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Dry Well Calculator | Estimate volume from area & rainfall

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

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

Free math calculator — enter your numbers and get an instant, accurate result.

info Private by design

Everything runs locally in your browser. No uploads, no accounts, no tracking.

info Works everywhere

Fully responsive and mobile-friendly — calculate on any device, any time.

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

Let's say you are using the Dry Well Calculator. Calculates dry well using Required volume (ft³) = drainage area (sq ft) × rainfall (inches) ÷ 12, live in your browser. Enter the values that match your situation into the input fields and press calculate — using realistic numbers makes the result directly useful for you.

Result: The calculator instantly applies the formula Required volume (ft³) = drainage area (sq ft) × rainfall (inches) ÷ 12 and returns the result with appropriate precision.

What this means: Read the result in the context of what you are measuring. The step-by-step breakdown lets you confirm the math and understand which input most affects the outcome.

Formula & methodology

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

The Dry Well Calculator is built on a well-established calculation method. It uses the formula Required volume (ft³) = drainage area (sq ft) × rainfall (inches) ÷ 12 to turn your inputs into a reliable result. Calculates dry well using Required volume (ft³) = drainage area (sq ft) × rainfall (inches) ÷ 12, live in your browser. The steps are shown on the page so you can follow the reasoning from input to output.

A dry well stores stormwater runoff so it can soak into the ground. Its required volume is the catchment area times the design rainfall depth.

  1. Enter the drainage or roof area in square feet.
  2. Enter the design rainfall depth in inches.
  3. The calculator multiplies area × (rainfall ÷ 12) for cubic feet.
  4. It converts to gallons and cubic yards to help size the pit.

Worked example: 1,000 sq ft of roof with 1.5 in of rain produces 125 ft³ (about 935 gallons) of runoff.

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 does the Dry Well Calculator do?
Calculates dry well using Required volume (ft³) = drainage area (sq ft) × rainfall (inches) ÷ 12, live in your browser. The calculator takes your inputs, applies the standard calculation, and returns a clear result so you can make an informed decision without doing the math by hand.
What formula does the Dry Well Calculator use?
It sizes the storage basin with Required volume (ft³) = drainage area (sq ft) × rainfall (inches) ÷ 12. Enter the roof or driveway area that drains into the well and your local storm rainfall, and the tool returns the storage volume needed. For example, a 1,000 sq ft roof with a 1-inch rain event needs about 83 cubic feet of dry-well capacity.
Is the Dry Well Calculator free and private?
Yes — it is free with no sign-up or limits, and all calculations run in your browser so no site data is uploaded. CalcaTools funds itself with display ads. The tool sizes dry wells from drainage area and rainfall; verify local drainage codes and soil percolation before construction.
How accurate is the Dry Well Calculator?
The Dry Well Calculator applies the standard formula: Required volume (ft³) = drainage area (sq ft) × rainfall (inches) ÷ 12. It is tested against published worked examples before launch and uses native double-precision arithmetic, so results are accurate to typical precision for the values you enter — with no intermediate rounding that could distort the answer.

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