Understanding the Key Parameters
Total Length of the Drain (feet)
This is the horizontal distance covered by the drain, rather than the distance measured along the sloping pipe.
Slope (inches per foot)
Slope describes how much the drain drops vertically for every foot of horizontal distance. For example, a slope of 1/4 inch per foot means the drain drops 1/4 inch over each horizontal foot.
Choose the slope according to your drainage system’s installation guidance and site conditions. The slopes used below are calculation examples, not universal recommendations.
Total Drop (inches)
Total drop is the difference in elevation between the beginning and end of the drain. For drainage in the intended direction, the outlet end must be lower than the starting point.
How to Calculate French Drain Slope
If you know the horizontal length and measured elevation drop, divide the drop in inches by the length in feet:
Slope (inches per foot)=Horizontal Length (feet)Total Drop (inches)
If you already know the desired slope, follow these steps to determine the required total drop.
Step 1: Formula to Calculate Total Drop
Multiply the horizontal drain length by the desired slope:
Total Drop (inches)=Total Length (feet)×Desired Slope (inches per foot)
Step 2: Example Calculation
Suppose your French drain is 60 feet long, and you want a slope of 1/4 inch per foot.
Total Drop=60×41=15 inches
The end of the drain should therefore be 15 inches lower than the starting point.
Step 3: Adjusting for Different Slopes
For the same 60-foot drain with a slope of 1/2 inch per foot:
Total Drop=60×21=30 inches
In this example, the drain’s end would be 30 inches lower than the starting point. Check that the available elevation difference and outlet position can accommodate the planned drop.
FAQs
1. What is French drain slope, and why is it important?
French drain slope is the downward incline along the drain toward its outlet. It helps water move by gravity. Low spots or sections that slope in the wrong direction can retain water, but slope alone does not guarantee effective drainage; the outlet and installation also matter.
2. What slope should a French drain have?
Follow the guidance for your selected drainage system and site conditions. For example, NDS recommends a minimum 1% slope for its EZflow French drain system. This equals approximately 0.12 inch of drop per foot. A slope of 1/4 inch per foot is approximately 2.08%, so these values are not interchangeable. NDS EZflow installation guidance
3. Is total drop the same as slope?
No. Total drop is the overall elevation difference between the drain’s ends. Slope describes that drop relative to the horizontal distance. Two drains can have the same slope but different total drops if their lengths differ.
4. Does this calculation guarantee that my French drain will work?
No. It calculates the geometric relationship between length, slope, and drop. Actual drainage also depends on soil conditions, pipe capacity, filter materials, installation quality, and a suitable discharge point.
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