Introduction
The Well Volume Calculator tells you how much water is sitting inside your well. Type in the casing size, how deep the well goes, and how far down the water starts. The tool does the math and gives you the answer in gallons, liters, cubic feet, and cubic meters.
It also figures out your purge volume. Before you test well water, you need to pump out the old water first. Most rules say to remove 3 to 5 well volumes. Pick your multiplier and the calculator shows you the exact amount to pump.
There are two modes. Simple Mode uses just three numbers and works great for most home wells. Advanced Mode adds the bore hole size, casing outer diameter, and screen details, so you get the gravel pack and screen volumes too.
You can work in inches and feet or in millimeters and meters. A cross-section drawing shows your well, and a step-by-step section shows every formula used, so you can check the work or use it for a report. If you need volumes for other round or square shapes, the Pipe Volume Calculator and the Tank Volume Calculator handle those jobs.
How to use our Well Volume Calculator
Enter your well size, depth, and water level, and the calculator tells you how much water is in the well in gallons, liters, cubic feet, and cubic meters. It also shows how much water you need to purge before testing.
Mode: Pick Simple Mode for a quick well volume from the casing only. Pick Advanced Mode to also add the bore hole, gravel pack, and screen.
Unit System: Choose Imperial (inches and feet) or Metric (millimeters and meters). Your entered numbers convert on their own. For quick side conversions, try the Meters to Feet Calculator, the Inches to Mm Calculator, or the Volume Conversion Calculator.
Well Casing Inner Diameter: Type the width inside the casing pipe, not the outside. A common home well is 4 inches.
Total Well Depth: Type how deep the well goes, from the top of the casing to the bottom. Check your well report or use a sounding tape.
Depth to Static Water Level: Type how far down the resting water sits when the pump is off. Use 0 if water is right at the top.
Bore Hole Diameter (Advanced): Type the width of the drilled hole. It is bigger than the casing because of the gravel pack. If you are planning the dig itself, the Excavation Calculator helps with spoil volume.
Casing Outer Diameter (Advanced): Type the outside width of the casing pipe. This is used with the bore hole to find the ring-shaped space around the pipe.
Well Screen Diameter (Advanced): Type the inside width of the slotted screen. It is often the same as the casing inner diameter.
Well Screen Length (Advanced): Type how tall the slotted screen section is. This is on your well completion report.
Purge Multiplier: Pick how many well volumes you plan to pump out, like 3× or 5×. Most sampling rules ask for 3 to 5.
Custom Multiplier: If you picked Custom, type your own number, like 3.5 well volumes.
Chart Volume Unit: Choose gallons, liters, cubic feet, or cubic meters for the bar chart.
Press Calculate to see your well volume, purge volume, a well cross-section drawing, and the full math steps. Press Reset to start over.
Well Volume: What It Means
Well volume is the amount of water standing inside a well right now. It is the water that sits in the casing (the pipe in the ground) from the top of the water down to the bottom of the well. That water only fills part of the well. The space above the water is dry pipe, so it does not count.
The Three Numbers You Need
- Casing inner diameter – the width inside the pipe, not the outside. A common home well is 4 or 6 inches.
- Total well depth – from the top of the well down to the bottom. This is on the well report the driller gave you.
- Depth to static water level – how far down the water sits when the pump is off. You measure this with a water level tape.
Depth minus water level gives the water column height. That height, plus the pipe width, gives the volume.
How the Math Works
A well is just a tall round tube of water. So the volume is the circle area times the height:
Volume = 3.1416 × radius² × water column height
The radius is half the inner diameter. If you work in feet, the answer comes out in cubic feet. Multiply cubic feet by 7.48 to get U.S. gallons, or by 28.32 to get liters. This is the same formula used by the Cylinder Volume Calculator, and the Circle Area Calculator handles the cross-section part on its own. To turn dimensions straight into cubic footage, the Cubic Feet Calculator works well too.
Gallons Per Foot of Water
Plumbers often use a quick shortcut. Find your casing size, then multiply by the feet of water in the well.
| Casing inner diameter | Gallons per foot |
|---|---|
| 2 in | 0.16 |
| 4 in | 0.65 |
| 6 in | 1.47 |
| 8 in | 2.61 |
| 10 in | 4.08 |
| 12 in | 5.88 |
Why Well Volume Matters
Shock chlorination. When you disinfect a well after flooding, repairs, or a bad bacteria test, the bleach dose is based on well volume. Too little does nothing. Too much can harm the pump parts and the septic system. The Dilution Calculator and the PPM Calculator help you land on the right chlorine strength.
Purging before a water test. Water that has sat in the pipe is stale. It does not match the water in the ground. Sampling rules usually say to pump out 3 to 5 well volumes first, so fresh groundwater is what gets tested. Three well volumes is the common choice for slow, low-flow sampling. Pair the purge total with the Flow Rate Calculator to see how many minutes of pumping that takes.
Pump and treatment work. Knowing the volume helps you size a purge tank, plan how long to run a pump, and figure how much treatment chemical to add. For pump sizing and head loss, look at the Pump Power Calculator, the Friction Loss Calculator, and the Pipe Flow Calculator.
Screen and Gravel Pack
Many wells have more water than just the pipe holds. The screen is the slotted section near the bottom that lets water in. Around the casing there is often a gravel pack in the drilled hole, called the annulus. Water fills the gaps between the gravel bits. Only about 30% of that ring is open space, so the gravel pack holds less water than its size suggests. For careful purge planning on monitoring wells, this extra water gets added in. If you are ordering the pack material itself, the Gravel Calculator and the Pea Gravel Calculator give you tonnage.
Tips for Good Numbers
- Measure the water level with the pump off for at least a few hours. A running pump pulls the level down.
- Water levels move with the seasons. Check again in a dry month.
- Use the inside pipe width. Using the outside width makes your volume too big.
- Keep your well report. It lists depth, casing size, and screen length.
- Round your purge amount up, never down.
Working on other water projects around the property? The Pool Volume Calculator, the Pond Liner Calculator, and the general Volume Calculator cover most of what comes next.