What Is the French Drain Calculator?
The French Drain Calculator is a free online tool designed for contractors and DIY builders who need quick, accurate calculations in the construction and building space. By entering your drain length, trench width, trench depth, you get instant results including gravel needed, pipe length, landscape fabric. No formulas to memorize, no spreadsheets to build — just enter your numbers and get the answer in seconds. Whether you're a beginner or experienced professional, this calculator saves you time and eliminates guesswork.
Why This Calculation Matters
Getting gravel needed right can make the difference between success and costly mistakes. In construction and building, small errors compound quickly. Manual calculations are error-prone and time-consuming, especially under pressure. This calculator applies proven formulas used by contractors and DIY builders worldwide, giving you confidence that your numbers are correct. Use it to plan your projects with precision and avoid common pitfalls that trip up beginners.
When Should You Use This Calculator?
This tool is most useful when you know your drain length and need to find the right gravel needed. It's also great for quick estimates before committing to a decision, and to double-check manual calculations or professional quotes, and when comparing different scenarios side by side. Bookmark this page and come back whenever you need a fast, reliable answer — the calculator is always free and requires no signup.
French Drain Calculator
Estimate materials for French drain installation.
French Drain Materials by Length
12" wide × 18" deep trench, 4" pipe
| Drain Length | Gravel (tons) | Pipe (ft) | Fabric (sq ft) | Est. Cost |
|---|---|---|---|---|
| 25 ft | 0.9 | 25 | 88 | $41 |
| 50 ft | 1.8 | 50 | 175 | $81 |
| 75 ft | 2.6 | 75 | 263 | $117 |
| 100 ft | 3.5 | 100 | 350 | $158 |
| 150 ft | 5.3 | 150 | 525 | $239 |
| 200 ft | 7.0 | 200 | 700 | $315 |
How to Use This Calculator
- Enter Your Drain Length (feet): Start by entering your drain length — this is the primary input for the calculation.
- Fill In Additional Details: Complete the remaining fields: trench width, trench depth, pipe diameter, gravel price. Each value refines the calculation for greater accuracy.
- Click Calculate: Hit the Calculate button to run the numbers. Results appear instantly below.
- Review Your Results: Check your gravel needed, pipe length, landscape fabric. Use these figures to inform your next decision or compare against alternative scenarios.
How It Works
This calculator estimates French drain materials by computing trench volume, then calculating gravel fill (minus pipe displacement), fabric liner requirements, and perforated pipe length.
The basic rule:
- Trench volume (cu ft) = length × width × depth (all in feet)
- Gravel volume = trench volume minus pipe volume (pipe displaces ~10% of trench)
- Gravel weight: washed drainage gravel weighs approximately 1.4 tons per cubic yard
- Landscape fabric: enough to line the bottom and sides with 6" overlap at seams
- Minimum drain slope: 1% grade (1/8" per foot, or 1" per 8 feet)
Use clean, washed gravel (3/4" to 1.5" stone) — not crushed limestone or pea gravel, which can clog. The perforated pipe should sit on 2-3 inches of gravel, holes down (if using rigid pipe) or surrounded by gravel. Wrap the entire gravel bed in landscape fabric to prevent silt infiltration.
Tips & Considerations
- Double-check your drain length before calculating — even small input errors can significantly change your results.
- Run the calculator with different values to compare scenarios and find the optimal approach for your situation.
- Pay attention to both gravel needed and pipe length — they work together to give you the full picture.
- Bookmark this page for quick access next time you need to plan your projects.
- If you're unsure about your gravel price, start with a conservative estimate and adjust from there.
Frequently Asked Questions
How deep should a French drain be?
Residential French drains are typically 18-24 inches deep. For footing drains, go to the depth of the footing (often 36-48 inches). The drain must be deep enough to be below the water problem — water flows down to the drain by gravity. Minimum practical depth is 12 inches.
What size gravel for a French drain?
Use 3/4" to 1.5" clean, washed gravel (river rock or granite). Avoid fine gravel, pea gravel (too small, clogs), crushed limestone (produces fines), or recycled concrete (clogs over time). The gravel should have void space for water to flow through.
How much does a French drain cost per foot?
DIY French drains cost $8-$15 per linear foot for materials. Professional installation runs $20-$50 per foot for exterior drains, or $40-$100+ per foot for interior basement drains (due to concrete cutting and waterproofing). A 50-foot DIY drain costs $400-$750 in materials.
Does a French drain need landscape fabric?
Yes, always use non-woven geotextile landscape fabric. It prevents soil from migrating into the gravel and clogging the drain over time. Line the entire trench before adding gravel, then fold the fabric over the top before backfilling with soil.
Should perforated pipe holes face up or down?
This is debated, but for most residential applications, holes face DOWN. Water rises into the pipe from below as the gravel bed saturates. The pipe then carries water away by gravity slope. Some manufacturers recommend holes up for intercepting surface water.
Where should a French drain discharge?
French drains should discharge to: a storm drain, a dry well, a rain garden, daylight (lower point on property), or a sump pump pit. Never discharge onto a neighbor's property or into a sanitary sewer. Check local codes for allowed discharge points.