CalcStack — free, instant, no signup. The interactive calculator loads automatically.

French Drain Calculator

The burrito method, quantified.

French drains fail in three predictable ways: too little slope, the wrong gravel, or no fabric wrap. This calculator handles the material side completely — trench volume minus pipe displacement gives the true gravel tonnage in washed #57 stone, plus perforated pipe footage and the geotextile burrito wrap — and checks your available fall against the 1% gravity minimum before you dig.

Why people use this

Landscapers and drainage contractors use it to quote materials in one pass — gravel tonnage with pipe displacement already out, fabric for the burrito wrap, and the 1% slope check that decides whether the job needs a sump instead. Homeowners use it to learn why the "holes down" detail and the fabric wrap are the difference between a 30-year drain and a clogged trench.

How it works

1. Enter drain length and trench width × depth (12 in × 18–24 in is the residential standard).
2. Pick 4-inch or 6-inch perforated pipe — displacement is subtracted from the gravel.
3. Enter the fall available to your outlet for the slope check (minimum 1% = 1 inch per 8 feet).
4. Read gravel tons and yards, pipe footage, fabric square feet, and estimated material cost.

Common questions

How much gravel does a 50-foot French drain need?

A 12 in wide × 18 in deep trench is 75 cu ft gross; subtract the 4-inch pipe's displacement (about 4.4 cu ft) and you need roughly 70.6 cu ft — 2.6 cubic yards, or about 3.7 tons at 1.4 tons per yard. Round the order up; leftover stone always finds a use.

Do the pipe holes face up or down?

DOWN — this is the most-made mistake. A French drain collects subsurface water that rises up through the gravel from below; holes on the bottom let water enter and flow away. Holes facing up clog and defeat the purpose.

What slope does a French drain need?

A minimum of 1% — 1 inch of fall per 8 feet of run, so a 50-foot drain needs at least 6 inches of drop to its outlet. Two percent is better where the terrain allows. No fall at all means the water needs a sump basin and pump instead of a gravity outlet.

Browse all 500+ free calculators →