Dry Well Calculator
Last updated: June 2026 · Free · No sign-up required
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.
Quick reference
| Area | Rainfall | Volume |
|---|---|---|
| 500 sq ft | 1 in | ≈ 42 ft³ |
| 1,000 sq ft | 1.5 in | 125 ft³ |
| 2,000 sq ft | 2 in | ≈ 333 ft³ |
functions Shows the working, not just the answer
For students and teachers, the final number is only half the value. Every math tool here exposes the formula it applied, the intermediate steps, and the rounding rule, so you can follow along, check your homework, or use the answer in a proof or report with confidence.
calculate Accurate to the spec
Calculations use 64-bit floating point with sensible rounding for the domain (currency to 2 decimals, percentages to 4 decimals, algebra to 6 significant figures). Where exact rational arithmetic matters — fractions, factorials, simplification — we use a dedicated BigNumber path so 1/3 + 1/6 returns ½, not 0.49999.
school Free for classroom use
Educators are welcome to link to any math calculator on CalcaTools from a class site, Google Classroom, or worksheet. The pages are mobile-friendly, free, ad-supported (so we can keep them free) and have no sign-up wall — students just click and use them in class or at home.
tips_and_updates Pair with the spoke articles
Below the calculator we link a small set of plain-English explainer pages — "What is a percentage?", "Why does PEMDAS matter?", and so on. They cover the underlying concept in 4–6 short paragraphs. Read those before the calculator if the topic is new, or after if you want the extra context.
Interpretation guide
| Output | What it tells you |
|---|---|
| Required storage | Runoff volume the dry well must hold, in cubic feet. |
| Gallons | The same volume in US gallons (× 7.48). |
| Cubic yards | Useful for sizing the gravel pit (÷ 27). |
Formula & methodology
Formula: Required volume (ft³) = drainage area (sq ft) × rainfall (inches) ÷ 12
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.
- Enter the drainage or roof area in square feet.
- Enter the design rainfall depth in inches.
- The calculator multiplies area × (rainfall ÷ 12) for cubic feet.
- 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.