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Yard Drainage Fixes

Catch Basin Calculator: Basin, Grate & Outlet Pipe Size

Catch basin calculators size a grate. In a yard the grate is rarely what limits the drain — the outlet pipe and its slope are. This sizes the three together: the basin and grate, de-rated for the leaves a lawn sends, the pipe at the fall you actually have, and how many basins the low spots need — then lists what to buy. For when a basin is the right tool at all, see catch basins for yard drainage.

What drains to the low spot?Square feet of each surface that sheds toward the basin(s). Leave what doesn't apply at 0.
Lawn soil

Clay sheds more of the rain as runoff; paving and roofs shed almost all of it.

Design storm (peak intensity)
Pipe material

Water arriving

29 GPM lawn 12 · paving 17

9-inch square basin with a 4-inch outlet

Your catchment sends about 29 GPM in this storm. A 4-inch smooth PVC pipe at 1% carries about 64 GPM with a margin for the fittings and grit — that outlet, not the grate, is usually what limits a yard basin.

Basin & grate

9-inch square

9-inch square grate: rated 96 GPM at half an inch of ponding, taken as 58 GPM usable once leaves arrive.

Set the grate a quarter to half an inch below the turf so water falls in, not around. A 12-to-24-inch-deep body gives the sump room.

Outlet pipe

4-inch PVC

  • 4-inch at 1%: about 64 GPM usable
  • 6-inch at 1%: about 189 GPM usable

Shopping list

  • 1 × 9-in catch basin with grate
  • 4 ten-foot sticks of 4-inch pipe (30 ft plus 5%)
  • Per basin: an outlet adapter, two elbows or a wye, a knock-out plug
  • 4 half-cubic-foot bags of gravel to bed the basin
  • A pop-up emitter or a daylight grate at the outlet

Planning ranges

DIY $110–$250

Installed $600–$2,000

Installed figures assume a modest run per basin; long runs, concrete cutting and lawn repair are extra. Planning estimates, not quotes.

Price the whole plan →

How it goes together

Yard catch basin cross-sectionCross-section of a yard catch basin. The lawn slopes down from both sides to a grate set just below turf level at the lowest point. Below the grate is a box basin; the 4-inch outlet pipe leaves the side of the basin several inches above its floor, so debris settles in a sump below the outlet, and the pipe falls at about one percent toward a lower discharge point. The basin sits on a bed of gravel.sump belowthe outlet4-in outlet, ≥1% fallgrate just below turf, at the low pointgravel bed

What this doesn't tell you

This is a planning size for a residential basin. It uses a short, intense design storm and published grate ratings de-rated for leaves; a grate buried in mulch or a pipe with a belly in it will do worse than any number here. It assumes the water actually reaches the grate — a basin set high, or six feet from the real low point, drains nothing — and that the outlet has somewhere to go. Measure the fall to the outlet before you buy anything.

How to read your results

  • Water arriving is the peak flow in a short, intense storm — the moment the basin has to keep up, not the total rain. Paving and roofs dominate it; a lawn sheds a fraction of what lands on it.
  • Basin and grate is the smallest size whose de-rated capacity covers each basin's share. Lawns get bumped to 9 inches regardless, because a 6-inch box has no room for a sump.
  • Outlet pipe is the smallest size that carries the total flow at your slope with a margin. When the verdict says the slope is the problem, no basin fixes it — water needs at least 1 percent of fall to a real outlet.
  • Two cost figures. DIY is the shopping list. Installed is the site's planning range per basin with a modest pipe run; long runs, concrete cutting and lawn repair are extra.

The formula — check it yourself

Q (cfs) = C × i × A, the rational method: C is the runoff factor (0.9 for paving, 0.95 for a roof, 0.15 to 0.35 for lawn by soil), i the rainfall intensity in inches per hour, A the area in acres (square feet ÷ 43,560). Multiply cubic feet per second by 448.8 for gallons per minute. A 1,500 sq ft patio at 6 in/hr: 0.9 × 6 × 0.0344 = 0.186 cfs, about 84 GPM. Pipe capacity comes from Manning's equation at full flow, the same calculation the French drain sizing calculator uses, taken at 75 percent for fittings and grit.

