Differential Pressure Calculator

What is differential pressure?

Differential pressure (delta P or dP) is the difference in static pressure between two points in a fluid system. It drives flow, indicates restriction, signals filter condition, and forms the basis of the most widely used industrial flow measurement principle. Any time fluid moves through a constriction, a valve, a filter, or along a pipe, one side is at a higher pressure than the other, and that gap is the differential pressure. In practice, dP is measured with a differential-pressure transmitter that senses both sides simultaneously and outputs the difference, often as a 4-20 mA signal proportional to the span.

Four ways this calculator finds delta P

Basic mode simply subtracts the low-side reading from the high-side reading and converts the result to your chosen pressure unit, supporting Pa, kPa, MPa, bar, mbar, psi, inH2O, mmHg, and atm. Orifice / venturi mode inverts the ISO 5167 flow equation to calculate volumetric flow rate from a known differential pressure across an orifice plate or venturi tube, using the discharge coefficient, pipe diameter, orifice bore, and fluid density. Darcy-Weisbach mode computes friction pressure drop along a straight pipe section from the friction factor, fluid density, velocity, and pipe geometry. Filter mode applies Darcy's Law for porous media: delta P equals the product of dynamic viscosity, flow rate, and medium thickness divided by the product of permeability and face area, making it the standard method for sizing HVAC filters, cleanroom panels, and industrial filter elements.

Orifice and venturi meters: ISO 5167 in detail

When a fluid accelerates through a constriction, static pressure drops proportionally to velocity squared (Bernoulli's principle). An orifice plate or venturi tube creates a controlled constriction so that measuring delta P gives flow rate. The ISO 5167 volumetric flow formula is Q = Cd times A2 times the square root of (2 times dP) divided by (rho times (1 minus beta to the fourth)). Cd is the discharge coefficient (around 0.61 for sharp-edged orifice plates, 0.95-0.99 for long-form venturis), A2 is the throat area, rho is fluid density, and beta is the diameter ratio d/D. The beta ratio should stay between 0.2 and 0.75 for the ISO uncertainty budgets to hold. Outside that range, use a different primary element or a direct flow meter.

Darcy-Weisbach pipe pressure drop

The Darcy-Weisbach equation, dP = f times (L/D) times (rho times v squared divided by 2), is the fundamental relation for friction pressure loss in full-flowing circular pipes. The Darcy friction factor f depends on Reynolds number and relative roughness and is found from the Moody chart or the Colebrook-White equation. For fully turbulent flow in commercial steel pipe, f is typically 0.01 to 0.025 depending on diameter. For laminar flow (Re below 2300), f = 64/Re exactly. System engineers add the equivalent pipe lengths of all fittings (elbows, tees, valves) to the actual pipe length before computing the total friction drop, then add minor losses and static head to get system resistance.

Filter differential pressure and Darcy's Law

For fibrous, granular, or membrane filter media, Darcy's Law gives the pressure drop as dP = (mu times Q times t) / (K times A), where mu is dynamic viscosity (Pa-s), Q is volumetric flow rate (m3/s), t is medium thickness (m), K is intrinsic permeability (m2), and A is face area (m2). Permeability spans many orders of magnitude: HEPA-grade glass-fibre media sits around 1e-12 to 1e-10 m2, while coarse pre-filters are 1e-9 to 1e-8 m2. As a filter loads with captured particles, K falls and delta P rises at constant flow. Monitoring delta P in service is therefore the standard method for scheduling filter replacement without opening equipment.