Differential Pressure Calculator

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

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.

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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.