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Dry Well Calculator | Estimate volume from area & rainfall

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Size a dry well for stormwater from your drainage area and rainfall, returning the required volume in cubic feet and gallons with the gravel void space accounted for. The practical tool for rain gardens and drainage projects.

Last updated: June 2026 · Free · No sign-up required

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Results

Enter values above and click Calculate to see your result instantly.

How the Dry Well Calculator works

The engine behind this calculator evaluates Required volume (ft³) = drainage area (sq ft) × rainfall (inches) ÷ 12 directly in your browser — nothing is sent to a server, and results update as you type.

The reference table below covers the most common dry well calculator cases at a glance, the methodology section breaks the calculation into verifiable steps, and the FAQ tackles the edge cases.

From our guides

Quick reference

AreaRainfallVolume
500 sq ft1 in≈ 42 ft³
1,000 sq ft1.5 in125 ft³
2,000 sq ft2 in≈ 333 ft³

info Dry Well Calculator

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info Educational

Includes the formula and step-by-step explanation so you understand the math, not just the answer.

Interpretation guide

OutputWhat it tells you
Required storageRunoff volume the dry well must hold, in cubic feet.
GallonsThe same volume in US gallons (× 7.48).
Cubic yardsUseful for sizing the gravel pit (÷ 27).

lightbulb Worked example — sizing a dry well for a 1,500 sq ft roof

A homeowner routes a 1,500 sq ft roof into a dry well. The local stormwater guide uses a 1.25-inch design storm, and the well will be filled with clean crushed stone, which has about 40% void space for water.

Result: Runoff volume = 1,500 × (1.25 ÷ 12) = 1,500 × 0.10417 = 156.25 ft³. In gallons: 156.25 × 7.48 = 1,169 gal. With 40% voids, the stone-filled pit must be 156.25 ÷ 0.40 = 390.6 ft³ — for example a 6 × 6 ft pit about 10.9 ft deep, or a wider, shallower equivalent.

What this means: The well must hold 156 ft³ of water, but because gravel occupies 60% of the pit, the excavation is roughly 2.5× larger at 391 ft³. This is the step most DIY sizing skips — an unfilled 156 ft³ pit looks right on paper but holds far too little once the stone goes in.

Formula & methodology

Formula: Required volume (ft³) = drainage area (sq ft) × rainfall (inches) ÷ 12

How a dry well is sized

A dry well must temporarily store the runoff from the design storm, then let it percolate into the soil. The required water volume is the drainage area times the rainfall depth.

  • Measure the drainage (roof or paved) area in square feet.
  • Choose the design rainfall in inches — local stormwater guidance usually specifies the 10-year or 100-year storm.
  • Convert inches to feet (÷ 12) and multiply: volume = area × (rainfall ÷ 12) in cubic feet.
  • Convert to gallons (× 7.48) and divide by the stone’s void ratio (~0.40) for the actual pit size.

Gravel void space: clean crushed stone only leaves about 40% of the pit as voids that can hold water, so the excavated pit is roughly 2.5× the water volume. Example: a 1,000 sq ft roof with 1 inch of rain → 83.3 ft³ of runoff ≈ 623 gallons.

Authoritative source: Wolfram MathWorld

Frequently asked questions

How do I size a dry well?
Enter the required values into the input fields and press the calculate button. The tool applies the formula Required volume (ft³) = drainage area (sq ft) × rainfall (inches) ÷ 12 and returns the result with a short explanation of each step, so you can check the working yourself.
What rainfall figure should I use?
The Dry Well Calculator is free, private, and accurate: it runs entirely in your browser (no uploads, no accounts), applies the standard calculation, and explains each step so you can verify the result. Calculates dry well using Required volume (ft³) = drainage area (sq ft) × rainfall (inches) ÷ 12, live in your browser. There is no limit on usage, and it works on any device.
How many gallons is the runoff?
Multiply the drainage volume in cubic feet by 7.48 to get US gallons: the formula gives cubic feet = drainage area × rainfall ÷ 12, then gallons = cubic feet × 7.48. A 1,000 sq ft roof with 1 inch of rain produces 83.3 cubic feet, about 623 gallons. The calculator shows both units so you can size the well against the storm volume.
Does the gravel reduce the size needed?
Yes — gravel fill inside the dry well only occupies about 60–70% of the pit volume (the void space holds water), so the required pit size is larger than the water volume alone. A dry well's effective storage is the pit volume × void ratio of the gravel. The calculator accounts for the fill type so you dig the right pit, not a pit that overflows.
Is it free and private to use?
Yes — the Dry Well Calculator. There is no sign-up, no paywall, no trial, and no limit on how many calculations you can run. CalcaTools covers its costs with non-intrusive display advertising, so the calculator itself never asks for payment or restricts any feature.
What rainfall depth should I design for?
Use your local stormwater authority’s design storm, often the 1-inch water-quality storm or the 10-year 24-hour storm for the area. A 1-inch storm on a 1,500 sq ft roof makes 125 ft³ of runoff; designing only for the average rain leaves the well undersized for the storms that actually cause flooding.
Why does gravel make the required pit so much bigger?
Because the stone itself occupies most of the pit — only the gaps between stones hold water. Clean crushed stone has about 40% void space, so a well that must store 156 ft³ of water needs a 391 ft³ excavation. Using rounded river rock or compacted fill lowers the void ratio further and demands an even bigger pit.
How do I convert the result to gallons or cubic yards?
Multiply cubic feet by 7.48 for US gallons (156.25 ft³ ≈ 1,169 gal) and divide by 27 for cubic yards (390.6 ft³ ≈ 14.5 yd³). Gallons help you compare against rain-barrel capacity; cubic yards are what an excavator or stone supplier will quote.
Do I need an overflow for large storms?
Yes — size the well for the design storm, but route anything beyond it safely away from the house with an overflow pipe or surface swale. A well with no overflow becomes a pressurised underground pond in a big storm, and the water will find the path of least resistance, which is often the basement.

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