What This Solves
Sizes a dry well (seepage pit) for stormwater infiltration by calculating the required storage volume, infiltration capacity, and drawdown time.
Best Used When
- You need a vertical underground infiltration system for roof or driveway runoff
- You want to calculate the required dry well dimensions to infiltrate a design storm within a target time
- You are evaluating whether site soils can support a dry well system
Do NOT Use When
- You need a linear subsurface drain rather than a point infiltration system — Use French Drain Calculator
- You are designing a surface bioretention basin with vegetation — Use Rain Garden Calculator
- You need an underground detention system with a controlled outlet rather than infiltration — Use Underground Detention Calculator
Key Assumptions
- Infiltration occurs through the sidewalls and bottom of the dry well
- Soil infiltration rate is constant and does not decrease with saturation
- No groundwater mounding interferes with infiltration capacity
- The dry well structure (perforated ring, gravel-filled pit) provides the specified void volume
- Setbacks from foundations, property lines, and water sources are met
Input Quality Notes
Field percolation testing is essential — table-based infiltration rates can be off by an order of magnitude. Test at the actual depth of the proposed dry well bottom. Check for seasonal high groundwater.
Calculate Dry Well Design
For educational purposes only. Not a substitute for professional engineering judgment.
Dry Well Design Overview
Dry wells (infiltration pits) provide temporary storage and ground infiltration for stormwater runoff. They are commonly used for roof drainage, parking lots, and residential stormwater management.
- Storage Volume - Total volume available in backfill voids
- Infiltration Area - Bottom and sidewall area for water to infiltrate
- Drawdown Time - Time to drain stored volume (24-72 hours typical)
- Safety Factor - Applied to soil infiltration rate for conservative design
Soil Infiltration Rates
| Soil Type | Min (in/hr) | Typical (in/hr) | Max (in/hr) |
|---|---|---|---|
| Gravel | 4 | 8 | 20 |
| Sand | 2 | 4 | 8 |
| Loamy Sand | 1 | 2.4 | 4 |
| Sandy Loam | 0.5 | 1 | 2 |
| Loam | 0.25 | 0.5 | 1 |
| Silt Loam | 0.15 | 0.3 | 0.6 |
| Sandy Clay Loam | 0.1 | 0.2 | 0.4 |
| Clay Loam | 0.05 | 0.1 | 0.2 |
| Silty Clay Loam | 0.04 | 0.08 | 0.15 |
| Sandy Clay | 0.02 | 0.05 | 0.1 |
| Silty Clay | 0.01 | 0.03 | 0.06 |
| Clay | 0.005 | 0.02 | 0.05 |
Source: ASCE MOP 77 (2006), adapted from NRCS data
Backfill Void Ratios
| Backfill Type | Min | Typical | Max |
|---|---|---|---|
| Gravel (3/4" - 1.5") | 0.30 | 0.35 | 0.40 |
| Crushed Stone (#57) | 0.35 | 0.40 | 0.45 |
| Open-Graded Aggregate | 0.38 | 0.42 | 0.48 |
| Perforated Chambers/Crates | 0.90 | 0.95 | 0.97 |
Source: ASCE MOP 77 (2006), Design and Construction of Urban Stormwater Management Systems
Dry well sizing chart
Quick-reference dry well sizes for common roof and impervious areas. These figures assume a design storm that produces 1 inch of runoff from the connected impervious area — a widely used water-quality / first-flush depth for on-lot infiltration. The required stored volume is therefore:
Stored volume (cf) = impervious area (sq ft) × (1 in ÷ 12 in/ft) = area ÷ 12
| Impervious area | Runoff to store (1" storm) | ≈ Volume in gallons | Excavation if filled with #57 stone |
|---|---|---|---|
| 500 sq ft | 41.7 cf | ≈ 312 gal | ≈ 104 cf |
| 1,000 sq ft | 83.3 cf | ≈ 623 gal | ≈ 208 cf |
| 1,500 sq ft | 125 cf | ≈ 935 gal | ≈ 313 cf |
| 2,000 sq ft | 166.7 cf | ≈ 1,247 gal | ≈ 417 cf |
| 2,500 sq ft | 208.3 cf | ≈ 1,558 gal | ≈ 521 cf |
Runoff to store is the design volume to enter in the calculator above. Excavation assumes the pit is filled with clean crushed #57 stone at a typical void ratio of 0.40 (stored volume ÷ 0.40), so only the void space holds water. Using open plastic infiltration chambers (void ratio ≈ 0.95) cuts the required excavation by more than half. 1 cubic foot ≈ 7.48 US gallons.
