French Drain Calculator

French Drain Calculator for Pipe, Gravel, and Fabric

This French drain calculator estimates the basic materials for a straight drainage trench. Enter the drain run, trench width and depth, gravel depth, and pipe diameter. It calculates perforated pipe sections, gravel volume and bags, estimated aggregate weight, filter fabric area and rolls, waste, and optional material cost. Use the result to make an early shopping list, then check it against the drainage plan and material labels for your site.

A French drain is a gravel-filled trench that uses a perforated pipe to collect and move water toward an outlet or another approved drainage location. Some projects use filter fabric to separate soil from the stone. The correct layout depends on where water comes from, soil conditions, slope, local rules, utilities, and the discharge point. A quantity calculator can estimate materials, but it cannot determine whether a French drain is the right drainage solution for a property.

How Much Gravel for a French Drain?

Gravel quantity is based on the length of the trench, its width, and the depth of the stone layer. Multiply those dimensions to estimate the gravel bed volume. This calculator subtracts an approximate volume for the pipe, adds the selected waste allowance, and converts the order volume into bags using the bag coverage you enter. It also estimates weight from an adjustable bulk density, which can help when arranging delivery or handling.

For example, a 30-foot trench that is 12 inches wide and has a 20-inch gravel layer holds roughly 50 cubic feet before subtracting the pipe volume. At a half-cubic-foot coverage per bag, that is about 100 bags before waste. The exact estimate changes with the pipe diameter, fill depth, aggregate packing, and product coverage. For a bulk order, compare the cubic-foot result with the supplier's cubic-yard measurement and ask how the aggregate is sold.

Measure the Drain Length and Trench

Measure the full pipe route from its starting point to the planned outlet. Include changes in direction if you will build the drain as connected straight sections. The calculator treats the entered length as a single run. It does not add fittings, cleanouts, end caps, catch basins, or connection lengths, so include those items separately in a detailed takeoff.

Measure trench width and depth at the expected gravel cross-section. A trench that changes size along its route can be split into sections, calculated separately, and totaled. If the trench has a sloped bottom or different gravel depth along the run, an average dimension may be useful for a rough order, but it can hide local differences. Check your actual excavation and drainage plan before ordering large quantities.

Choose Perforated Drain Pipe

The pipe run usually follows the trench route, but the type of pipe and how it is installed depend on the system design. Enter the pipe's outside diameter so the calculator can estimate its displacement in the gravel. The pipe section count is based on the stock length you enter and includes the waste allowance. Choose the pipe length that matches the product available from your supplier.

Perforated pipe products differ in material, wall stiffness, perforation pattern, and whether they are supplied with a sock or wrap. Some drainage designs use solid pipe for the outlet section and perforated pipe only where water collection is intended. The calculator counts one type of pipe along the full entered length. If your design has both perforated and solid portions, calculate those lengths separately and include couplings and fittings in your order list.

Pipe stock is sold in different lengths and may be rigid or flexible. Flexible corrugated pipe can follow curves, while rigid pipe sections may need elbows or couplers at changes in direction. Select a product that matches the design and expected loading. For a driveway or area with vehicle traffic, pipe strength and cover can matter. Follow the manufacturer's installation notes for bedding, connections, and access points, and do not assume every perforated pipe can be used in every location.

Estimate Gravel Depth and Pipe Displacement

Enter the depth of gravel you expect around the pipe. The gravel depth cannot be greater than the trench depth in this tool. The trench volume is length multiplied by width and gravel depth. It then subtracts the volume occupied by a round pipe based on its outside diameter and the full trench run. This is an approximation because the pipe may sit at a different elevation or have a different profile than a simple cylinder.

Drainage aggregate is often selected for its size, cleanliness, and ability to let water pass through, but the correct specification depends on the project. This calculator does not choose stone gradation or determine whether a particular gravel is suitable. Use the stone size and material requirements on the project plans or from a qualified drainage professional. Ask your supplier how its bag coverage or bulk volume is measured.

Avoid using the calculator's example density as a substitute for the aggregate supplier's data. Rounded stone, crushed rock, and mixed gravel can have different void spaces and bulk weights. Moisture can also affect delivered weight. If stone is sold by the ton, ask the supplier to convert the estimated cubic yards using the product density they use for sales. Include a little extra only when your measurements and installation plan justify it.

Filter Fabric Width and Roll Count

Filter fabric may line the trench and wrap around the gravel to separate stone from surrounding soil. A simple fabric cut width includes the trench bottom, both trench sides, and an overlap allowance across the top. The calculator estimates this width as trench width plus twice the trench depth plus the overlap. It multiplies the result by drain length and applies the waste percentage to estimate fabric area.

The roll count is calculated from the total fabric area divided by the area of one roll. This area-based method assumes the fabric can be cut and joined to fit. Actual roll use also depends on roll width, overlap at seams, trench shape, and installation sequence. If the required wrap width is greater than the roll width, you may need additional seams or a different product. Check the roll dimensions and installation instructions before ordering.

