Dry Well Installation Guide: How to Size, Dig and Install a Dry Well

By the Landscape Drainage Proz team, an authorized NDS dealer. Product specs from NDS technical documents. Last updated September 24, 2026.

A dry well is an underground chamber that collects runoff from downspouts, drains and low spots, holds it, and lets it soak slowly into the soil. To size one, work out how much water a storm sends it: one inch of rain on 1,000 sq ft of roof is about 623 gallons. Then install it with its top about 8 in. down and its bottom at least 2 ft above the water table.

This guide covers when a dry well works, the sizing math, how deep to dig, where it can go, and each installation step. Product specs come from NDS, the manufacturer of the Flo-Well dry well we sell.

Downspout piped underground to an NDS Flo-Well dry well surrounded by drain rock, with water soaking into the soil — A downspout piped to a Flo-Well. The unit holds the water while it seeps out through the side holes into the surrounding rock and soil.

What Is a Dry Well?

Rain from a roof, driveway or yard drain flows through a pipe into the dry well. The well fills, then empties over hours or days as water seeps through its sides and bottom into the soil. Unlike a French drain, it does not need a downhill outlet. It stores the water where it is and returns it to the ground.

The old way is a pit filled with stone. Modern plastic dry wells such as the NDS Flo-Well are open chambers, so almost all of the space holds water.

Is a Dry Well Right for Your Yard?

A dry well is a good fit when:

Downspouts dump water next to the foundation and there is no downhill place to send it
A French drain or catch basin needs an outlet and the yard is flat
Your town wants roof runoff kept on your property

It is a poor fit in heavy clay, over shallow bedrock or where the water table is high, because the water has nowhere to soak in.

Quick soil test: dig a hole to the depth the dry well will sit, fill it with water and let it drain to soak the soil. Fill it again and time it. Many homeowner guides say a dry well only belongs in soil that drains within 24 hours. If you have no test result, Penn State's dry well guide says to assume a slow 0.5 in. per hour.

How to Size a Dry Well

A dry well has to hold the runoff from one storm while it soaks in. Many stormwater rules size for the first 1 in. of rain. Your town may use a different number.

Runoff (gallons) = roof or paved area (sq ft) x rainfall (in.) / 12 x 7.48

Area drained	Runoff, 1 in. rain	Flo-Wells alone*	Flo-Wells in rock**	Gravel pit***
250 sq ft	156 gal	4	1	1.9 cu yd
500 sq ft	312 gal	7	2	3.9 cu yd
1,000 sq ft	623 gal	13	4	7.7 cu yd
1,500 sq ft	935 gal	19	6	11.6 cu yd
2,000 sq ft	1,247 gal	25	8	15.4 cu yd

*50 gal per Flo-Well. **NDS says one Flo-Well in a 4 ft wide x 4 ft deep hole backfilled with drain rock holds about 165.5 gal, because the rock adds up to 40% void space. ***Stone needed for a gravel-filled pit at 40% voids. All figures ignore water soaking out during the storm, so they are conservative. Your soil, backfill and local design storm change the answer. Use the NDS Flo-Well calculator for a site-specific count.

Example: a downspout drains 500 sq ft of roof. One inch of rain sends about 312 gallons. Two Flo-Wells set in drain rock (about 331 gal) cover it. The gravel-only version would need about 3.9 cubic yards of stone.

Last, check the drain time. The well should empty within 72 hours so it is ready for the next storm and does not breed mosquitoes. Slow soil means you need a wider footprint (more units side by side), not just a deeper hole.

How Deep and Where to Put It

Rule	Guideline	Source
Depth to the top of the unit	At least 8 in. of cover (rated for light vehicles, loads up to 175 psi, with a bottom panel); up to 8 ft deep	NDS Flo-Well FAQ
Bottom of the dry well	At least 2 ft above the seasonal high water table or bedrock	Penn State PHRC
Distance from foundations	10 ft recommended; some cities allow less for small plastic units	Penn State PHRC, Oregon State
Spacing between Flo-Wells	At least 18 in.	NDS Flo-Well FAQ
Stacking	No more than 4 high, with a bottom panel and a center support pipe	NDS Flo-Well FAQ
Drain time	Empty within 72 hours	Penn State PHRC

Keep dry wells away from septic systems and drinking-water wells too. Distances vary widely, so ask your town or county before you choose a spot. Place the well downhill of the house when you can.

