A dry well estimate needs four numbers: the pit's length, width, and depth, plus the void ratio of the fill. If the dry well is a prefabricated tank or chamber, record the manufacturer's stated storage capacity instead of calculating a pit. Take the measurements on the ground before ordering material.
What measurements does a dry well need?
Measure the structure itself, not the design storm. The runoff calculation that sizes a dry well is a separate step; this page covers measuring the pit or tank so the storage volume is correct.
| Measurement | How to take it | What it controls |
|---|---|---|
| Pit length | Tape the longest inside dimension at the bottom of the excavation | Gross pit volume |
| Pit width | Tape across the pit at right angles to the length | Gross pit volume |
| Pit depth | Read from finished grade to the pit floor | Gross pit volume and fall check |
| Pit diameter | Tape across round pits at two perpendicular points and average | Gross pit volume for cylindrical pits |
| Tank outside dimensions and stated capacity | Read the product label; measure the tank to confirm the model | Storage volume of prefabricated systems |
| Fill void ratio | Use the project record or supplier data for the specific stone | Effective storage of gravel-filled pits |
| Inlet pipe outside diameter | Tape or calipers around the pipe body | Displacement inside the pit |
How do you measure a gravel-filled pit?
Calculate the gross pit volume first, then multiply by the void ratio of the fill. The spaces between the stones hold the water, so a gravel pit stores only a fraction of its excavated volume.
Gross pit volume (rectangular) = length × width × depth
Gross pit volume (cylinder) = pi × (diameter ÷ 2)² × depth
Effective storage = gross pit volume × void ratio of the fill
A void ratio of 0.30 to 0.40 is a typical range for clean washed gravel, not a project value. Confirm the actual ratio from the project record or the aggregate supplier, because crushed stone with more fines holds less water than open-graded washed stone.
Worked imperial example
A rectangular pit is 6 ft long, 6 ft wide, and 6 ft deep, filled with clean washed stone at a 0.35 void ratio.
- Gross pit volume: 6 × 6 × 6 = 216 ft³.
- Effective storage: 216 × 0.35 = 75.6 ft³.
- Convert: 75.6 × 7.48052 = 565.5 gal.
Result: the pit stores about 565.5 gallons. The 0.35 void ratio is a worked-example assumption; use the supplier's value for the stone actually delivered.
Worked metric example
A cylindrical pit is 1.5 m in diameter and 2.0 m deep with a 0.35 void ratio.
- Gross pit volume: 3.1416 × (1.5 ÷ 2)² × 2.0 = 3.534 m³.
- Effective storage: 3.534 × 0.35 = 1.237 m³.
- Convert: 1.237 × 1,000 = 1,237 L.
How do you measure a tank or chamber system?
For a prefabricated dry well tank or chamber, use the manufacturer's stated storage capacity. Measure the tank's outside length, width, and height and confirm they match the labelled model before using the stated figure.
Where a tank sits inside a stone surround, the surround's voids add storage. Calculate the stone envelope volume minus the tank's outside volume, multiply by the void ratio, and add the tank's stated capacity.
Subtract the inlet pipe's displacement when a long run of pipe sits inside the pit. A 4 in perforated pipe over a 10 ft run displaces 3.1416 × (4 ÷ 12 ÷ 2)² × 10 = 0.87 ft³, which is small but worth removing on tight quantity records.
What mistakes change the quantity?
- Counting the gross pit volume as the storage volume and skipping the void ratio entirely.
- Applying one universal void ratio to every stone; crushed, dusty, or dirty stone has a lower ratio than washed open-graded stone.
- Measuring nominal tank dimensions instead of the stated capacity and the actual model on site.
- Taping sloughed or collapsed pit walls as the finished dimension instead of the excavated record.
- Forgetting that bulk stone is sold with its voids already present; do not shrink the stone order by the void percentage. The bank and loose volume guide keeps material states separate.
- Mixing the design-sizing step (runoff area, rainfall, percolation rate) with the structure measurement. 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. Excavation carries cave-in and utility-strike hazards; keep spoil and equipment away from the pit edge and take measurements from a safe position using the project's approved method.
This page covers measuring the dry well structure. Hydraulic sizing, percolation testing, setback rules, permits, and discharge design follow the project documents, the governing authority, and qualified people.
Sources and source scope
- Number Analytics, Ultimate Dry Well Installation Guide: dry well storage relation V = Q × t / n with gravel porosity typically 0.30 to 0.40.
- Alpha Environmental, Residential Dry Well Tank Systems: percolation test role, runoff volume from impervious area, and setback context.
- NIST Handbook 44, 2026 Appendix C: exact cubic-foot to gallon, cubic-meter to liter, and cubic-yard relationships.
- 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 formulas measure stated pit, tank, and pipe quantities. The 0.30 to 0.40 gravel void range is labelled as typical, not universal; the project record or aggregate supplier controls the value used for ordering. 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.
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.
