Dry Well Size Calculator
Roof and soil to dry well storage verdict.
What it does
Downspouts dump shocking volumes: a thousand square feet under one inch sheds over six hundred gallons in an hour. Dry wells swallow that surge into stone voids, then bleed it to soil over hours. Both sides need sizing.
The tool above multiplies roof by storm for runoff gallons, pit volume by 40 % voids for storage, and runoff over bottom-area infiltration for drain hours. Adequate means storage covers runoff and draining finishes inside 72 hours.
University stormwater guides with extension publications teach this same volume-and-perc method with clean-stone void ratios and mosquito-safe drain limits. Clays, high water tables, and tiny lots fail the math before digging starts.
Site it ten feet from foundations with overflow daylighted somewhere safe. Pretreat roof grit with filters, keep septics and wells clear per code, and enlarge until both storage and drain time pass.
How to use
- Enter roof footprint in sq ft.
- Enter design storm in inches.
- Enter soil perc rate.
- Enter pit length, width, depth.
- Read runoff, storage, and drain time.
- Enlarge until the verdict reads adequate.
Formula
- roof
- Roof footprint draining to the pit, sq ft.
- storm
- Design storm depth in inches.
- perc
- Soil perc rate in inches per hour.
- L, W, D
- Pit dimensions in feet.
Stone voids assumed 40 % for clean fill. Drain time divides runoff by bottom-area infiltration (area times perc rate times 0.623). Adequate needs storage covering runoff with drain inside 72 hours.
Worked example
Example: 1000 sq ft, 1 in storm
1000 sq ft roof, 1 inch storm, 8x8x4 ft pit in 1 in/hr soil.
- Roof is 1000 sq ft, storm drops 1 in.
- Runoff: 1000 x 1 x 0.623 = 623 gal.
- Pit 8 x 8 x 4 ft at 40 % voids: 256 x 7.48 x 0.4 = 766 gal.
- Storage covers runoff with margin.
- Drain: 623 / (64 x 1 x 0.623) = 15.6 hr: adequate.
Reference table
| Soil | Perc rate | Verdict |
|---|---|---|
| Sand | Over 8 in/hr | Excellent |
| Sandy loam | 1 to 8 in/hr | Good |
| Silt loam | 0.5 to 1 in/hr | Marginal |
| Clay | Under 0.5 in/hr | Pick another practice |
Limits
- Planning math at 40 % voids and stated perc.
- Real perc tests beat guesses.
- Setbacks and codes rule placement.
- Clays and high water fail dry wells.
Common mistakes
- Undersizing storage versus runoff.
- Guessing perc instead of testing.
- Ignoring the 72-hour mosquito limit.
- Placing near foundations or septics.
- Skipping pretreatment filters.
Try it yourself
Quick checks. Answers sit tucked under each question.
Same storm, 6x6x3 pit in 0.25 in/hr soil?
Runoff is still 623 gal. Storage is 108 x 7.48 x 0.4 = 323 gal. Drain is 623 / (36 x 0.25 x 0.623) = 111.1 hr: enlarge.
2 in storm, 10x10x5 pit in 2 in/hr soil?
Runoff is 1000 x 2 x 0.623 = 1,246 gal. Storage is 500 x 7.48 x 0.4 = 1,496 gal. Drain is 10 hr: adequate.
Questions
What is a dry well?
A buried stone pit that catches roof water and bleeds it into soil. Runoff fills voids between stones; percolation empties them. Sizing balances both sides.
How much water does stone hold?
About 40 % voids for clean washed stone, the standard planning figure. Mixed or dirty stone holds less; this page assumes clean fill.
How fast must it drain?
Within 72 hours to stop mosquitoes and keep capacity ready. Sandy soils drain overnight; clays may never qualify for dry wells.
How much runs off a roof?
Roof area times inches times 0.623 gallons. A 1000 sq ft roof under 1 inch sheds 623 gallons, which surprises first-time builders.
What if the soil is clay?
Bigger pit, pretreatment filter, or a different practice for tight clays, high water tables, and small lots. Dry wells fail gracefully into puddles when undersized.
Where can I put one?
Ten feet from foundations, away from septics and wells per local code, with overflow daylighted safely. Setbacks protect structures first.