French drain sizing logic
Runoff gallons come from drainage area, rainfall depth, and runoff factor. Trench storage equals trench volume x stone void space, and gravel order adds waste on top of the stone volume.
RainYard Tools
Trench sizing
French Drain Calculator
Estimate runoff, trench storage, gravel, and pipe length for a French drain run.
Runoff to manage635 galSelected storm volume from the drained area.
Stone storage539 galOnly the void space counts as temporary storage.
Storage balance-97 galIncrease length, depth, storage, or overflow capacity.
Gravel order7.33 cu ydPipe length is roughly the trench run.
Example
A 1,200 sq ft problem area in a 1 inch storm with 85% runoff creates about 636 gallons. A 60 ft trench that is 1.5 ft wide, 2 ft deep, and 40% void space stores about 539 gallons, so it still needs more storage or overflow help.
Why this page is different from the gravel calculator
The gravel page answers how much stone to buy. This page answers whether the trench size is even close to the storm volume before you price gravel and pipe.
Storage alone is not the whole design
A French drain still depends on a safe discharge point, clean stone, filter fabric where needed, and a layout that does not collect roof water against the house.
Questions
Common checks.
Can a French drain solve basement water?
Sometimes it helps intercept surface water, but basement water can also come from grading, waterproofing, footing drains, or groundwater pressure.
What void space should I use?
Open-graded clean stone is often screened around 35-45% void space. If the stone is dirty or mixed, storage drops.
What if the storage balance is negative?
Add trench length, add depth where safe, split the flow, or route part of the water to another practice such as a swale, basin, or dry well.
Helpful referencesCall 811 before diggingEPA National Stormwater Calculator