A dry well quantity is three numbers, not one. Gross pit volume is the hole you dug. Effective storage is the water space left between the stones. The stone order is the loose bulk stone that fills the hole, voids included.
This calculator keeps those three lines separate for one gravel-filled pit with a uniform cross-section. Enter the pit dimensions and the supplier-confirmed void ratio, and it returns gross volume, storage in gallons or litres, and the stone order. Use the dry well measurement guide to record the field dimensions, the calculation-method guide to check that a gravel pit is the right method for your structure, and the worked dry well example to check a full record.
How much water does a gravel-filled dry well hold?
It holds the gross pit volume multiplied by the stone void ratio. A 6 ft by 6 ft pit that is 5 ft deep holds 180 ft3 of space, but at a void ratio of 0.35 only 63 ft3 of that space is water, or 471.3 gallons. The other 117 ft3 is stone.
Record gross volume first, storage second, and the loose stone order last. Each stage has a different source and can change for a different reason.
| Result | Calculation | What you must confirm |
|---|---|---|
| Gross pit volume | Measured area × pit depth | Shape, dimensions, depth, and units |
| Effective storage | Gross pit volume × stone void ratio | Exact stone product and supplier-confirmed ratio |
| Stone order | Gross pit volume, rounded up only at the supplier increment | Selling unit, minimum order, and delivery increment |
This page covers a stormwater dry well: a pit that receives runoff and lets it soak into the ground. It does not cover a drilled water well, where the volume is a water column in a borehole. If your structure is a water well, this calculator gives the wrong answer.
Which measurements do you need?
You need the pit plan dimensions, the pit depth, and the stone void ratio for the exact product being placed. Measure in the units printed beside the fields and record where each dimension came from.
Length and width
Measure the usable inside limits of the pit at the depth where stone will sit. Excavation width can include working space, side slopes, or overbreak that stone never fills. Split an abrupt width change into a separate section.
Diameter
Measure a circular pit's full width through its centre. A radius is half the diameter, so entering a radius in the diameter field reduces the calculated area to one quarter of the intended circle.
Known area
Use known-area mode when a drawing, survey, or separate shape calculation already gives the pit area. Confirm whether that area includes openings, pipes, or chamber footprints that displace stone before entering it.
Pit depth
Enter the depth of the stone-filled pit, from the top of the stone zone to the pit bottom. A soil cap, chamber lid, or cover above the stone is not part of this depth. Take enough readings to represent the pit, and split the pit where depth changes clearly.
Stone void ratio
Enter the void ratio for the exact stone product and condition, confirmed by the supplier or the project record. Clean washed gravel is typically 0.30 to 0.40. That range is a typical range, not a project value, so the calculator leaves the field blank rather than prefilling a number that could be wrong for your stone. Crushed, dirty, or mixed stone can fall below the clean range, and the ratio choice alone can change storage by about one third on the same pit.
What formulas does the calculator use?
The calculator finds plan area, multiplies by pit depth, and then multiplies the gross volume by the void ratio. It retains full JavaScript precision during the calculation and displays up to 3 decimal places.
Rectangle area = length × width
Circle area = π × (diameter ÷ 2)²
Gross pit volume = area × pit depth
Effective storage = gross pit volume × stone void ratio
Stone order = ceiling(gross pit volume ÷ supplier increment) × supplier increment
When no supplier increment is entered, the stone order stays at the unrounded gross volume. An exact increment multiple stays unchanged.
| Displayed conversion | Relationship | Calculator use |
|---|---|---|
| Cubic feet to cubic yards | 27 ft³ = 1 yd³ | Imperial gross volume and stone order |
| Cubic feet to gallons | 1 ft³ = 7.48052 gal | Imperial storage equivalent |
| Cubic metres to litres | 1 m³ = 1,000 L | Metric gross and storage equivalents |
| Cubic yards to cubic metres | 1 yd³ = 0.764554857984 m³ | Checking a supplier unit conversion |
NIST Handbook 44 Appendix C supplies the cited volume relationships. Unit switching converts populated dimensions and the order increment, clears the displayed result, and requires a fresh calculation in the selected units.
Worked imperial example
A rectangular pit is 6 ft long, 6 ft wide, and 5 ft deep, filled with stone at a supplier-confirmed void ratio of 0.35.
- Area: 6 × 6 = 36 ft².
- Gross volume: 36 × 5 = 180 ft³.
- Gross volume in cubic yards: 180 ÷ 27 = 6.667 yd³.
- Effective storage: 180 × 0.35 = 63 ft³.
- Storage in gallons: 63 × 7.48052 = 471.3 gal.
