A dry well is either a pit filled with gravel, a prefabricated chamber set in the ground, or a chamber surrounded by gravel. Each structure needs its own volume method, and the wrong method gives a storage number that can be off by a factor of two or three.
Which dry well volume method should I use?
Match the method to the structure. There are three cases, and each one measures something different.
| Structure | Method | What you measure |
|---|---|---|
| Gravel-filled pit | Gross pit volume × void ratio | Pit dimensions and the stone void ratio |
| Prefabricated chamber | Manufacturer stated capacity, plus stone-surround voids | Chamber capacity and surround dimensions |
| Chamber in a gravel surround | Chamber capacity + (surround gross × void ratio) | All three: capacity, surround, ratio |
The void ratio is the share of the pit that stays open water space once the stone is placed. Clean washed gravel typically falls between 0.30 and 0.40; the supplier or the project record sets the real value for the delivered stone.
How do you calculate a gravel-pit dry well?
Storage = length × width × depth × void ratio.
Fixture: a 6 × 6 × 6 ft pit with a stated void ratio of 0.35. Gross volume = 6 × 6 × 6 = 216 ft³. Storage = 216 × 0.35 = 75.6 ft³. Convert to gallons: 75.6 × 7.48052 = 565.5 gal.
The 565.5 gal figure is the water the pit can hold. The 216 ft³ gross figure is the hole, not the storage; using it as the storage number overstates capacity by nearly three times. The mistakes guide shows how often that error shows up on real orders.
How do you calculate a prefabricated chamber dry well?
Start from the manufacturer's stated water capacity, then add the voids in the stone surround. Use the stated capacity rather than computing it from the chamber's outside dimensions; wall thickness and internal structure change the real number.
Fixture: a chamber with a stated capacity of 50 gal, set in a 4 × 4 × 4 ft stone surround with a void ratio of 0.35. Surround gross = 4 × 4 × 4 = 64 ft³. Surround storage = 64 × 0.35 = 22.4 ft³. Convert: 22.4 × 7.48052 = 167.6 gal. Total storage = 50 + 167.6 = 217.6 gal.
The chamber body displaces stone, so the surround is the pit minus the chamber footprint, not the full pit. On small chambers the difference is minor; record it anyway so the record stays consistent.
How much does the void ratio change the answer?
More than most people expect. Take the same 6 × 6 × 6 ft pit from the first fixture:
- At a void ratio of 0.30: 216 × 0.30 = 64.8 ft³ = 484.7 gal.
- At a void ratio of 0.40: 216 × 0.40 = 86.4 ft³ = 646.3 gal.
That is a 161.6 gal swing from one ratio assumption. The ratio is not a free choice; confirm it with the stone supplier or the project record before the number goes into the order.
What if the pit is round or irregular?
Use the cylinder formula for a round pit: volume = π × radius² × depth, then multiply by the void ratio as before.
Fixture: a round pit 1.5 m in diameter and 2.0 m deep, void ratio 0.35. Gross = π × 0.75² × 2.0 = 3.534 m³. Storage = 3.534 × 0.35 = 1.237 m³ = 1,237 L.
For an irregular pit, take several tape readings along each side and average them, then use the rectangular method. Record the readings so the average can be checked later. The measuring guide gives the field procedure.
What do these methods not cover?
These methods measure quantity. Sizing the well for a design storm is a design task and belongs to a qualified professional with the local rainfall data and soil information.
Pipe displacement is usually small next to the pit volume; subtract it when the inlet pipe run inside the pit is long. Call 811 a few business days before digging so utilities are marked.
Where do the worked numbers come from?
A full end-to-end worked example with reconciled imperial and metric fixtures is in the dry well volume worked example.
Sources and source scope
- Number Analytics, dry well installation guide: effective storage = gross volume × void ratio; gravel porosity typically 0.30 to 0.40; reviewed 2026-10-18.
- Evaluating drywells for stormwater management and enhanced aquifer recharge (PMC): chamber geometry, including sedimentation chamber, overflow pipe, and gravel-packed bottom chamber, determines total volume and maximum head; accessed 2026-10-18.
- NIST Handbook 44, 2026 Appendix C: exact conversion constants used in the fixtures (1 ft³ = 7.48052 gal; 1 yd³ = 27 ft³; 1 m³ = 1,000 L).
- 811 Before You Dig: the free national before-you-dig service; contact a few business days before digging and wait for utility marking.
Scope: every dimension, depth, and void ratio in the fixtures is a stated fixture used to demonstrate the arithmetic; none is a stone, depth, or sizing recommendation. The 0.30 to 0.40 void range is a typical range for clean washed gravel, not a project value. The governing drawings, survey and measurement method, supplier certificate, soil information, contract provisions, applicable law, and responsible qualified people control the accepted project quantity.
Review responsibility: Saleem Sial owns the published calculation, source, and editorial checks. Waseem Sial, External Reviewer, 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. For stone and soil decisions, see the dry well volume guide. 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.
