French Drain Calculator
Calculate Trench Gravel, Drain Pipe & Costs
Tired of wet spots or standing water in your lawn? A French drain is an effective, time-tested drainage solution. This calculator helps you determine exactly how much crushed stone, perforated pipe, and landscape filter fabric you need for a successful install.
What Is a French Drain and Why Does It Matter?
Excess rainwater and poor yard drainage can cause muddy lawns, basement leaks, and damage to structural foundations. A French drain is a classic, effective, and low-maintenance gravity drainage system designed to redirect surface water and groundwater safely away from your home.
The system consists of a sloped trench filled with clean gravel (usually 3/4-inch round washed stone) and a perforated drain pipe wrapped in a water-permeable filter fabric envelope. The fabric is crucial because it lets water in while blocking fine clay and soil particles from clogging the gravel voids over time.
For residential drainage, trenches are typically dug 12 inches wide and 18 to 24 inches deep, with a minimum slope of 1% (a 1-foot drop per 100 feet of run). Water flows through the soil, into the gravel bed, and is collected by the perforated pipe, discharging at a safe low point, bioswale, or dry well.
French Drain Trench & Gravel Sizing Guide
| Trench Width x Depth | Gravel Per 100 Linear Ft | Pipe Size Recommend | Best Application |
|---|---|---|---|
| 12 x 18 inches | 5.2 tons (3.7 cu yd) | 4-inch Perforated | Standard lawn and garden bed runoff |
| 12 x 24 inches | 7.2 tons (5.1 cu yd) | 4-inch Perforated | Near patio boundaries and driveway edges |
| 18 x 24 inches | 11.1 tons (7.9 cu yd) | 6-inch Perforated | Heavy slope bases and home foundation footing protection |
| 8 x 12 inches | 2.1 tons (1.5 cu yd) | 3-inch Perforated | Shallow planter boxes and landscape drainage |
How to Use This French Drain Calculator
Enter the total length of the drain trench, the desired width and depth in inches, the price of washed gravel per ton, and the unit cost of perforated pipe. Click Calculate to immediately get quantities and material cost estimations.
Frequently Asked Questions
Which direction should the pipe holes face?
The holes in the perforated pipe should face downward. Water rises from the bottom of the trench into the pipe and is carried away; facing them up allows sediment and soil to fall in easily.
What type of gravel is best for a French drain?
Washed gravel or crushed stone (3/4-inch to 1.5-inch size, ASTM C33 standard) is ideal. Do not use gravel with fines or sand mixed in, as it will compact and block water flow.
How much slope does a French drain need?
A French drain needs a minimum slope of 1% (1 inch of drop for every 8 feet of run, or 1 foot drop per 100 feet) to ensure water drains by gravity.
- USDA Natural Resources Conservation Service - Drainage design standards and groundwater control.
- ASTM International (ASTM) - Specification C33 for concrete aggregates and drainage washed stone.