Size a swale from three numbers
Timebox: one hour at a table with the three numbers in front of you. Output: a channel you can dig — bottom width, depth, length, and grade — or the honest news that a swale is not your fix. The math below is yard-scale arithmetic, not an engineered design; if your code or site demands one, the arithmetic still tells you what to ask for.
Checked 2026-09-10 · procedure re-verified against sources below
The arithmetic
- Design storm. The desk uses 1 inch of rain over your catchment:
volume (gal) = catchment (sq ft) × 1 ÷ 12 × 7.48. - Soak rate. The wetted bottom soaks at the perk rate:
gallons/hour = bottom area (sq ft) × rate (in/hr) ÷ 12 × 7.48. - Storage. Pick a bottom width (18–36 in) and depth (6–12 in). Cross-section area (sq ft) of the trapezoid with 3:1 sides. The channel length must hold the storm:
length (ft) = volume (cu ft) ÷ cross-section (sq ft). - The 72-hour drain check. Soak capacity per square foot of bottom over 72 hours:
rate (in/hr) × 72 ÷ 12 × 7.48gallons. Bottom area must pass:bottom area ≥ volume ÷ capacity— see the 72-hour rule. - Dimensions and grade. Grade 1–4% along the run, or check dams step it. A berm on the downhill side buys depth without digging.
- The gate. Rate under 0.5 in/hr: the swale is a carrier. Size the channel for flow, add armor, and give it a real outlet. It will not soak; don't size it as if it will.
Worked example
| Input | Value |
|---|---|
| Catchment | 1,480 sq ft |
| Perk rate | 0.8 in/hr (passes the 0.5 gate) |
| Grade along the run | 2% (no dams needed) |
| Design storm volume | 1,480 × 1/12 × 7.48 ≈ 922 gal (123 cu ft) |
| Channel cross-section (2-ft bottom, 8-in deep, 3:1 sides) | ≈ 2.7 sq ft |
| Length to hold the storm | 123 ÷ 2.7 ≈ 46 ft |
| Drain check | 92 sq ft of bottom drains 922 gal in ≈ 20 h — inside 72 h |
That is a real swale — 46 feet of channel — not a “drainage feature” by the mailbox. If the yard cannot give 46 feet, the fix was never a swale: the volume needs a dry well or conveyance. The arithmetic exists to deliver exactly this news before the shovel does.
Where people stop
- Sizing by feel. “A swale about this big” is how undersized swales get born and overflow in year one.
- Skipping the gate. On 0.3-in/hr clay, soak math is theater. Size for carry and armor the channel.
- Forgetting the overflow. Every swale meets a bigger storm eventually. Give it a spill point that leads somewhere legal, not into the crawlspace.
Where to go next
- Swale — the object this page sizes.
- Build check dams — when the grade says so.
- Plant a swale — what keeps the sized channel where you dug it.
Sources
- Minnesota Stormwater Manual, swale sizing — stormwater.pca.state.mn.us
- USDA NRCS — nrcs.usda.gov
- US EPA NPDES — epa.gov/npdes