Swale Calculator
— How Much Do You Need?

A swale is a shallow, wide channel built into the landscape to collect and redirect stormwater runoff. Unlike a steep drainage ditch that rushes water away as fast as possible, a swale is designed with gentle slopes that slow the flow of water down, giving it time to seep into the ground or travel safely toward a lower point on your property. You'll find swales along roadsides, at the edges of driveways, between adjacent yards, and around the perimeters of commercial properties. They're one of the most practical and cost-effective drainage solutions available for managing water that would otherwise pool around your foundation, flood low-lying areas, or wash out your landscaping. Lining a swale with the right gravel makes it drain faster, last longer, and perform reliably through heavy rain events.

These three terms get mixed up often, and it's worth understanding how they differ before you decide which solution fits your property. A swale is a broad, shallow, above-ground channel with gently sloped sides. Its job is to slow water down, spread it out, and encourage it to infiltrate the soil rather than rush away. A ditch is typically narrower and steeper, designed to move large volumes of water quickly rather than let it soak in. Ditches are common along roads and agricultural land where moving water fast is the priority. A French drain is a completely different system. It works underground using a perforated pipe wrapped in filter fabric and surrounded by gravel to intercept subsurface water before it saturates the soil or reaches your foundation. A swale manages water on the surface. A French drain manages water that has already entered the ground. Each one solves a different drainage problem. If you have surface runoff flooding your yard or washing across your driveway, a gravel-lined swale is usually the right starting point.

Yes, lining a swale with gravel is one of the best upgrades you can make for its long-term performance. A gravel-lined swale drains faster than a bare earth or grass swale, resists erosion during heavy rain events, and eliminates the regular mowing and vegetation maintenance that a vegetated swale requires. It also clears standing water more quickly, which reduces the conditions that attract mosquitoes. The best materials for lining a swale are clean, washed stone in the 1.5 to 3 inch range, such as river rock or clean crushed stone. These sizes are large enough to stay in place under flowing water but still allow water to pass through efficiently. Smaller pea gravel or fine sand can wash out or compact over time, reducing drainage and requiring frequent replacement. A heavier, open-graded stone gives you the best combination of stability and drainage performance. If you are unsure which stone size makes the most sense for your specific swale size and flow rate, the Hello Gravel team is happy to help you choose the right material before you place your order.

The amount of gravel you need depends on the length of your swale, its width, and how deep you plan to place the stone. A standard approach is to line the bottom and lower sides of the swale to a depth of about 6 inches of compacted stone. As a practical example, a 50-foot swale that is 3 feet wide lined at 6 inches deep would require roughly 2.5 to 3 cubic yards of stone, which translates to approximately 3 to 4 tons depending on the stone type. For longer or wider swales, that number climbs quickly. A 100-foot swale at the same width and depth would put you closer to 6 to 8 tons. These are starting estimates only. Your actual quantity will depend on the exact shape of your channel and how deeply the sides are sloped. The most reliable way to get an accurate number is to use the Hello Gravel calculator. Plug in your dimensions and it will give you a project-specific estimate so you are not left short or sitting on a pile of extra material.

A properly functioning swale should drain completely within 24 to 48 hours after a rain event. If water is still sitting in your swale 3 or more days later, something is not working the way it should. The most common causes of slow drainage are an insufficient slope, a blockage from accumulated sediment or debris, or soil with poor natural infiltration. Standing water that persists beyond 48 to 72 hours also becomes a breeding environment for mosquitoes, so it is worth addressing the problem rather than waiting it out. A gravel-lined swale with a consistent downward slope of at least 1 to 2 percent will typically drain well within that 24 to 48 hour window. If yours is not meeting that standard, adding or refreshing the gravel layer and checking the grade of the channel floor are the first two places to start.