The best rain garden depth for most yards is 4 to 8 inches (10 to 20 centimeters) from the basin bottom to the top of your overflow berm. This range holds enough water to capture a normal storm. It also stays low enough that the garden doesn't look like a pond or become hard to care for. Most state extension services land on this same window in their homeowner guides. You don't need to go deeper than 8 inches for a home project.
I dug my first rain garden basin with a flat shovel and a wheelbarrow one Saturday morning. The rain garden excavation took about four hours for a 150-square-foot (14-square-meter) space dug to 6 inches deep. The best trick I learned at a county workshop was to stretch a string level across the hole on stakes. That string showed me if the bottom stayed flat. Without it, water pools on one side and leaves the other side dry. My rain garden excavation went much faster once I stopped guessing the grade and trusted the string. I wish someone had told me that tip before I started my first attempt.
Your rain garden basin depth choice sets up a trade between footprint and digging work. A deeper basin at 8 inches (20 centimeters) holds more water per square foot. You can build a smaller garden and still catch the same storm volume that way. A basin at 4 inches (10 centimeters) needs a wider footprint for the same job but takes less digging. If your yard is tight on space, go deeper. If you have room to spread out, keep it low and save your back. In my case, 6 inches hit the sweet spot for a mid-sized suburban lot.
Your soil type should drive the final rain garden basin depth choice. Sandy soil drains fast, so 4-5 inches (10-12.5 centimeters) works well. Water soaks away before the basin fills to the brim. Clay soils drain much slower. You need 6-8 inches (15-20 centimeters) paired with an amended soil mix so water doesn't sit for days. If you swapped out the clay for a sand-compost blend, 6 inches (15 centimeters) gives you a solid middle ground. Run a percolation test before you pick your depth. Dig a small hole, fill it with water, and time how fast it drains. That tells you what your soil can handle.
Build a berm on the downhill side to hold water in the basin. Pack the berm soil tight. Make it at least 2 inches (5 centimeters) taller than your planned water line so heavy storms don't overtop it. Cut a small notch in the berm for overflow and aim it away from your house. Keep the basin bottom dead flat so water spreads across the whole garden. Check your level in three spots across the bottom before you add soil and plants. A few extra minutes with a string level during the dig saves you from drainage headaches for years to come. Your future self will thank you for taking the time to get the grade right. I've seen gardens where the owner skipped this step and ended up with a soggy corner and a dry corner. That kind of problem is tough to fix once plants are in the ground. You'd have to pull everything out, re-grade, and start over. Do it right the first time and you won't deal with that headache.
Read the full article: Rain Garden Guide for Homeowners
