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Yard Drainage Fixes

Dry Well Sizing Calculator

Most dry well calculators either size a manufacturer's own product or make you guess the dimensions. This one works the other way round: tell it the water and the soil, and it sizes the well for you — as a gravel pit and as an open chamber — then checks whether your ground will actually drain it. For how a dry well works and installs, see dry wells.

What drains into it?
Design storm
Soil it sits in
Pit depthDeeper stores the same water in a smaller footprint — but stay above the water table.

Water to store

310 gal ≈ 41.6 cu ft

Gravel-filled pit

5.5' × 5.5' × 4'

Holds the water in its stone voids. Needs 4.5 yd³ (~6.3 tons) of #57 stone and about 136 sq ft of filter fabric.

Open chamber

3.5' × 3.5' × 4'

A hollow dry-well unit stores water directly, so the hole is smaller — but you buy the chamber instead of stone. Still wrap it in fabric.

The gravel pit is bigger because stone fills most of the hole — water lives only in the roughly 40% gaps between the stones. A hollow chamber stores nearly all its volume, so it needs less digging for the same water.

Workable, but on the slow side

empties in ~33 hrs

It drains within the 24-to-72-hour window a dry well needs, but there's little margin. Make the pit a bit larger, or add an overflow so a second storm before it empties has somewhere to go.

How it goes together

Yard dry well cross-sectionCross-section of a yard dry well. Water from a downspout or area drain flows through an inlet pipe into a gravel-filled pit set at least 10 feet out and downhill from the house. The pit is wrapped in filter fabric; an overflow near the top carries excess away; arrows show water soaking out through the bottom and sides into the surrounding soil.10 ft min, downhillInlet from downspoutOverflow#57 stonein fabricsoaks into soil
The well sits at least 10 ft out and downhill from the house. Roof or surface water enters through the inlet, is held in the stone voids, and soaks into the soil; an overflow handles anything the soil can't take fast enough.

What this doesn't tell you

This is a planning estimate. It sizes the well to hold a full design storm, using rough infiltration rates for each soil — your real soil, water table, and how much water infiltrates during the storm all shift the number. It can't confirm the ground below drains, that you're clear of the water table, or that the location is legal and far enough from the house. Do a percolation test and check local rules before you dig.

How to read your results

The tool gives you the water to store, then two ways to store it:

  • Gravel-filled pit. A hole packed with #57 stone. It's the cheapest and most common approach, but the stone fills most of the hole — water only occupies the roughly 40% gaps between stones — so the pit has to be about two and a half times the volume of the water it holds. That's why the gravel footprint is the larger of the two, and it's the figure most people get wrong.
  • Open chamber. A hollow, perforated dry-well unit stores water directly, so it needs a smaller hole for the same storm — you're trading digging and stone for the cost of the unit. Either way you still wrap it in fabric and bed it in some stone.

The drain-down check is the part a product seller won't give you: it estimates how long your soil takes to empty the well and flags when a dry well simply won't work.

The formula — check it yourself

Required storage is the runoff the well has to swallow:

gallons = area (sq ft) × rainfall (in) × 0.623

The 0.623 is the gallons in an inch of rain over a square foot — the same constant behind the roof runoff calculator, which is why you can size a well straight from a roof figure. Divide the gallons by 7.48 for cubic feet of water, then by about 0.4 for the size of a gravel pit (an open chamber skips that step). Multiply the pit's cubic yards by roughly 1.4 for tons of #57 stone.

Will a dry well even work here?

A dry well is a storage tank that empties by soaking into the ground, so it only works if the ground actually drains. The standard is that it should empty within 24 to 72 hours. Sandy and loamy soils usually clear that easily; tight clay often can't, and a dry well in clay just stays full and backs up. Dig a test hole, fill it with water, and time how far it drops in an hour — that's a rough percolation rate you can enter above. If the soil won't drain, the water needs a different plan: regrading to move it away, a drain to a lower daylight point, or a rain garden designed for slow soils. The French drain vs. dry well comparison walks through when each fits.

Placement & materials

Put the well at least 10 feet from the house and downhill, clear of utilities, septic fields, and property lines. Use clean, angular #57 stone — not pea gravel, which packs and leaves less void space. Wrap the stone in geotextile fabric so soil can't silt the voids, and add an overflow near the top so a storm that outpaces the soil has a safe path away rather than pooling at the surface. To work out how much water each downspout delivers, start with the roof runoff calculator.

Frequently asked questions

How big a dry well do I need?
Size it from the water it must hold: catchment area (sq ft) × storm rainfall (inches) × 0.623 = gallons. Because a gravel-filled well only stores water in its roughly 40% voids, divide the water volume by about 0.4 to get the pit size. One downspout off 250 square feet of roof in a 1-inch storm makes about 156 gallons — roughly a 4½ × 4½ × 3-foot stone pit.
How much gravel does a dry well need?
Enough to fill the whole pit, because the stone is the structure and the water lives in the gaps. A 4½ × 4½ × 3-foot pit is about 60 cubic feet — a little over 2 cubic yards, or roughly 3 tons of #57 clean crushed stone. The calculator gives the cubic yards and tons for your size.
How deep should a dry well be?
Usually 3 to 6 feet. Going deeper stores the same water in a smaller footprint, which helps on tight lots — but the bottom must stay above the seasonal high water table, or the well fills from below and never drains. If you hit the water table, go wider and shallower instead.
How far from the house should a dry well be?
At least 10 feet, and downhill. The whole point is to move roof and surface water away from the foundation; a well too close just re-saturates the soil you're trying to keep dry, and can push water back against the wall. Farther is better if the grade allows it.
Will a dry well work in clay soil?
Often not. A dry well only works if the soil can empty it between storms — the rule of thumb is 24 to 72 hours. Tight clay drains far too slowly, so the well stays full and backs up. Do a simple percolation test first; if the water won't clear in a few days, regrading, a drain to daylight, or a rain garden built for slow soils is the better answer.
How long should a dry well take to drain?
It should empty within about 24 to 72 hours so it's ready for the next storm. If water sits longer, the well is undersized for your soil or the soil simply doesn't drain — either make it larger or reconsider whether a dry well fits. The calculator estimates the drain-down time from your soil and size.
Does a dry well need filter fabric?
Yes. Wrap the stone (or the chamber) in geotextile fabric, against the soil on the bottom and all sides. Without it, fine soil particles wash into the voids and silt the well up, and a clogged dry well can't be cleaned out — it has to be dug up and rebuilt.

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