When performing a 2 hour pressure test on a waterline, the formula for calculating the allowable water loss over the 2 hours is given on the testing form. But, the form asks for the actual water loss. Of course, you know the pressure loss, but it asks for the water loss in gallons. This can be accomplished by using a pump to force water back into the waterline until you reach the starting pressure you had when the test started and having a way to measure the amount of water you used to re-fill the waterline. I have came across several contractors that can't do this, so I thought I would calculate the actual water loss knowing the pressure loss over the 2 hours. I believe the calculation would incorporate the temperature of the water being tested, the initial and final pressure, the initial and final density of water, and the bulk modulus of water. With a little research, I found formulas to calculate the change in density of the water per the change in pressure. With the change of density known, I then can calculate the change in volume and thusly the gallonage. But, the gallonage values I am getting seem very low and not in accord with the allowable water loss quantity. I am wondering if these principals of water are based on a closed system in which the change in density is calculated per the change in temperature and pressure, and doesn't account for the case where water escapes the system. I am also wondering if with the loss in pressure being usually small, the calculation of gallonage loss is highly inaccurate. Anyway, I came to this forum to ask if anyone else knows of a formula for accurately calculating the water loss in gallons, or if you would just tell me I am wasting my time calculating it and to just have the contractor figure out a way of re-filling the waterline to physically see how much water it takes. Thank you for your time.