French Drain Gravel Calculator
Calculate the exact amount of drainage gravel, washed stone, or river rock needed to fill your French drain trench. Enter trench length, width, and depth to get volume and tonnage estimates instantly.
💧 French Drain Cross-Section
Washed Stone FillHow Much Gravel Do I Need for a French Drain?
French drain gravel requirements depend on the trench length, width, and depth. Most residential French drains use 1–1.5 cubic yards of gravel per 10 linear feet of trench.
Measure Your Trench
Measure the total length of your drain trench. For complex systems with multiple branches, calculate each run separately and add them together.
Set Trench Width & Depth
Enter your trench width (typically 1–2 ft) and depth (typically 1–2 ft for residential drains). Deeper trenches require more gravel but can handle greater water volume.
Choose Washed Drainage Stone
Select River Rock, Pea Gravel, or Crushed Stone — any clean, washed stone that allows water to flow freely and doesn't clog. Avoid materials with fines (dust).
Calculate Total Volume
The calculator gives you the total cubic yards and tons of gravel needed to fill your trench. This accounts for the full trench volume.
Add 10% Buffer
Always order 10% more to account for settlement, spillage, and any over-excavation. French drain gravel should slightly mound above grade before compaction.
French Drain Volume Formula
A French drain trench is filled with clean, washed gravel around a perforated pipe. The gravel volume is calculated from the trench dimensions (length × width × depth). Since the pipe occupies a small fraction of the volume, we recommend the full trench fill calculation.
Types of French Drain Gravel
Click any application card below to highlight specific specs and project guidelines.
#57 Washed Stone
3/4" clean, washed crushed stone is the gold standard for French drains. It offers ideal void space for water flow and resists clogging.
Pea Gravel
Smooth, 3/8" pea gravel drains well and is easy to work with. Use it in lighter-duty residential drains where flow rates are modest.
River Rock (1"–2")
Larger river rock provides excellent drainage capacity for high-volume situations but costs more per ton. Ideal around foundation walls.
#2 Crushed Stone
Coarser 1.5"–2" crushed stone is used in large commercial French drains and detention basin outflows where maximum flow rate is needed.
French Drain Installation Tips
Interactive guidelines to ensure a flawless installation and avoid common planning mistakes.
Always Use Filter Fabric
Wrap the entire trench with landscape filter fabric before filling with gravel. This prevents fine soil particles from migrating into the gravel and clogging the drain over time.
Maintain Proper Slope
Your trench must slope at least 1% (1 inch drop per 8 feet) to ensure water flows toward the outlet. Use a laser level or water level to verify.
Cap the Perforated Pipe
Cover the perforated pipe inlet with filter sock (a mesh sleeve) to prevent gravel from entering and blocking the pipe perforations over time.
Mark Underground Utilities
Call 811 (USA) before digging to have underground utilities marked. French drains are typically installed at 18–36 inch depths, well within the range of buried lines.
French Drain Gravel Calculator — FAQs
Find answers to common questions regarding calculations, material selection, and installation.
Q1 How much gravel do I need for a French drain?
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For a typical 50-foot French drain with a 12-inch wide × 18-inch deep trench: Volume = 50 ft × 1 ft × 1.5 ft = 75 ft³ = 2.78 cubic yards ≈ 3.9 tons. Most residential French drains use 1.0–1.5 cubic yards of drainage stone per 10 linear feet of trench.
Q2 What size gravel is best for a French drain?
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#57 washed stone (3/4" clean crushed stone) is the industry-preferred choice for French drains. It provides large voids for water flow while being too large to pass through filter fabric. Pea gravel (3/8") also works for lighter applications, as does 1"–2" river rock for high-volume drainage.
Q3 How deep should a French drain be?
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Residential French drains are typically 18–36 inches deep. Surface drainage drains (catching runoff) need only 12–18 inches. Foundation drains should extend below the footing level — usually 24–36 inches. Commercial applications may go 4–6 feet deep. Slope the bottom of the trench at least 1% toward the outlet.
Q4 How wide should a French drain be?
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A standard residential French drain trench is 12–18 inches wide. Narrow 6-inch wide drains exist for tight spaces but have limited capacity. Commercial or high-volume drains may be 24–36 inches wide. Wider trenches require significantly more gravel but can handle greater water flow.
Q5 How long does gravel in a French drain last?
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A properly installed French drain with filter fabric can last 30–40 years before needing maintenance. Without filter fabric, the gravel voids fill with soil fines within 5–10 years, requiring the drain to be excavated and the gravel replaced. Annual inspection of the inlet and outlet is recommended.
Q6 Do I need a perforated pipe in a French drain?
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Not always. A traditional French drain uses only gravel-filled trenches to redirect water — no pipe needed. Adding a 4-inch perforated PVC or corrugated pipe increases the flow capacity dramatically and is recommended for foundation drainage, areas with heavy clay soil, or any situation with significant water volume to manage.