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Size a gravel-filled dry well for a downspout: storm volume, stone in yards and tons, pit size and fabric.
With these inputs:
| Result | Value |
|---|---|
| Stone for the dry well | 7.72 yd³ (10.8 tons) |
| Runoff to hold | 623 gal (83.3 ft³) |
| Pit footprint | 69 sq ft — about 8.3 × 8.3 ft |
| Filter fabric to wrap it | 239 sq ft |
| Drainage area | 1,000 sq ft |
| Storm depth | 1 in |
| Void space in clean #57 stone | 40% |
| Pit volume (stone) | 208.3 ft³ |
| Pit depth | 3 ft |
| Stone density | 1.4 tons per yd³ (#57) |
Test how fast the soil drains before you dig: a dry well in clay can stay full for days. Wrap the stone in non-woven filter fabric so soil does not clog the voids, and follow your local rules on setbacks from foundations, property lines and septic systems.
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.
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.
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.
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.
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.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.