Dry Well Calculator
— How Much Do You Need?

For most homeowners dealing with excess water from downspouts, sump pump discharge, or low spots that tend to pool after rain, a dry well is one of the most practical and cost-effective drainage solutions available. Instead of letting water sheet across your yard, erode your lawn, or push toward a foundation, a dry well channels it underground and lets it slowly percolate back into the soil where it belongs. Dry wells work best in areas with good soil permeability. Sandy loam and gravelly subsoils absorb water quickly, so a properly sized dry well can handle even heavy rain events without backing up. In clay-heavy soils, a dry well still provides real benefit, though it will drain more slowly and may need to be larger to compensate. If you have persistent standing water after rain, a single downspout that dumps into a soggy corner of the yard, or a sump pump with nowhere efficient to discharge, a dry well is often the most direct fix. It is also a common first step before investing in a full French drain system or significant regrading. The key is sizing it correctly for your drainage volume and making sure the gravel fill is clean and properly chosen so the well drains efficiently from day one.

Most residential dry wells are installed at a depth of 4 to 6 feet, though the right depth for your project depends on a few important factors. The most critical rule is that the bottom of your dry well should sit at least 2 feet above the seasonal high water table. If the well reaches too close to the water table, the surrounding soil is already saturated and has no capacity to absorb more water, which means your well will back up and stop working exactly when you need it most. Beyond the water table, you want the bottom of your pit to reach permeable soil. If you hit sandy loam, gravelly subsoil, or loose subsoil, you are in good shape. If you are digging through dense clay the entire way down, a dry well alone may not be enough, and you might need to consider alternative drainage strategies or combine your dry well with other solutions. Before you break ground, check with your local building department. Many municipalities require a permit for dry well installations, and some have minimum depth requirements or setback rules from property lines, septic systems, and wells. Confirming those details before you dig also helps you nail down the exact dimensions of your pit, which is what you will plug into the Hello Gravel calculator to get your gravel quantity estimate.

Yes, gravel is the essential fill material for a dry well, and choosing the right kind makes a significant difference in how well your system performs over time. The gravel creates open void space inside the pit that temporarily holds water while it seeps into the surrounding soil. Without that void space, water has nowhere to go and the well backs up quickly. The most important characteristic to look for is cleanliness. You want washed crushed stone with minimal fines, meaning little to no clay, silt, or sandy particles mixed in. Those fine materials migrate into the gravel voids over time, clog them, and dramatically reduce drainage capacity. A clean stone maintains its permeability for years. For a traditional all-gravel pit, the gravel itself serves as both the drainage medium and the structural fill that supports the surrounding soil. For installations that use a prefabricated plastic chamber or barrel, gravel is packed around the chamber to hold it in place and extend the drainage area beyond the chamber walls. Either way, the quality and volume of gravel you use directly determines how well your dry well performs and how long it lasts. Hello Gravel delivers bulk gravel by the truckload, which makes it straightforward to get the right amount of clean stone delivered right to your site on the day that works best for your project schedule.

Clean, washed crushed stone in the 1.5-inch to 2.5-inch size range is the most widely used and recommended gravel for dry well fill. The angular shape of crushed stone creates more consistent void space than round stones because the irregular edges lock together without fully nesting, which means more room for water to move through and temporarily collect. The single most important quality to look for is that the stone is washed and free of fines. Fines are the small particles, dust, clay, and silt that can be present in unprocessed or poorly screened aggregate. Even a relatively small percentage of fines can migrate into the void spaces over time and reduce your dry well's drainage capacity. A clean stone that has been washed and screened holds its permeability far longer. Some installers use larger clean stone for deeper fill areas where structural support matters most, and a slightly smaller clean gravel near the top of the pit. What you want to avoid is any material that is blended with soil, contains significant clay content, or has not been properly washed. If you are unsure which product is the right fit for your drainage conditions, reach out to the Hello Gravel team and we can point you toward the right material for your project.

The amount of gravel you need depends on the dimensions of your pit and whether you are building a traditional all-gravel dry well or surrounding a prefabricated chamber. For a traditional gravel-filled pit that is 4 feet in diameter and 4 feet deep, you are looking at roughly 2 cubic yards of clean stone to fill it. A deeper pit at the same diameter, say 6 feet deep, pushes that closer to 3 cubic yards. If your design calls for gravel backfill around a chamber and a layer of stone across the top before you replace the soil, add another half to full cubic yard depending on the chamber size. Keep in mind that dry well projects sometimes include connecting trench runs back to downspouts or a sump pump discharge point, and those trenches also need to be filled with clean gravel around the pipe. If you are running a 50-foot trench at 12 inches wide and 18 inches deep, that adds another cubic yard or more to your total. Because the specific dimensions of your pit and any connected trench work vary from project to project, the Hello Gravel calculator is the most reliable way to get a precise estimate. Plug in your dimensions and get a number you can order with confidence. If your project involves multiple dry wells or a larger drainage layout, our team is happy to help you think through the quantities before you place your order.