Gravity Fall Engineering
French Drain Slope & Elevation Drop Calculator
Find total fall, drop per 10 feet and the outlet depth you need, then verify your yard's natural grade from line-level or laser readings.
Pitch calculator
Units
Pitch standard
Total fall
6.00in
Drop per 10 ft
1.20in
Outlet depth
24.0in
Outlet depth assumes flat ground. On a sloped yard, subtract the natural drop below.
Elevation helper (line level / laser)
Measure from a level string or laser line down to grade. A larger reading at B means the ground drops toward B.
Natural drop
6.00in
Natural slope
1.00%
People Also Ask
What is the minimum slope for a French drain?
The minimum recommended French drain slope is 1 percent, which equals 1/8 inch of drop per linear foot or 1.2 inches per 10 feet. Half a percent can work for smooth PVC over short runs, but 1 to 2 percent prevents silt buildup and standing water.
How much fall does a 100 foot French drain need?
At the standard 1 percent pitch, a 100-foot French drain needs 12 inches of total fall from inlet to outlet. At 0.5 percent it needs 6 inches, and at 2 percent it needs 24 inches. On flat ground that fall must come from deepening the trench.
Can a French drain be level?
A level French drain will still collect water but cannot move it, so the trench stays saturated and silts up over time. Always establish gravity fall of at least 1/8 inch per foot toward a daylight outlet, dry well or approved storm connection.
How do I check yard slope with a line level?
Stake a string between point A and point B, level it with a line level, then measure from the string down to the ground at each stake. Subtract the A reading from the B reading to get the natural drop, then divide by the distance to get slope.