Measured result: 180 ft³ gross, 63 ft³ or 471.3 gallons of storage. The stone order is the gross volume, 6.667 yd³ unrounded, or 7 yd³ when rounded up to a 1 yd³ supplier increment. Ordering 63 ft³ of stone would under-order by nearly two thirds, because that number is the water space, not the stone.
Worked metric example
A rectangular pit is 2.0 m long, 2.0 m wide, and 1.5 m deep, at a void ratio of 0.35.
- Area: 2.0 × 2.0 = 4 m².
- Gross volume: 4 × 1.5 = 6.0 m³.
- Effective storage: 6.0 × 0.35 = 2.1 m³.
- Storage in litres: 2.1 × 1,000 = 2,100 L.
Measured result: 6.0 m³ gross, 2.1 m³ or 2,100 litres of storage, and a 6.0 m³ loose stone order before any supplier increment rounding.
How much does the void ratio change the answer?
More than most pits allow for. Take the imperial pit above, 180 ft³ gross, and change only the ratio.
| Void ratio | Effective storage | Storage in gallons |
|---|---|---|
| 0.30 | 54 ft³ | 403.9 gal |
| 0.35 | 63 ft³ | 471.3 gal |
| 0.40 | 72 ft³ | 538.6 gal |
The stone order does not change in this table, because the order is the gross volume in every row. Only the storage changes. Record the ratio, its product, and its source beside the result so a later reviewer can see which assumption moved.
What does this calculator not do?
The calculator measures quantity for a pit you already have. It does not decide whether that pit is big enough, suitable, or legal for your site.
| Condition | Why one run does not fit | Quantity action |
|---|---|---|
| Prefabricated chamber | Capacity is a manufacturer-stated figure, not pit geometry | Use the stated capacity, plus surround voids from the dry well volume methods guide |
| Chamber in a gravel surround | Chamber body displaces stone | Add chamber capacity to surround gross × void ratio, with the chamber footprint deducted |
| Irregular, sloped-wall, or stepped pit | Cross-section is not constant | Split into uniform sections and add unrounded volumes |
| Pipe or fitting inside the pit | It displaces stone and storage space | Measure a separate deduction when the occupied length is material |
| Drainage sizing | Rainfall, contributing area, soil infiltration, and groundwater decide adequacy | Use a drainage design from the responsible designer or authority |
| Soil suitability, setbacks, water-table clearance, and permits | Local rules and site conditions govern | Check the project record and local requirements before digging |
A quantity result does not make an excavation safe. Locate and control underground installations before digging. In the United States, contact 811 a few business days before digging so utilities are marked.
Common dry well volume mistakes
The common dry well volume mistakes guide explains each of these errors with its project consequence.
- Using gross pit volume as the storage number.
- Ordering the storage volume as if it were the stone quantity.
- Using one fixed void ratio for every stone product and condition.
- Entering radius in the diameter field.
- Using one average depth across abrupt changes.
- Including a soil cap or cover depth in the stone pit depth.
- Computing a chamber from outside dimensions instead of its stated capacity.
- Rounding every section before adding the product total.
- Treating a quantity result as proof of drainage sizing, soil suitability, or excavation safety.
What should a dry well quantity record include?
- Project, pit, drawing, and revision identifiers.
- Shape, section limits, dimensions, depth readings, and units.
- Gross pit volume before any ratio is applied.
- Stone product, void ratio, ratio source, and date.
- Effective storage in cubic feet or cubic metres, plus gallons or litres.
- Selling unit, minimum order, increment, rounded order, and quote date.
- Separate chamber, pipe, surround, backfill, spoil, and restoration calculations.
- Design, utility, excavation, delivery, access, and unresolved checks.
Sources and scope
- NIST Handbook 44 (2026), Appendix C: cubic-foot, cubic-yard, gallon, litre, and cubic-metre relationships.
- Number Analytics dry well installation guide: gravel porosity typically 0.30 to 0.40; read 4 November 2026. Its sizing formula is design context only and is not used by this calculator.
- Evaluating drywells for stormwater management: chamber geometry determines total volume and maximum head for prefabricated chambers.
- 811 Before You Dig: free national utility-marking service; contact a few business days before digging.
Source scope: NIST supports the stated conversions. Number Analytics supports the typical void-ratio range only; the project value must come from the supplier or project record. PMC supports the chamber-geometry limitation. Project drawings, specifications, surveys, supplier records, soil and groundwater information, local rules, and responsible people control the product, depth, material state, order, and work plan.
Review note: Calculation logic and independent automated fixtures passed internal review. Saleem Sial is the research and editorial owner. Waseem Sial's external review remains ongoing. The published page is indexable.