How to Install a Dry Well

Call 811 and check permits. Get utility lines marked and confirm local rules and setbacks.
Dig the hole. For one Flo-Well, NDS uses a hole about 4 ft across and 3-1/2 to 4 ft deep. Keep the bottom level and do not compact it. Compacted soil drains slower.
Add the rock base. Line the hole with non-woven filter fabric, then place 6 to 11 in. of 3/4 to 1 in. drain rock in the bottom.
Assemble the Flo-Well. Snap the 3 side panels together. Punch out the 1 in. weep holes with a hammer. NDS says to remove all 30 when the unit is wrapped in fabric.
Wrap and set it. Wrap the unit in filter fabric so rock and soil stay out, and set it on the rock base. Add the bottom panel if it will be under light traffic or stacked.
Connect the inlet. Run solid pipe from the downspout, catch basin or French drain into the unit, sloped toward the well. Attach the cover, or the 6 in. surface inlet grate if the well also collects surface water.
Backfill. Surround the unit with about 1 ft of drain rock. Where there is no traffic, NDS says EZ-Flow bundles can wrap the unit instead for extra storage. Fold fabric over the rock, then add about 8 in. of soil and sod.
Add an overflow. Give big storms somewhere to go, such as a pop-up emitter lower than the inlet. A gutter guard or leaf screen keeps debris out of the well.

Download the Flo-Well installation manual (PDF) and the Flo-Well FAQ (PDF) from NDS.

How Much Does a Dry Well Cost?

Here is what the dry well units cost at our current prices (September 2026). Each kit includes 3 side panels and a cover.

Flo-Well kits	Storage, unit only	Delivered price
1	50 gal	$155.87
2	100 gal	$311.74
4	200 gal	$623.48
18	900 gal	$2,085.66 (free shipping)

Each kit is $115.87 plus $40 shipping; orders over $2,000 ship free. Filter fabric, drain rock, pipe, the bottom panel and the surface inlet are extra, as is labor. Ordering 20 or more kits for a subdivision or commercial site? Request a quote with your ZIP code.

Shop the Flo-Well Dry Well Or get a written quote

Dry Well vs. French Drain

	French drain	Dry well
What it does	Collects water from wet ground and moves it	Stores water and lets it soak in
Needs a lower outlet?	Yes	No
Shape	Long, shallow trench	One deep hole
Best for	Soggy lawns, foundations, slopes	Downspouts and flat yards with no outlet
NDS product	EZ-Flow	Flo-Well

They work well together. A French drain or catch basin collects the water, and the dry well is where it ends up. See our French drain installation guide.

Common Dry Well Mistakes

No soil test. In clay the well fills and stays full.
Too close to the house. Water soaking in next to the foundation can end up in the basement.
Undersizing. One 50 gal unit on a 1,000 sq ft roof overflows in the first inch of rain.
Skipping the fabric. Soil washes into the rock and the unit, and the well slowly clogs.
No overflow. Every dry well eventually meets a storm bigger than it is.
Stacking too high. NDS recommends no more than 4 stacked units and prefers connecting them side by side.

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Frequently Asked Questions

Is a dry well a good idea?

It is when water has nowhere downhill to go and your soil drains. A dry well works best in sandy or loamy soil that soaks up water within about a day. In heavy clay, or where the water table is high, it fills up and stays full.

How deep do you dig a dry well?

For one NDS Flo-Well, a hole about 4 ft across and 3-1/2 to 4 ft deep, per NDS. That leaves 6 to 11 in. of drain rock under the unit, about 1 ft of rock around it and 8 in. of soil on top. Keep the bottom at least 2 ft above the seasonal high water table or bedrock.

How big should a dry well be?

Big enough to hold the runoff from your design storm. Roof area (sq ft) times rainfall (in.) divided by 12 gives cubic feet; multiply by 7.48 for gallons. One inch of rain on 1,000 sq ft of roof is about 623 gallons. NDS also has a free Flo-Well calculator that accounts for soil and backfill.

Should a dry well be filled with gravel?

A traditional dry well is a pit full of stone, and only about 40% of that pit holds water. A plastic dry well like the NDS Flo-Well is open inside, which NDS says gives 250% more storage than a gravel dry well the same size. Drain rock goes around the outside, not inside.

Can you run a French drain to a dry well?

Yes. It is one of the most common setups when a yard has no downhill outlet. The French drain collects water across the yard and carries it to the dry well, which stores it while it soaks in.

How much does a dry well cost?

An NDS Flo-Well kit is $115.87 at our prices and holds about 50 gallons. Shipping is $40 per kit, and orders over $2,000 ship free. Add drain rock, filter fabric, pipe and labor. Most homes need more than one unit per downspout area.

Do you need a permit for a dry well?

Often, yes. The EPA treats stormwater dry wells as Class V injection wells, and many states and towns register or permit them and set distances from foundations, septic systems and wells. Check with your town or county before you dig.

How far should a dry well be from the house?

Penn State's Housing Research Center recommends 10 ft from building foundations to reduce the risk of basement seepage. Some cities allow less for small plastic units. Your local code decides.