Rain Garden Calculator
— How Much Do You Need?

A rain garden is a shallow, planted depression built to capture stormwater runoff from rooftops, driveways, and other hard surfaces. Instead of letting that water rush into storm drains or cause erosion across your yard, a rain garden directs it into a specially designed basin where it slowly soaks into the ground. The result is less flooding, fewer pollutants reaching local waterways, and healthier groundwater recharge on your own property. What makes a rain garden work is not just the plants, it is the engineered layers underneath them. A properly built rain garden depends on a clean gravel drainage layer at the bottom, a well amended sandy soil mix in the middle, and often decorative stone around the inlet and overflow channels to control fast moving water. Without the right aggregate foundation, even a beautifully planted rain garden will sit waterlogged and fail to drain the way it should. That is where Hello Gravel comes in. We supply the drainage gravel, coarse sand, and decorative stone that give your rain garden its structure and performance.

The standard recommendation is to place your rain garden at least 10 feet from your home's foundation. Any closer and you risk directing water toward your basement or crawl space, which defeats the purpose entirely. You should also keep the garden at least 25 feet from any septic system or private well, and always check for buried utility lines before you dig. Beyond those minimum distances, the best spot for a rain garden is a natural low point in your yard where runoff already tends to flow. If your yard has a slope, positioning the garden to intercept water as it moves downhill makes the whole system more efficient. Just avoid any area that stays wet for days after a normal rainstorm, since that is a sign the native soil beneath it drains poorly. A good site drains on its own within a day or two after rain, which means your rain garden will perform reliably instead of staying flooded.

A typical rain garden is excavated to a total depth of 18 to 24 inches. That depth is not arbitrary. It is what you need to fit the three functional layers that make the garden work. The bottom layer is a drainage gravel bed, usually 6 to 12 inches deep. This is where most of the water storage and infiltration happens, and it needs to be built with clean, washed crushed stone or gravel so water can move through it freely. Above that goes 12 to 18 inches of amended planting soil, typically a blend of coarse sand, native topsoil, and compost. This layer supports plant growth while still allowing good drainage. The top inch or two is usually mulch or decorative stone to protect the soil surface and slow incoming water during heavy rain events. Getting the depth right matters more than most people realize. Too shallow and water will pond on the surface. Too deep and the planting zone becomes difficult to establish and maintain. Use the Hello Gravel calculator on this page to work out your material quantities once you have your dimensions planned out.

There are three types of aggregate that go into a well built rain garden, and Hello Gravel can supply all of them in bulk. The first is drainage gravel for the bottom layer. You want a clean, washed crushed stone in the 3/4 inch to 1.5 inch size range. It should be angular rather than smooth so the pieces interlock and hold their position, and it needs to be free of fine particles that would clog the drainage spaces over time. This layer is the backbone of your rain garden's performance. The second material is coarse sand, which forms the major component of your amended planting soil. A proven rain garden soil mix runs about 50 to 60 percent coarse sand, blended with compost and topsoil. That blend drains quickly during storms while still holding enough nutrients and structure for plants to establish healthy root systems. The third material is decorative stone or river rock, which is used at the inlet where water enters the garden and at the overflow outlet where excess water exits. These high velocity zones need stone protection to prevent erosion and keep the structure intact over time. If you leave those areas as bare soil or mulch, water will eventually carve channels through them and undermine the whole design.

Your quantities depend on the size and depth of your specific rain garden, so it helps to run the numbers against your actual dimensions. That said, here are some general figures to give you a realistic starting point. For a 200 square foot rain garden with a 6 inch drainage gravel layer, you are looking at roughly 2 to 3 tons of crushed stone. If you build that gravel layer out to 10 or 12 inches, which is the better choice for larger drainage areas, expect to need 4 to 5 tons. For the amended soil layer, if you are blending a 15 inch planting zone from scratch over that same 200 square feet, you may need 3 to 5 tons of coarse sand depending on how much native topsoil you are incorporating into the mix. These are starting estimates, not exact figures. Your actual numbers will shift based on your excavation depth, the shape of your garden, and how you are mixing the planting soil. The Hello Gravel calculator on this page will give you a more precise result once you enter your measurements. All materials are delivered by the truckload, so it is worth dialing in your quantities before you place your order.