Bioswale Calculator
— How Much Do You Need?

A bioswale is a shallow, vegetated channel designed to manage stormwater runoff. When rain falls on hard surfaces like driveways, parking lots, or rooftops, water picks up speed and carries pollutants like oil, fertilizer, and sediment along with it. A bioswale intercepts that runoff, slows it down, filters out contaminants through engineered layers of aggregate and soil, and allows the cleaner water to soak back into the ground. You will find bioswales along roadsides, at the edges of parking lots, around commercial buildings, and in residential yards where drainage is a recurring problem. They are one of the most practical tools in stormwater management because they work with the natural water cycle rather than fighting it. If your property deals with pooling water, runoff erosion, or water quality concerns, a bioswale is worth serious consideration.

A standard swale is simply a shallow, graded channel that redirects water from one location to another. It might just be shaped earth or a strip of turf. A bioswale does the same job but adds layers of engineered aggregate and carefully selected vegetation that actively filter pollutants and encourage water to infiltrate the soil rather than just move along the surface. Think of a swale as a graded path for water, and a bioswale as a graded path that also acts as a natural filtration system. If you are dealing with runoff that carries sediment or pollutants, or if you want the water to soak in rather than simply redirect somewhere else, a bioswale is the better-performing choice. The aggregate layers are what make the real difference in how well it works.

Both features manage stormwater and improve water quality, but they function differently. A bioswale is a linear channel meant to carry water from one area to another while filtering it as it moves. A rain garden is a planted depression that collects water from a single point and holds it in place until it soaks into the ground. If you need to manage runoff across a long stretch of landscape, like along a driveway or the edge of a property, a bioswale is typically the right fit. If you have a single downspout or a concentrated low spot where water collects, a rain garden may serve you better. Many larger stormwater systems use both together, with a bioswale feeding into a rain garden at the end of the drainage path.

A properly built bioswale relies on multiple layers of aggregate working together. The bottom layer is usually a drainage zone filled with clean, washed crushed stone, typically in the 3/4 inch to 1.5 inch range, which allows water to move freely toward a perforated underdrain pipe. Above that sits a filtration layer of coarse sand or sandy growing media that filters fine particles while supporting plant root systems. At the inlet and outlet points where water enters and exits fastest, riprap, river rock, or larger crushed stone prevents erosion and scour from damaging the channel walls. Each layer has a specific job, so using the right aggregate in the right place matters more than most people expect. Hello Gravel can deliver all of these aggregate materials in bulk by the truckload, so you can get everything your project needs without dealing with multiple suppliers.

The quantity depends on the dimensions of your bioswale and how deep each aggregate layer is designed to be. As a rough starting point, a bioswale that is 50 feet long, 4 feet wide, and 18 inches deep would require approximately 2 to 4 tons of drainage stone for the underdrain layer alone, with additional material needed for the sand filtration layer and any inlet or outlet protection stone. Keep in mind that bioswale depths and layer thicknesses vary based on your site's drainage requirements and local design standards, so those numbers can shift significantly from one project to the next. The best way to get an accurate number is to use the Hello Gravel online calculator with your specific measurements. If you are not sure where to start, the Hello Gravel team is happy to help you work through the quantities for each layer of your build.