Dry Well Sizing: Start With Capture Volume

Storage is only half of the problem

A dry well needs temporary storage for the runoff you intend to capture, but surrounding soil must also accept that water. A large underground void in poorly infiltrating soil can behave like a buried pond.

Step 1: define the contributing area

Only count surfaces that actually drain to the dry well. A roof branch serving one downspout may represent a fraction of the total roof, not the whole house. For roof catchment, use the horizontal projected/plan area draining to that branch rather than the sloped roof-surface area.

Step 2: choose the storm and capture target

A system does not automatically need to hold every drop from every storm. The relevant target may be set by local rules, an overflow strategy or a homeowner's planning goal.

Step 3: separate water volume from excavation volume

Stone-filled pits store water in voids between aggregate. The tool therefore divides target water volume by the assumed void fraction to show a gross stone-zone volume. Manufactured chambers use their own verified storage specifications.

Step 4: verify the soil instead of guessing it

A textural label such as “loam” or “clay” is not a substitute for a field observation at the proposed infiltration location. Use repeated drain-down readings to calculate an observed rate, then check whether your local stormwater authority requires a different formal method, safety factor or seasonal test.

Checks that can invalidate the storage math

• Poor infiltration or a restrictive soil layer.
• High seasonal groundwater or shallow bedrock.
• Insufficient setbacks from foundations, wells, septic systems or property lines.
• No safe overflow path when the feature fills.
• Buried utilities or local restrictions on infiltration.