# Calculating water pressure

1. Oct 26, 2004

### rick326

Given only the rate of flow in gpm and the inside diameter of a hose in inches, is it possible to calculate the pressure of water leaving the hose?
I have no experience or training in this area but it seems to me that more information would be required. Any help will be greatly appreciated.

2. Oct 26, 2004

### niehls

it's been a while, but i believe that Bernoulli's equation could help. Check that one out

3. Oct 26, 2004

### Chi Meson

Without getting into very advanced fluid dynamics, it is assumed that the water pressure, after the water has left the hose, is same as atmospheric pressure.

If you are referring to the pressure of the water against a surface (as the water splashes on a surface like your hand, after it leaves the hose) then the "pressure" would be the force of the water/cross-sectional-area of the water stream (assumed to be the area of the hose?) P=F/A

From the volume flow rate (gal/min) you can calculate the "mass-flow rate" (mass/sec), and assuming all the water is stopped, the mass flow rate times the change in velocity will produce the force.
Ft = m[delta]v , so F=(m/t) x [delta]v

So, yes, the pressure of this stream of water against a surface can be calculated from the information given.

Last edited: Oct 26, 2004
4. Jan 21, 2009

### srntcbd

I am trying to calculate the force exerted on a body during high pressure water blasting. The numbers are 36,000psi pump pressure, 5gpm flow rate. lbs of force back pressure is the number I'm looking for. HELP!

5. Jan 21, 2009

### Staff: Mentor

You need the velocity, then you use Bernoulli's equation to find the pressure. What is the area of the nozzle?

6. Jan 21, 2009

### srntcbd

There are two nozzles. Both are 1/8" in diameter.