Dry Well Calculator
A dry well is a pit filled with clean stone that holds a storm's runoff while it soaks into the ground. Only the gaps between the stones hold water — about 40% of the pit for clean #57 stone — so the pit has to be about two and a half times the volume of the water. Enter the roof area draining to it, the storm depth and the pit depth for the water volume, the stone in cubic yards and tons, the pit size and the filter fabric to wrap it.
What you'll need
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How a dry well works
A dry well takes the water from a downspout, holds it in the gaps between clean stones, and lets it soak into the soil over the following hours. Only the voids hold water — about 40% of a pit of clean, uniformly graded stone such as #57, which is the design value stormwater manuals such as Pennsylvania's use — so the pit must be about two and a half times the volume of the storm it has to hold.
Worked example: 1,000 sq ft of roof, 1 inch storm, 3 ft deep
| Step | Working | Result |
|---|---|---|
| Runoff | 1,000 × 1 ÷ 12 | 83.3 ft³ = 623 gallons |
| Stone volume | 83.3 ÷ 0.40 | 208.3 ft³ = 7.72 yd³ |
| Stone weight | 7.72 × 1.4 | 10.8 tons of #57 |
| Pit footprint | 208.3 ÷ 3 | 69.4 sq ft, about 8.3 × 8.3 ft |
| Fabric | top + bottom + four sides | about 239 sq ft |
The runoff figure is the USGS one: an inch of rain on an acre is 27,154 gallons, or 0.623 gallons per square foot. Use the roof's footprint, not its sloped area — rain falls vertically.
Before you dig
- Do a percolation test: dig a hole to the pit depth, fill it with water, and time how fast it drains. Slow clay soil may need a bigger pit or a different solution.
- Wrap the stone in non-woven filter fabric so soil does not wash into the voids.
- Follow your local rules for setbacks from the house, property lines and septic systems, and call 811 before digging.
- Use only washed stone — the #57 stone calculator prices it for other jobs, and a French drain is the answer when water needs carrying away rather than holding.
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Frequently asked questions
- How big should a dry well be?
- Big enough that its voids hold the design storm. Water volume = drainage area × rainfall (in) ÷ 12; stone volume = water ÷ 0.40 for clean stone. A 1,000 sq ft roof in a 1 inch storm sheds 83.3 cubic feet (623 gallons), so the pit needs 208 cubic feet of stone — about 8.3 × 8.3 ft at 3 ft deep.
- How much gravel do I need for a dry well?
- Pit volume ÷ 27 for cubic yards, × 1.4 for tons of #57 stone. The 208 cubic foot example is 7.72 cubic yards, or about 10.8 tons. A prefabricated dry well chamber holds more water per cubic foot and needs far less stone around it.
- How many gallons of water come off my roof in a storm?
- About 0.623 gallons per square foot per inch of rain — the USGS figure of 27,154 gallons for one inch on an acre, divided by 43,560 sq ft. A 1,000 sq ft roof sheds about 623 gallons in a 1 inch storm; the sloped area does not matter, only the footprint.
- What gravel is best for a dry well?
- Clean, washed, uniformly graded stone such as #57 or #3. Stormwater design manuals such as Pennsylvania's use a 40% void ratio for this kind of stone. Never use crusher run or anything with fines — the fines fill the voids and the well stops working.
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