What is differential pressure?

Differential pressure (often written dP or ΔP) is simply the difference between two pressure readings, calculated as P1 minus P2. It is used throughout engineering and building services to measure things such as the pressure drop across a filter, the flow through an orifice plate, the level in a sealed tank, or the pressure loss along a length of pipe or duct. A single absolute pressure reading tells you very little on its own; the difference between two points tells you about flow, restriction, or level.

How do you convert differential pressure between units?

Differential pressure uses the same units as absolute or gauge pressure: pascals (Pa), kilopascals (kPa), bar, pounds per square inch (psi), or millimetres of water column (mmH2O). The common conversions are 1 kPa = 1,000 Pa, 1 bar = 100 kPa = 14.5038 psi, 1 psi = 6.89476 kPa, and 1 kPa is approximately equal to 101.97 mmH2O. This calculator converts your ΔP result into all of these units automatically.

How does differential pressure relate to flow rate?

For flow through a fixed restriction such as an orifice plate, venturi, or filter, flow rate is approximately proportional to the square root of the differential pressure across it (Q2/Q1 = sqrt(dP2/dP1)), assuming the fluid, restriction geometry, and discharge coefficient stay the same. This means doubling the differential pressure only increases flow by about 41%, not 100%. This calculator uses that relationship to estimate the new flow rate if you know a reference flow and pressure drop and want to find the flow at a different dP.

Differential Pressure Calculator NZ

This calculator works out the differential pressure (ΔP) between two pressure readings, a measurement used across building services and engineering to judge flow, restriction, or blockage without needing an absolute pressure reading. Select your working unit (kPa, Pa, bar, psi, or mmH2O), then enter the higher upstream pressure (P1) and the lower downstream pressure (P2); the calculator subtracts P2 from P1 and returns ΔP in your chosen unit, with instant conversions into kPa and psi, plus a full breakdown into pascals, kilopascals, bar, psi, and mmH2O. If you also know a reference flow rate at a known differential pressure, from a pump curve or filter datasheet, enter both figures in the optional second section and the tool estimates the flow rate at your calculated ΔP using the square root law for flow through a fixed restriction such as an orifice, venturi, or filter. The detail panel breaks down your reference flow, reference ΔP, the ratio between the two, and the resulting estimated flow, while a plain-language summary flags if P2 is higher than P1, which usually means the pressure points or flow direction have been mixed up. This is a useful first check when sizing ductwork, monitoring filter loading, or troubleshooting a pressure drop, but the flow estimate is indicative only and does not replace a proper orifice or venturi sizing calculation using the correct discharge coefficient for your installation.

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Standard engineering formula ΔP = P1 - P2. Flow ratio uses the square root law for flow through a fixed restriction.

1. Pressure Readings

Units for P1 and P2

Higher pressure (P1) upstream / inlet

Lower pressure (P2) downstream / outlet

2. Optional: Flow Estimate

If you know a reference flow rate at a known differential pressure (for example, from a pump curve or filter datasheet), enter it here to estimate the new flow rate at your calculated ΔP.

Reference flow rate any unit, e.g. L/min

Reference differential pressure same units as above

Differential Pressure Result

ΔP (P1 - P2)
40.00
kPa (your input unit)

ΔP in kPa

40.000

Kilopascals

ΔP in psi

5.802

Pounds per sq inch

Estimated Flow

40.00

At this ΔP (square root law)

Full Unit Conversion

Pascals (Pa)40,000.0 Pa

Kilopascals (kPa)40.000 kPa

Bar0.40000 bar

psi5.802 psi

mmH2O4,078.9 mmH2O

Flow Estimate Detail

Reference flow100.00

Reference ΔP250.00 kPa

Your ΔP40.00 kPa

ΔP ratio (yours / reference)0.1600

Estimated flow at your ΔP40.00 (same units as reference flow)

Summary: Enter your two pressure readings above.

What Differential Pressure Measures

Differential pressure, written as ΔP or dP, is the difference between two pressure readings taken at two different points in a system. It is one of the most widely used measurements in engineering and building services because it tells you about flow, restriction, or fluid level without needing to know the absolute pressure at either point. Common applications include measuring the pressure drop across an air filter or heat exchanger, sizing an orifice plate to measure flow rate, checking blockage in a pipeline, and monitoring cleanroom or hospital ward pressurisation.

The Formula

The calculation itself is simple subtraction: ΔP = P1 minus P2, where P1 is the higher (usually upstream) pressure and P2 is the lower (usually downstream) pressure. The result carries the same pressure units as your inputs. What makes differential pressure useful in practice is converting that number into consistent units and, where relevant, relating it to a flow rate.

Pressure Unit Conversions

From	To	Multiply by
kPa	Pa	1,000
kPa	bar	0.01
kPa	psi	0.145038
kPa	mmH2O	101.972
bar	psi	14.5038
psi	kPa	6.89476

Differential Pressure and Flow Rate

For flow through a fixed restriction such as an orifice plate, venturi meter, or filter element, the relationship between flow rate and differential pressure follows a square root law: flow is proportional to the square root of ΔP, provided the fluid, temperature, and restriction geometry stay the same. This means that if you double the differential pressure across a fixed orifice, the flow rate only increases by a factor of about 1.41 (the square root of 2), not by a factor of 2. This calculator applies that relationship: enter a known flow rate at a known reference ΔP, and it estimates the new flow rate at the ΔP you have calculated.

This square root approximation is accurate for turbulent flow through a fixed restriction. It does not apply to positive displacement flow (such as most water meters or gear pumps), where flow rate is largely independent of differential pressure across the normal operating range. Always check the datasheet or standard (such as ISO 5167 for orifice plates) for the exact discharge coefficient and geometry factors in a real design calculation.

Related Calculators

Sources: Standard fluid mechanics reference formulae for pressure difference and orifice flow (square root law), consistent with ISO 5167 principles for differential pressure flow measurement. Unit conversion factors from NIST Special Publication 811.

This calculator provides indicative estimates only. The flow estimate assumes a fixed restriction geometry and turbulent flow; it is not a substitute for a proper orifice or venturi sizing calculation using the discharge coefficient for your specific installation. Always refer to equipment datasheets or the relevant standard for critical design work.