French Drain Gravel Calculator: How Much to Order
A French drain gravel calculator estimates stone from trench length, width, and gravel depth, then adds a small waste allowance; for 4 in. pipe, many trenches use about 6 in. of gravel under the pipe and about 6 in. above it. Underordering leaves the pipe poorly supported, delays the job, and can force a second material run. We show the volume formula, pipe-clearance math, cover-depth choices, and how to convert cubic feet to cubic yards, tons, and bags.
How much gravel do I need for a French drain?
The gravel amount starts with trench volume, then pipe displacement if the drain uses pipe. A French drain gravel calculator uses trench length, width, and depth to find cubic feet, subtracts pipe volume when needed, and then adds waste. That gives an order quantity that fits the trench, not a rough guess.
Start with the trench dimensions
Enter trench length, width, and depth first. Those three numbers define the excavated volume. If the trench is 40 ft long, 1 ft wide, and 1 ft deep, the raw trench volume is about 40 cubic ft before any pipe is counted.
Know when the pipe changes the answer
A piped French drain needs gravel around and over the pipe, but the pipe itself takes up space. That means the gravel total is lower than the full trench volume. A pipeless gravel trench uses more stone because there is no pipe displacement to subtract.
Builder’s Calculator gives a standard French drain depth of about 12 inches.
How to calculate French drain gravel in cubic yards
Use trench volume in cubic feet, subtract pipe displacement when the drain has pipe, then divide by 27 to convert to cubic yards. That is the cleanest way to move from geometry to order quantity. The conversion matters because suppliers usually sell loose aggregate by cubic yard or ton.
Trench volume formula
Trench volume (cu ft) = length × width × depth. If the trench is 30 ft long, 12 in. wide, and 12 in. deep, convert inches to feet first: 12 in. = 1 ft. The volume is 30 × 1 × 1 = 30 cubic ft.
Subtract pipe displacement when pipe is included
Pipe size must be part of the estimate, not an afterthought. The calculator can accept pipe size options from 2 inches to 48 inches, and a custom outside diameter can also be entered. For a 4-inch perforated PVC pipe, the actual outside diameter is 4.215 inches, so the gravel space is smaller than the nominal pipe size suggests.
Convert cubic feet to cubic yards
Cubic yards = cubic feet ÷ 27. A 30 cubic ft estimate is a little over 1 cubic yd. If waste is added, multiply first and convert after. For example, 30 cubic ft × 1.10 = 33 cubic ft, and 33 ÷ 27 = 1.22 cubic yd.
Pipe size, slope, and gravel clearance
Piped French drains need enough gravel below the pipe, beside the pipe, and above it. The slope also matters because water has to move toward the outlet. Builder guidance commonly uses at least 1/8 in. per ft of slope, and the drain should run to daylight or to another outlet point.
Perforated pipe outside diameter and standard lengths
The calculator supports standard pipe lengths of 10, 20, 30, and 40 ft, and it also accepts a custom outside diameter. That matters because gravel clearance is based on the pipe’s actual outside size, not only the named diameter. A 4-inch pipe is not 4 inches outside to outside once fittings and wall thickness are counted.
How much gravel goes above a 4-inch perforated pipe?
For a 4-inch drain pipe, Builder’s Calculator says the setup allows about 5 inches of gravel above the pipe, and it also recommends 3 inches of top soil or grass above the drain. In practice, that means the gravel section should fill the trench around the pipe, then leave enough cover for settling and surface protection.
Minimum slope in plain terms
A French drain pipe should slope at least 1/8 in. per ft toward the outlet. That is a small drop, but it keeps water moving instead of sitting in the pipe. If the run ends at daylight, the outlet should stay open and lower than the trench invert so gravity can do the work.
| Project example | Trench size | Pipe | Gravel volume | Cubic yards | With 10% waste |
|---|---|---|---|---|---|
| Pipeless trench | 30 ft × 12 in. × 12 in. | None | 30.00 cu ft | 1.11 cu yd | 33.00 cu ft / 1.22 cu yd |
| Piped drain | 30 ft × 12 in. × 12 in. | 4 in. perforated pipe | Less pipe displacement | Lower than trench volume | Add 10% after pipe subtraction |
| Longer run | 50 ft × 12 in. × 12 in. | 4 in. perforated pipe | 50.00 cu ft before pipe | 1.85 cu yd before pipe | Order higher after waste |
What size gravel is best for a French drain?
Washed angular gravel is the default choice for most perforated-pipe French drains because it drains well and resists movement better than rounded stone. Pea gravel is usually a poor choice for pipe drainage because it can clog with silt more easily. Filter fabric helps, but it does not turn the wrong stone into the right one.
Washed angular gravel versus river rock
Washed angular gravel gives more void space and better lock than smooth, rounded stone. River rock can work in some drainage layouts, but it usually leaves less stable support around the pipe. For pipe-backed drains, a consistent washed aggregate is the safer estimating default.
Why pea gravel is usually a poor choice
Pea gravel has smaller, rounded pieces and can trap fines in the voids. Builder’s Calculator warns not to use pea gravel because it can clog with silt. That warning matters most where the trench sees fine soil, roof runoff, or slow drainage from clay ground.
How fabric changes clog control
Filter fabric or geotextile lines the trench and reduces soil migration into the stone. The calculator explicitly includes filter fabric needed, so fabric area becomes part of the order when the trench is lined. Fabric can reduce long-term clogging risk, but it does not replace good stone selection or proper slope.
