A dry well's storage comes from two things you cannot see from the surface: the voids in the stone fill and the soil's ability to drain. Check both before ordering material, because the wrong stone or the wrong soil shrinks the real storage by a third or more.
How do material and soil change a dry well's volume?
Effective storage equals the pit's gross volume multiplied by the fill's void ratio, then reduced by anything that blocks drainage. Clean washed gravel stores 30 to 40 percent of the pit volume; crushed or dirty stone stores less. Tight clay or a high water table slows how fast water leaves, so the pit must hold the water longer. Both facts change the pit size you dig and the stone you order.
Which stone holds the most water?
Clean, washed, open-graded stone holds the most water. A void ratio of 0.30 to 0.40 is typical for clean washed gravel, not a project value. Crushed stone with fines or dirty aggregate falls below that range because the small particles fill the spaces between the stones.
Ask the aggregate supplier for the void ratio of the specific product delivered. Do not treat one number as universal; the ratio sets the storage quantity, and the result moves with it.
| Stone | Typical void ratio | Storage in a 216 ft3 pit |
|---|---|---|
| Clean washed gravel | 0.30 to 0.40 | 64.8 to 86.4 ft3 |
| Crushed stone with fines | Varies; ask the supplier | Lower than clean washed gravel |
Why line the pit with geotextile?
Line the pit sides with geotextile fabric before placing the aggregate. The fabric stops fine soil particles from migrating into the stone voids, which would clog the reservoir and reduce storage over its life. The fabric adds no storage; it protects the storage the stone provides.
How much does the void ratio change the storage?
A rectangular pit measures 6 ft long, 6 ft wide, and 6 ft deep: 6 × 6 × 6 = 216 ft3.
- At a 0.30 void ratio: 216 × 0.30 = 64.8 ft3; 64.8 × 7.48052 = 484.7 gal.
- At a 0.40 void ratio: 216 × 0.40 = 86.4 ft3; 86.4 × 7.48052 = 646.3 gal.
Result: the same pit stores 484.7 or 646.3 gallons depending on the stone. The void ratio is a worked-example input here; use the supplier's value for the stone actually delivered.
How does soil drainage change the design?
The soil's drainage rate decides how long water stays in the pit. Run a percolation test at the pit location: dig a hole 12 in deep and 12 in wide, fill it completely, let it drain fully to saturate the ground, then refill and measure how far the level drops in one hour. A rate of 0.5 in per hour is acceptable and 1 in per hour is excellent; if the water takes more than 24 hours to drain, the spot is unsuitable for a dry well.
Keep the pit bottom at least 4 ft above the seasonal high water table, bedrock, or hardpan. In slow-draining soil, the well must be deeper or wider to act as a reservoir while the ground absorbs the water. Sizing the well for a storm is a separate design step for qualified people. These thresholds are commonly recommended; the local building code and the project documents govern the accepted values.
What mistakes change the quantity?
- Applying one universal void ratio to every stone instead of the supplier's value for the delivered product.
- Counting the gross pit volume as the storage volume and skipping the void ratio entirely.
- Ordering dirty or fine-rich stone while assuming clean-gravel storage.
- Shrinking the stone order by the void percentage. Bulk stone ships with its voids included; order the gross pit volume in loose stone. The bank and loose volume guide keeps material states separate.
- Skipping the percolation test or placing the pit near the water table, which leaves the storage unused.
- Mixing the storm-sizing step (runoff area, rainfall, percolation rate) with the structure quantity. Sizing follows the project documents and a qualified person.
Safety and professional scope
Contact 811 a few business days before digging and wait for buried utilities to be marked. Keep spoil and equipment away from the pit edge and take measurements from a safe position. Placement setbacks are commonly 10 ft from building foundations, 25 ft from septic systems, and 50 to 100 ft from water supply wells; local building codes govern the exact distances.
This page covers material and soil quantity considerations. Hydraulic sizing, design percolation testing, permits, and discharge design follow the project documents, the governing authority, and qualified people.
Sources and source scope
- Engineer Fix, How Deep Should a Dry Well Be for Proper Drainage?: percolation test method and rate thresholds, 4 ft water table clearance, geotextile practice, and setback distances; reviewed 2026-10-17.
- Number Analytics, Ultimate Dry Well Installation Guide: dry well storage relation V = Q × t / n with gravel porosity typically 0.30 to 0.40.
- NIST Handbook 44, 2026 Appendix C: exact cubic-foot to gallon relationship.
- 811 Before You Dig: the free national before-you-dig service; contact a few business days before digging and wait for utility marking.
Scope: the 0.30 to 0.40 void-ratio range is typical for clean washed gravel, not universal; the supplier's value for the delivered product controls the quantity. Perc rates, clearances, and setbacks are commonly recommended; the governing drawings, manufacturer data, soil information, contract provisions, safety plan, applicable law, and responsible qualified people control the accepted dimensions and final project quantity.
Review responsibility: Saleem Sial owns the published calculation, source, and editorial checks. Waseem Sial, External Reviewer and Engineer, remains listed for ongoing external review; no completed external-review date is claimed.
For the measuring steps, see How to Measure for a Dry Well Volume. Next, return to the Earthwork & Drainage Calculators hub for the drainage tool set.
Use the Dry Well Volume Calculator to run the gross volume, storage, and stone order for your pit dimensions.
