How well do you know your, er, well?

When you drill a new well, or repair, sampling it is important to ensure the water is safe to drink. And one of the common challenges in sampling your well is determining the well volume. The good news is that your well volume isn’t hard to figure out, if you know a little math or have a well volume calculator — like the one provided by Hydro-Terra — handy. But if you don’t have the Internet handy, you just need to know the equation and how it works to get the right amount of water.

Why Would I Need To Know My Well Volume?

Part of owning a well is knowing how much water you can get out of it. Being able to calculate your well volume will allow you to compare volume to the overall water usage and ensure that you’re using your well properly and that you aren’t putting too much strain on your water table. Similarly, if you need to repair your well or sink a new well, sampling the water will be important to ensure your water hasn’t been contaminated by coliform bacteria or other possible groundwater contaminants.

You’ll need to pump out three times the well volume to ensure you’re getting enough water for a proper test. So, both for your own safety and to test your well, you’re going to have to do a little math.

The Information You Need

First, you’ll need to have some basic information about your well. Start with the diameter; most wells are drilled so as to be circular. Take the diameter and divide it by two; that gives you the radius of your well. This can be as simple as pulling out a tape measure, but do your best to be precise.

Next, you’ll need to know both the overall depth of your well, and the depth to water, in feet. Finding the depth to water is simple: Just tie a fishing weight to a long piece of string and lower it down. When your weight reaches the water, mark off the string. You can use the same method to determine the overall depth of your well, but be sure to use a weight that won’t get snagged on pipes.

A deep well is just the start

Doing The Math

First, subtract the depth to water from the overall depth of the well. For example, if our well was ten feet deep and it was a foot to water, we’d have nine feet of water. Next, use the following equation: pi (or 3.15 if you’re not using a calculator) times the square of your radius times the height of your well.

Say our example well has a two foot diameter. We’d divide that by two, and get a one foot radius. Multiply that by pi, and then the height of nine feet, and you’ve got the well volume: Approximately 28.3 cubic feet. Multiply that cubic feet by 7.47, and you’ve got the total number of gallons in your well volume, 211 gallons.

A few tips: Err on the side of rounding up, especially when sampling, and be sure to keep your units consistent, and if your radius is in inches, for example, convert your height to inches, or your approximation will be off. But once you’ve got that figured out, the rest is down to the pumping.