French Drain Calculator
Estimate trench volume and gravel needed for a French drain.
French Drain Material Estimate
Multiply trench length, width and depth to estimate gross fill volume. Subtract pipe volume for a more detailed estimate.
How French Drain Gravel Volume Is Estimated
A French drain estimate uses trench length, width and gravel depth to calculate the volume of stone required. If a perforated pipe occupies part of the trench, its displacement can be considered separately for a more precise material estimate.
French Drain Example
A trench 50 ft long, 1 ft wide and 1.5 ft deep has 75 cubic feet of gross volume, or about 2.78 cubic yards before subtracting pipe volume or adding extra material.
Drainage Planning Factors
Trench slope, soil conditions, filter fabric, pipe diameter and the amount of gravel below and above the pipe all affect the final material requirement. Proper outlet design is just as important as the gravel quantity.
Frequently Asked Questions
What type of gravel is used?
Clean, washed angular or rounded stone is commonly used because fines can reduce drainage performance.
Should I include the pipe volume?
For small projects the difference may be modest, but subtracting pipe displacement improves accuracy.
Does a French drain need slope?
A suitable slope toward a safe outlet is generally important for effective drainage.
Calculation Assumptions and Practical Limits
French-drain gravel volume depends on trench dimensions minus any space occupied by pipe or other components. Compaction, irregular excavation, settlement and waste can increase material ordered beyond the geometric volume.