These are planning estimates. Many jurisdictions require capturing more than 1 inch (or a specific design storm), and the well must also drain within 24–72 hours, which depends on soil infiltration rate and wetted area — use the calculator to confirm both the storage volume and the drawdown time for your soil. Source: void ratios and infiltration method per ASCE MOP 77 (2006).
How to size a dry well
Sizing a dry well is a two-step process:
- Find the runoff volume to store. Multiply the connected impervious area by the design runoff depth. Using a 1-inch storm, a 1,500 sq ft roof produces 1,500 ÷ 12 = 125 cubic feet (about 935 gallons) of runoff.
- Turn that into a physical well. If the excavation is filled with stone, only the voids hold water, so the hole must be bigger than the stored volume: excavated volume = stored volume ÷ void ratio. With #57 stone (≈ 0.40 voids), 125 ÷ 0.40 = ≈ 313 cubic feet of excavation. Pick a plan area that fits the site, then required depth = stored volume ÷ (void ratio × plan area).
The calculator above runs both steps and adds a drawdown-time check: it sizes the infiltration area from the soil rate (with a safety factor) and confirms the stored water drains within your maximum allowed time.
Frequently asked questions
What size dry well do I need?
Size the dry well to store the runoff from your design storm. A common rule for on-lot infiltration is to capture 1 inch of runoff from the connected impervious (roof or pavement) area. The required stored volume is simply: area (sq ft) divided by 12 = cubic feet. For example, a 1,000 sq ft roof needs about 83 cubic feet (roughly 625 gallons) of storage. If the well is filled with clean #57 stone (about 40% voids), you need about 2.5 times that as excavated volume — around 208 cubic feet for the same 1,000 sq ft roof. Always confirm the required design depth against your local stormwater code, since some jurisdictions require more than 1 inch.
How do you calculate dry well size?
There are two steps. First, find the runoff volume to store: V = impervious area x runoff depth. Using a 1 inch design depth, V (cubic feet) = area (sq ft) / 12. Second, convert that stored volume into a physical well. If you fill the excavation with stone, the usable storage is only the void space, so excavated volume = stored volume / void ratio (about 0.40 for clean #57 stone, or up to ~0.95 for open plastic infiltration chambers). The calculator above does both steps and also checks the drawdown time — the stored water should infiltrate within 24 to 72 hours so the well is empty before the next storm.
How deep and wide should a dry well be?
Pick a plan area (diameter or length x width) that fits your site, then solve for depth: required depth = stored volume / (void ratio x plan area). Most residential dry wells are 3 to 8 feet deep. Keep the bottom of the well at least 2 to 4 feet above the seasonal high groundwater table and bedrock, and set it back from foundations (typically 10+ feet) so infiltrating water drains away from the building rather than toward it.
How long should a dry well take to drain?
A dry well should fully drain (draw down) within 24 to 72 hours after a storm so its full storage is available for the next event and standing water does not become a mosquito or odor issue. Drawdown time = stored volume / infiltration capacity, where infiltration capacity depends on the soil rate and the wetted bottom and sidewall area. Sandy soils drain quickly; clay-rich soils may not infiltrate fast enough for a dry well at all, in which case a different practice (such as a storage-and-release detention system) is needed.
Dry well vs NDS Flo-Well — what is the difference?
A traditional dry well is an excavated pit filled with clean stone or gravel; only the void space between stones (about 30 to 40 percent) actually stores water, so it takes a large hole. An NDS Flo-Well (and similar plastic chamber/crate systems) is a hollow molded structure that is nearly all open space — roughly 90 to 95 percent void — so it stores far more water per cubic foot of excavation and is faster to install. In this calculator that difference is the void ratio: choose "Crushed Stone (#57)" (~0.40) for a stone-filled pit, or "Perforated Chambers/Crates" (~0.95) to model a Flo-Well-style product. The sizing target (the runoff volume to store) is the same; only the excavation size changes.
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Last verified: February 2026