Swale Volume & Dimensions Calculator

Size a swale two ways: tell it your planned depth, width and length to get the volume it'll hold, or tell it a target volume (say, the runoff you're trying to capture) to find the trench length you'd need to dig.

What do you want to find?ⓘSwitch depending on whether you already know the trench length or are sizing against a target volume.

Depth (m)ⓘHow deep the trench is dug, measured straight down from ground level.Base width (m)ⓘThe flat bottom width of the trench.

Top width (m)ⓘThe width at ground level, including the sloped sides.Swale length (m)ⓘThe length of trench you're planning or have already dug.

Volume1.35 m³

How to measure your inputs

Depth: how deep the trench is dug, measured straight down from ground level.
Base width: the flat bottom width of the trench.
Top width: the width at ground level, including the sloped sides.
Target volume: if you're sizing against expected runoff rather than a fixed length, get that number from a rainfall/catchment calculation first (Swales' RainAdvisor does this for a specific pinned site).

Methodology

Standard trapezoidal cross-section approximation: area = depth × (base width + top width) ÷ 2, volume = area × length. This is the same formula used across earthworks engineering for estimating channel/trench volume, and matches the cross-section math already validated in Swales' RainAdvisor — reimplemented independently here so this calculator has no dependency on site data or sign-in.

How to size a swale correctly

Why swale volume matters more than "how it looks"

A swale that looks right but holds too little water just overflows in the first real storm, sending runoff exactly where you were trying to stop it. Sizing starts with the volume you actually need to capture — from a catchment/rainfall calculation, or from Swales' free RainAdvisor if you've analyzed a specific site — and works backward to a depth, width, and length that hold that volume without needing a trench so long it eats your whole yard.

Typical swale dimensions, by scale

A small garden swale (0.2-0.4m deep, 0.5-0.8m wide) suits residential yards and short runoff distances. Larger earthworks swales (0.5-0.8m deep, 1-2m wide) show up on small farms and food forests handling bigger catchment areas. Depth is usually the limiting factor for hand-digging; width and length are cheaper to add than depth once you’re past about 0.6m.

From a number to a construction plan

Once you have depth, width, and length, mark the trench on the ground, check it follows a level contour (a swale that’s off-level channels water to one end instead of spreading it, which usually means an overflow point on the downhill side), and dig with slightly sloped sides for stability rather than vertical walls, which is what the base-width/top-width split in this calculator already accounts for.

Frequently asked questions

How deep should a swale be?

Most residential and small-farm swales run 0.2-0.6m deep. Deeper trenches hold more water per metre of length but take significantly more digging effort and are more prone to collapsing sides without reinforcement, so most builds stay in that range unless a specific volume target requires more.

What's the difference between a swale and a drainage ditch?

A swale is dug level along a contour line so water spreads and infiltrates along its whole length; a drainage ditch is dug with a slope so water flows through it to an exit point. Swales are for capturing and infiltrating water on-site; ditches are for moving it away.

How long does my swale need to be?

That depends on your target volume and how deep/wide you're willing to dig — use this calculator's second mode (enter a target volume) to solve for the length directly, rather than guessing a length and hoping it's enough.

Can a swale be too big?

Oversizing wastes digging effort but isn't otherwise harmful — unlike undersizing, which causes overflow exactly where you didn't want it. If you're unsure of the target volume, it's safer to round up.