects. The water number sits comfortably inside a standard DP transmitter range. Same physics, thirty-three times apart.
The v² term also explains a familiar quirk: every meter that infers flow from a pressure difference, orifice, venturi, pitot, is a square-root device. Halve the flow and the signal falls to a quarter. The consequences for turndown and low-flow accuracy are covered in our flow rate and pressure guide.
Total pressure and Bernoulli
Total pressure is the pressure you get where the flow is brought to rest, which is why aerodynamicists call the same quantity stagnation pressure. At the nose of a probe facing the stream, the local velocity is zero and the kinetic term has been converted back into pressure. Bernoulli’s equation for steady, incompressible, frictionless flow along a streamline says the sum is constant:
ps + ½ρv² + ρgz = constant
In a horizontal run the elevation term ρgz drops out and the working identity is simply p0 = ps + q. Where the fluid speeds up, static pressure falls; where it slows, static pressure recovers. A venturi throat is the textbook case: velocity peaks, static dips, and the dip is exactly what the DP transmitter measures.
Two honest limits. Real pipes have friction, so total pressure actually falls in the flow direction; that loss is the head a pump or fan must put back. And above roughly Mach 0.3 the incompressible formula understates the stagnation pressure of a gas; compressible corrections exist, but for process and HVAC velocities you will not need them.
Pitot-static tube ports
A pitot-static tube is two instruments in one body. The opening at the tip faces upstream; flow stagnates against it, so the inner passage carries total pressure p0. A ring of small holes on the side of the body sits parallel to the flow, so the outer passage carries static pressure ps. Pipe both passages to a differential pressure transmitter and the cell reads q = p0 − ps directly. Velocity follows from the formula rearranged:
v = √(2q / ρ)
Alignment matters less than most people fear, but it is not free. A standard pitot-static tube holds its reading within about 1 percent as long as the probe stays within roughly 11 to 13 degrees of the flow direction; the S-type probes used in dirty stack gas drift 2 to 4 percent inside a ±10 degree window. Past that, errors climb quickly, so probes get alignment marks and mounting bosses for a reason.
A single-point pitot reads one spot in the velocity profile. Industrial lines mostly use the averaging version, a bar spanning the pipe with several impact ports across the chord, sold as our averaging pitot tube flow meter. The V-cone flow meter applies the same Bernoulli trade in a different shape: a cone accelerates the flow and the static pressure dip across it becomes the signal. Both need a differential pressure transmitter matched to the dynamic head, which is where the 60 Pa versus 2 kPa arithmetic above becomes a purchasing decision.
Duct traverse worked example
A round supply duct, 500 mm inside diameter, carries air at 20 °C. A pitot-static traverse across the standard measuring points averages a dynamic pressure of 45 Pa. Velocity and flow follow in four steps:
- Air density at 20 °C and sea-level pressure: ρ = 101,325 / (287.05 × 293.15) = 1.204 kg/m³.
- Velocity: v = √(2 × 45 / 1.204) = √74.75 = 8.65 m/s.
- Duct area: A = π × (0.25)² = 0.1963 m².
- Flow: Q = v × A = 1.698 m³/s = 6,111 m³/h, or about 3,597 CFM.
The density step is the one people skip. If the same duct carries 80 °C air, density drops to about 1.00 kg/m³, and computing with the cold-air value reads velocity roughly 9 percent low. Correct ρ for temperature before taking the square root, not after. For the reverse calculation, velocity from a known flow and pipe size, use our pipe velocity calculator. And place the traverse in a straight run; the upstream diameters that DP-type meters need are tabulated in our straight run requirements guide.
Static pressure in HVAC
HVAC borrows the same words and gives them a system-level meaning, which is where much of the search traffic on this topic comes from. Duct static pressure, stated in inches of water column, is the resistance the fan must overcome to push air through filters, coils, dampers, and duct runs. Residential air handlers are commonly rated around 0.5 in WC of external static pressure; commercial systems run higher, and a clogged filter shows up directly as rising static.
Fan curves keep the trio separate. Fan static pressure is the wall-referenced rise the fan delivers against system resistance. Velocity pressure is the dynamic head of the air leaving the outlet, ½ρv² at outlet velocity. Fan total pressure is the sum, and mixing the two up is a classic commissioning error: a fan selected on total pressure against a static-pressure requirement will come up short of airflow once installed.
Static pressure on DP datasheets
One terminology trap deserves its own section. On a differential pressure transmitter datasheet, “static pressure” does not mean the fluid-mechanics quantity above. It means the line pressure both impulse lines share: a DP cell spanned at 10 kPa across an orifice may be doing that job on a line running at 4 MPa, and the datasheet calls that 4 MPa the static or working pressure.
Two ratings follow from it. The static pressure limit is the line pressure the cell body tolerates. The static pressure effect is the small zero and span shift per unit of line pressure, quoted as percent of span per MPa, and it matters precisely when the differential is small and the line pressure is high. When you size a DP transmitter for pitot or orifice service, state both numbers: the differential span and the line pressure it rides on.
Instrument selection
The selection collapses to a short map:
| To measure | Use |
|---|---|
| Static pressure | A gauge pressure transmitter on a wall tap; absolute reference where the reading must ignore weather and altitude |
| Dynamic pressure | A DP transmitter across the impact and static ports of a pitot-static probe; span it to the dynamic head, tens of Pa for air, kPa for liquids |
| Total pressure | A forward-facing impact port alone; in practice it is rarely wanted by itself outside test work |
| Flow from dynamic head | An averaging pitot tube for low permanent pressure loss, or a V-cone where straight run is short |
The full range of transmitters, sensors, switches, and gauges lives under pressure instruments.
Application example
Fuel and gas lines, West Africa. An engineering contractor asked us for a flow measurement that would add almost no permanent pressure loss on existing gas and liquid lines, and wanted the documentation before the order: datasheets, port arrangement, DP transmitter pairing. An averaging pitot probe fit the constraint, because the bar stagnates only a small fraction of the stream and returns most of the head downstream. We put together the datasheet package, the port arrangement, and the matched DP transmitter span worked out from their line velocities.
FAQ
What is the difference between static pressure and dynamic pressure?
Static pressure is what the fluid exerts on surfaces that do not face the flow, and it exists even when the fluid stands still. Dynamic pressure is the kinetic energy of motion expressed as a pressure, q = ½ρv², and it is zero at rest. A wall gauge reads static; only a probe facing the flow sees the dynamic contribution.
What is stagnation pressure?
Stagnation pressure is another name for total pressure: the pressure at a point where the flow has been brought to rest, converting its kinetic energy back into pressure. For incompressible flow it equals static plus dynamic pressure, p0 = ps + ½ρv². The term is standard in aerodynamics; process engineers usually say total pressure.
Does a pitot tube measure stagnation pressure?
The forward-facing impact port does: flow stagnates against the tip, so that passage carries stagnation (total) pressure. A pitot-static tube adds side holes that carry static pressure, and the differential between the two ports is the dynamic pressure from which velocity is calculated.
Do pressure gauges measure static or dynamic pressure?
Static. A gauge or transmitter mounted on a wall tap sits normal to the flow, so the kinetic term never reaches the diaphragm, moving fluid or not. To capture dynamic pressure you need an impact port facing the stream and a DP measurement against a static port.
Request a quote
Send us the fluid, line size, operating velocity or flow range, and line pressure. We will work out the dynamic head, confirm whether a gauge transmitter, DP transmitter, or averaging pitot package fits, and quote the matching build. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.