French Drain Gravel Calculator

Background.

Most of a French drain is rock. The perforated pipe gets the attention, but by volume it is a small part of the job: in an 80-foot trench 12 inches wide holding 18 inches of gravel, a 4-inch pipe takes up about 7 of the 120 cubic feet, under 6 percent. Everything else is washed stone, and the practical question before you dig is how much of it to order, in the unit your supplier actually sells.

The geometry is a long box with a cylinder running through it. Length times width times gravel depth gives the trench volume. Taking out the pipe leaves the space the stone has to fill, and a waste allowance covers what real trenches do: wander wider than planned, dip deeper around roots, and swallow stone into soft ground. The result is then expressed three ways. Landscape yards sell by the cubic yard, quarries and delivery trucks usually sell by weight, and home centres sell bags, so the calculator gives cubic yards, tons and a whole-bag count, plus a material cost if you enter a price.

Those units are not interchangeable. Cubic yards are pure geometry; tons depend on how heavy that particular stone is per yard, which changes with the rock, its size grading and how wet it is. That is why the weight figure needs a density from you, and why the 1.4 tons per cubic yard used by default is a starting point, not a property of all gravel. For the 80-foot example the order comes to about 4.6 cubic yards, 6.4 tons, or 249 half-cubic-foot bags. That last figure is worth a second look before you plan a weekend around it.

What the arithmetic does not do is design the drain. Where the water goes, how much fall the pipe needs, whether the outlet is allowed to discharge where you plan, and whether the problem calls for a dry well or foundation work are separate decisions. Have buried utilities marked before any digging; in the US that is a free call to 811.

What is french drain gravel calculator?

French drain gravel is the stone envelope that surrounds and covers the perforated pipe in a drainage trench. Water seeps through the surrounding soil into the open spaces between the stones, runs down to the pipe and is carried away. The stone is normally clean and washed, because fine particles fill those spaces and slow the drain, and it is often wrapped in a non-woven geotextile that keeps the soil out. In this calculator the gravel depth is the height of that stone layer only; any topsoil, sod or decorative cap above it is not counted.

How to use this calculator.

1. 01Measure the drain length in feet along the route the trench will actually follow, including bends and the run out to the outlet.
2. 02Enter the trench width and gravel depth in inches. Use the width the spade or trencher really cuts, and only the depth that will be filled with stone, not the soil you put back on top.
3. 03Enter the pipe's outside diameter and leave the pipe subtraction on. For a stone-only drain with no pipe, switch the subtraction off, since it needs a diameter greater than zero.
4. 04Set the waste allowance: the default 10% suits a reasonably neat trench; go higher for hand-dug trenches in rocky or soft ground, where the cut is rarely straight.
5. 05Ask your supplier for the tons-per-cubic-yard figure for the exact stone you are buying, and read the bag volume off the bag if you are buying retail.
6. 06To see a cost, enter the price your supplier quoted and pick whether it is per cubic yard, per ton or per bag. The figure covers stone only, not delivery, pipe or fabric.

The formula.

Width W and depth D are entered in inches, so each is divided by 12 to put everything in feet. The trench volume is V = L × (W ÷ 12) × (D ÷ 12) cubic feet, where L is the drain length. With the subtraction switched on, the pipe is treated as a solid cylinder of outside diameter d: its volume is π × (d ÷ 24)² × L, the d ÷ 24 being the radius in feet. The whole outside volume comes off, because stone can no more get inside the pipe than into the pipe wall.

Net gravel is V minus the pipe. The waste allowance w then multiplies it by (1 + w ÷ 100), so 10% means × 1.10. Note the order: waste is added after the pipe is taken out. Dividing by 27, the number of cubic feet in a cubic yard, gives the cubic-yard figure. Tons are cubic yards multiplied by the density you entered, so any gap between that density and your supplier's carries straight through to the weight. Bags are the adjusted cubic feet divided by the bag volume and rounded up to a whole bag.

The cost multiplies the price by the quantity in the unit you chose: cubic yards, tons or whole bags. Pricing by the bag therefore charges for the rounded-up count, and pricing by the ton inherits any error in the density. Width and depth scale the result in direct proportion, so a trench cut 50% wider needs nearly 50% more stone; the pipe term is small by comparison. The calculator refuses a pipe wider than the trench or deeper than the gravel depth, since that would give a negative volume. One thing is outside the formula: it treats the stone as the bulk volume you buy, ignoring the empty space between stones that matters for water storage.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. Environmental Protection Agency (2025). Types of Green Infrastructure. EPA.
2. [2]Philadelphia Water Department (n.d.). Subsurface Trenches. Green Stormwater Infrastructure.
3. [3]ASTM International (2020). ASTM D2321-20: Standard Practice for Underground Installation of Thermoplastic Pipe for Sewers and Other Gravity-Flow Applications. ASTM.
4. [4]Advanced Drainage Systems (n.d.). Single Wall HDPE Pipe Installation Guide. ADS.
5. [5]NDS (2026). EZflow Gravel-Free French Drain System with 3 in. Slotted Pipe and Engineered Aggregate. NDS.
6. [6]Advanced Drainage Systems (2019). ADS Drainage Handbook. ADS.