Inches of Water to GPM Calculator
Volumetric Flow Rate Result
What is Inches of Water (inH₂O)?
“Inches of Water” (often written as inH₂O) is a unit of pressure. It measures how much pressure is exerted by a column of water of a certain height. For example, 10 inches of water pressure means the force exerted by a 10-inch tall column of water at rest.
In HVAC, fluid dynamics, and process systems, inH₂O is commonly used to measure low-pressure differentials across filters, orifices, and ducts.
Why Convert Inches of Water to GPM?
Pressure values (inH₂O) are useful, but engineers often need flow rate to design, troubleshoot, and optimize systems.
Some examples where conversion is essential:
- HVAC Systems → Estimating air or liquid flow through ducts and nozzles.
- Industrial Processes → Calculating liquid flow through orifice plates or venturi nozzles.
- Fluid Testing → Determining how pressure drop translates into usable flow rates.
- Pump & Valve Sizing → Ensuring systems are neither under-designed nor oversized.
Formula Behind the Calculator
The calculator uses a generalized flow equation:
[
\text{GPM} = C \times C_d \times D^2 \times \sqrt{P}
]
Where:
- C = Base flow constant (19.6 for water, accounting for unit conversion & gravity)
- Cᴅ = Discharge coefficient (depends on orifice/nozzle type)
- D = Diameter of orifice or nozzle (inches)
- P = Pressure differential (inches of water column)
Example:
If you have:
- Pressure = 10 inH₂O
- Diameter = 1.5 in
- Orifice Type = Venturi nozzle (Cᴅ ≈ 0.98)
Then:
[
\text{GPM} = 19.6 \times 0.98 \times (1.5^2) \times \sqrt{10}
]
This equals approximately 136.33 GPM.
Instead of doing this math manually, the calculator gives you results instantly.
Orifice & Nozzle Types in the Calculator
The discharge coefficient (Cᴅ) is critical because it corrects for flow losses. The calculator lets you choose from common types:
- Sharp Edge Orifice (Cᴅ ≈ 0.61) → Simple, low-cost, but energy-wasting.
- Venturi Nozzle (Cᴅ ≈ 0.98) → High efficiency, minimal energy loss.
- Long Radius Nozzle (Cᴅ ≈ 0.80) → Smooth entry, moderate losses.
- Borda Mouthpiece (Cᴅ ≈ 0.51) → Older design, less efficient.
By selecting the right type, you get results closer to real-world conditions.
How to Use the Calculator
- Enter Pressure Differential (inH₂O) → e.g., 10.0
- Enter Diameter (inches) → e.g., 1.5
- Select Orifice/Nozzle Type → Sharp Edge, Venturi, Long Radius, or Borda
- Click Calculate → The flow rate in GPM will appear instantly.
- Reset anytime to clear inputs and start fresh.
The result also shows the formula used so you understand how the number was derived.
Key Benefits of the Inches of Water to GPM Calculator
Accuracy without complexity – No need for manual formula application.
Customizable – Accounts for nozzle type and diameter.
Time-saving – Get results in seconds.
Educational – Helps students, engineers, and technicians learn the concept.
Practical – Directly useful in HVAC, plumbing, and industrial flow measurement.
Limitations & Disclaimer
While powerful, this calculator provides estimated results. It assumes:
- Fluid = Water at standard conditions
- No consideration for temperature, viscosity, or pipe friction
- Simplified orifice plate model
For highly precise engineering projects, consult detailed fluid dynamics references or use CFD simulations.
