Differential Pressure Calculator

What Is Differential Pressure?

Differential pressure is the difference between two pressure readings at different points in a system. It tells you how much the pressure drops across a filter, valve, duct, or pipe segment. HVAC technicians, process engineers, and instrumentation specialists use ΔP to diagnose blockages and size equipment.

Differential Pressure Formula

$$\Delta P = P_1 - P_2$$

$P_1$ is the upstream (higher) pressure and $P_2$ is the downstream pressure. A positive ΔP means pressure drops in the flow direction. Example: $110{,}000\,\text{Pa}$ upstream and $100{,}000\,\text{Pa}$ downstream gives $\Delta P = 10{,}000\,\text{Pa} = 10\,\text{kPa}$.

Common Pressure Units

Pascals (Pa) are the SI unit. Kilopascals (kPa), pounds per square inch (psi), and bar are common in industry. One bar equals 100 kPa and roughly 14.5 psi.

Frequently Asked Questions

Which pressure should be P₁?

Use the upstream or higher reading as P₁. The calculator subtracts P₂ from P₁, so a positive result means pressure decreases in the flow direction.

What does a negative differential pressure mean?

A negative ΔP means P₂ is higher than P₁. This can happen if readings are swapped or if flow reverses direction.

How is differential pressure measured?

A differential pressure transmitter connects to two ports and outputs the difference directly. Manometers and pitot tubes also measure ΔP across filters and ducts.

When is ΔP important in HVAC?

Filter clogging, duct restrictions, and fan performance all show up as rising differential pressure. Monitoring ΔP helps schedule maintenance before airflow drops.

Is gauge pressure the same as absolute pressure?

No. Gauge pressure is relative to atmospheric pressure. Make sure both readings use the same reference (gauge or absolute) before subtracting.

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What Is Differential Pressure?

Differential Pressure Formula

Common Pressure Units

Frequently Asked Questions

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