French drain gravel worksheet and formulas
This worksheet separates piped and pipeless drains so the estimate matches the actual build. It also gives a waste allowance slider and a unit conversion path from cubic feet to cubic yards and tons. That prevents the common mistake of ordering one generic stone quantity for every trench.
Worksheet formulas
- Measure trench length, width, and depth.
- Convert inches to feet. Width in inches ÷ 12, depth in inches ÷ 12.
- Find trench volume. Length × width × depth.
- Subtract pipe displacement if the drain has pipe and the gravel space is reduced by the pipe outside diameter.
- Add waste. Multiply the gravel volume by 1.05 to 1.10, or more if the trench is irregular.
- Convert units. Cubic feet ÷ 27 = cubic yards.
- Convert to tons. Use the stone density assumed by the calculator for the selected gravel type.
Conversion table
| Unit | Formula | Example result |
|---|---|---|
| Cubic feet | Length × width × depth | 30 cu ft |
| Cubic yards | Cubic feet ÷ 27 | 1.11 cu yd |
| With 5% waste | 30 × 1.05 | 31.5 cu ft |
| With 10% waste | 30 × 1.10 | 33.0 cu ft |
How much waste should I add to my estimate?
Most French drain gravel estimates need 5% to 10% waste, with higher allowances for rocky soil, uneven trench walls, or hand-dug runs that vary in width. Short, straight trenches need less waste. Long runs, wet clay, and irregular excavations need more because the cut rarely stays perfectly uniform.
Waste allowance guide
| Project type | Suggested waste | Why |
|---|---|---|
| Short, straight trench | 5% | Predictable dimensions |
| Typical DIY run | 7% to 10% | Minor width changes and settling |
| Rocky or irregular trench | 10% to 15% | Extra fill from wall changes and cleanup |
How to round for delivery and pickup
Order a little above the math if the supplier sells in full cubic yards or by ton. A rounded-up delivery is cheaper than a second trip. For small jobs, bags can work, but the bag count should still come from the cubic-foot total after waste is added.
What changes the estimate: fabric, cover depth, and outlet choice
Fabric lining, cover depth, and outlet choice all affect how much stone and accessory material the job needs. A trench with soil cover and settling allowance needs more fill than one that stops at the pipe crown. A daylight outlet or catch basin can also change the gravel path and the total length to be filled.
Filter fabric and trench lining
If the trench is wrapped or lined, the fabric area should be added to the estimate. The gravel quantity itself may not change much, but the job total does because fabric needs to cover the trench bottom and sides. That is why a good calculator lists fabric separately instead of hiding it.
Soil or grass cover and settling allowance
Builder’s Calculator recommends 3 inches of top soil or grass above the drain. It also says a 4-inch drain pipe setup allows about 5 inches of gravel above the pipe. That cover depth matters because loose fill settles, and too little cover can expose the drain after rain or compaction.
Catch basin, daylight outlet, or no-pipe drainage runs
A catch basin usually means the run is collecting water at a low point, so the stone and fabric near the inlet may need extra attention. A daylight outlet needs a continuous slope to a visible discharge point. A no-pipe drainage run is a gravel trench only, so the full excavation volume is closer to the final order quantity.
How deep should a French drain trench be?
Builder’s Calculator gives a standard French drain depth of about 12 inches, with most installations using an 8-12 inch wide trench. That depth is enough for gravel around a typical pipe run plus soil cover above it. Deeper trenches may be needed where the outlet elevation, surface water, or soil conditions demand more room.
Can I build a French drain without a pipe?
Yes, a French drain can be built without a pipe when the goal is simply to move water through a gravel-filled trench. A pipeless trench uses more stone because there is no pipe displacement to subtract. It is best estimated as a full excavated gravel volume with waste added.
Frequently asked questions
How much gravel do I need for a French drain?
Use trench length, width, and depth to get cubic feet, then subtract pipe volume if a perforated pipe is included. Add 5% to 10% waste and convert to cubic yards by dividing by 27. That gives a realistic order size for a piped or pipeless drain.
How do I calculate French drain gravel in cubic yards?
Multiply length × width × depth in feet to get cubic feet, then divide by 27 for cubic yards. If pipe is part of the drain, subtract its outside diameter displacement first. Add waste before the conversion if the estimate needs a delivery cushion.
What size gravel is best for a French drain?
Washed angular gravel is the usual choice for perforated-pipe French drains because it drains well and holds shape better than rounded stone. Pea gravel is a weaker option for many installs because it can clog with silt. Fabric helps, but it does not replace the right stone.
Do I need to include pipe size when calculating French drain gravel?
Yes. Pipe outside diameter changes the amount of gravel space inside the trench, so it changes the order quantity. The calculator supports pipe sizes from 2 inches to 48 inches and also allows a custom outside diameter, which is important for accurate pipe displacement math.
How much gravel goes above a 4-inch French drain pipe?
Builder’s Calculator says a 4-inch perforated PVC pipe has an actual outside diameter of 4.215 inches, and the setup allows about 5 inches of gravel above the pipe. It also recommends 3 inches of top soil or grass above the drain to cover settling and protect the stone.
What slope does a French drain pipe need?
A French drain pipe should have at least 1/8 inch per foot of slope toward the outlet. That keeps water moving to daylight, a basin, or another discharge point. Without enough slope, water can sit in the pipe and reduce drainage performance.