Grate capacities used

Published ratings at half an inch of ponding, from manufacturer spec sheets, and the 60 percent figure the tool relies on once leaves arrive.

GrateRatedUsableBasin + grate, planning
6-inch round28 GPM17 GPM$15–$35
9-inch square96 GPM58 GPM$25–$60
12-inch square155 GPM93 GPM$40–$130
18-inch square264 GPM158 GPM$100–$250
24-inch square500 GPM300 GPM$200–$450

Sizing chart

One basin, Southeast design storm (6 in/hr), smooth PVC outlet at 1 percent.

Area draining inLawnPatio or driveway
500 sq ft8 GPM → 9-inch square, 4-in pipe28 GPM → 9-inch square, 4-in pipe
1,000 sq ft16 GPM → 9-inch square, 4-in pipe56 GPM → 9-inch square, 4-in pipe
2,000 sq ft31 GPM → 9-inch square, 4-in pipe111 GPM → 18-inch square, 6-in pipe
4,000 sq ft62 GPM → 12-inch square, 4-in pipe223 GPM → 24-inch square, null-in pipe

The lawn column is why most residential low spots need a 9-inch basin and a 4-inch pipe, and the paving column is why patios and driveways so often need more.

When a catch basin is the wrong fix

A basin drains a point. Water that pools in one identifiable spot is its job; water that seeps across a whole strip of lawn wants a French drain, and water that arrives from one downspout is better caught at the downspout with a downspout basin or a buried line to an emitter. A basin with nowhere to send the water is a decorative bucket; if the fall to daylight isn't there, a dry well stores it instead. And a basin that already exists and still floods is usually clogged, set too high, or piped flat.

Frequently asked questions

What size catch basin do I need?
Size it from the water reaching it. Flow in gallons per minute is about 0.0103 × runoff factor × rainfall intensity in inches per hour × area in square feet. A typical lawn low spot of 1,000 square feet in a 6-inch-per-hour downpour sends around 15 GPM and a 9-inch basin is plenty; a 1,500 square foot patio sends about 84 GPM and wants a 12-inch basin. Outlet pipe size usually limits capacity before the grate does.
How many catch basins do I need?
One per low spot — a basin only collects water that reaches its grate, so a single basin cannot drain two puddles. Enter the number of low spots and the tool splits the flow across them and sizes each grate for its share, while sizing the shared outlet pipe for the total. Very large areas are better split into more basins than served by one big one.
What size pipe should a catch basin drain into?
Four-inch is the residential standard and carries about 65 GPM usable in smooth PVC at a 1 percent fall; 6-inch is for several basins combined or a big paved area. Three-inch corrugated kits are sold for downspouts but silt up and cannot be rodded, so the tool never recommends them. Slope matters as much as diameter — the same 4-inch pipe carries about 40 percent more at 2 percent than at 1 percent, and below 1 percent it silts up whatever size it is.
How deep should a catch basin be?
Deep enough that the outlet leaves the side of the basin several inches above the floor, leaving a sump below it where grit and leaves settle instead of washing into the pipe. Common residential basins are 12 to 24 inches deep. Set the grate a quarter to half an inch below the surrounding turf or paving so water falls in rather than running around it.
Does a catch basin need gravel underneath?
A few inches of compacted gravel under the basin keeps it level and stops it settling and tilting as the backfill consolidates, which is what leaves a grate standing proud of a puddle a year later. It is bedding, not drainage — a catch basin with an outlet pipe does not rely on water soaking away below it.
Why does the calculator de-rate the grate?
Published grate capacities are measured clean, with half an inch of water ponded over them. The first autumn buries a lawn grate in leaves, and grit collects in the slots. Taking 60 percent of the rating as usable is a planning margin so the basin still keeps up when it is not brand new — and it is why 9 inches, not 6, is the practical minimum for a lawn.

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