Fabric placement varies by design. Some plans wrap the gravel completely, while others specify a liner arrangement or a fabric sock around the pipe. Follow the cross-section details instead of wrapping every component by habit. Keep fabric overlaps in the direction and position shown in the plans, and avoid leaving gaps where soil can wash into the stone. The calculator's simple wrap-width method is most useful for estimating one continuous trench with a consistent section.

Add Waste for Cuts and Connections

The waste allowance applies to pipe length, gravel volume, and fabric area. It gives extra material for trimming pipe, working around corners, handling fabric, and small differences between measured and excavated dimensions. The calculator rounds up to whole pipe sections, aggregate bags, and fabric rolls. A long straight trench may need less trimming than a route with multiple bends or branches.

Waste is a planning choice rather than a fixed construction rule. Consider the trench shape, number of fittings, stock lengths, fabric roll dimensions, and whether leftover gravel or fabric can be used elsewhere. For gravel delivered in bulk, suppliers may use their own minimum order or rounding increments. Confirm the sale quantity and delivery conditions before relying on a decimal volume estimate.

Gravel Bags, Bulk Volume, and Weight

Bagged gravel quantities are calculated from the estimated order volume divided by bag coverage. Enter the coverage listed on the product or supplier information. A bag label may show volume, weight, or both. The calculator uses bulk density to estimate total gravel weight; density varies with rock type, grading, moisture, and how tightly the aggregate is packed.

The results also show approximate cubic yards for imperial planning. One cubic yard equals 27 cubic feet, and bulk suppliers may quote by the cubic yard or ton. Do not convert a volume to a tonnage using an assumed density when the supplier provides a product-specific value. Confirm the actual weight, truck capacity, access route, and delivery drop location if ordering bulk stone.

French Drain Material Cost Estimate

Enter optional prices for a pipe section, gravel bag, and fabric roll to create a basic material cost estimate. The calculator multiplies each price by its rounded-up material quantity and adds the entered costs. If you are ordering gravel by the cubic yard instead of bags, use the displayed volume to request a local quote and record that cost separately.

The estimate excludes labor, excavation, disposal of soil, equipment rental, fittings, catch basins, outlet protection, permits, taxes, and delivery unless you include those items in your own budget. Prices vary by region, product, roll size, aggregate type, and supplier. Compare products by their dimensions and performance details as well as price, since two packages may cover different areas or volumes.

For a more complete budget, list excavation and spoil removal separately from the pipe, stone, and fabric. A trench may need hand digging near utilities or equipment access may be limited. Include a safe route for moving aggregate and a place to store pipe and fabric while work is underway. These site costs can be larger than the difference between two material brands, so consider access and delivery as part of early planning.

Plan Slope, Outlet, and Water Direction

A functioning drain needs a defined route for collected water. Slope, outlet location, soil type, groundwater, nearby foundations, and local rules all influence a drainage layout. This calculator does not calculate required slope or prove that water will flow to a safe location. Use a site plan or professional advice to choose a route and discharge point that complies with applicable requirements.

Before digging, identify underground utilities and follow the local locate process. Consider where water may go during heavy rain and whether it could affect neighboring land, sidewalks, septic systems, or building foundations. A French drain can move water, but it cannot correct every source of moisture. If water is entering a basement or causing persistent foundation problems, investigate the source and get site-specific guidance.

Metric and Imperial Units

Imperial mode uses feet for the drain run, inches for trench dimensions and pipe diameter, cubic feet for gravel bags, and pounds per cubic foot for density. Metric mode uses metres, centimetres, litres for bag volume, and kilograms per cubic metre for density. The calculator converts your current entries when you change unit systems so the dimensions continue to represent the same physical trench.

Check each field label before entering a value. Product packaging may use litres, cubic feet, kilograms, or pounds, and mixing those units can change a material estimate substantially. Conversions shown on screen may be rounded for readability. Use exact supplier coverage and stock lengths where possible, then recheck the result after converting dimensions from a drawing or field measurement.

How the French Drain Formula Works

The base gravel volume equals drain length multiplied by trench width and gravel depth, minus the estimated cylinder volume of the pipe. The calculator adds the chosen waste percentage and divides by the bag volume to estimate bag count. Bulk density converts order volume to a rough weight. Pipe length is increased by the same waste allowance and divided by stock pipe length to round up to sections.

Fabric cut width is trench width plus twice trench depth plus the overlap allowance. Fabric area equals that width times drain length, with waste applied. The roll count divides required area by roll area and rounds up. Each formula assumes a constant trench section and a continuous run. Fittings, branches, changing depth, seams, and field conditions can change the final quantities.

Limits of an Online French Drain Calculator

This tool estimates common materials for a simple trench; it does not design a drainage system or inspect the site. The diagram is a visual aid, not a construction drawing. It does not assess soil drainage, slope, water table, groundwater pressure, retaining conditions, foundation details, outlet capacity, or local stormwater rules. It also does not include every fitting or accessory.

Confirm the design, materials, and discharge route before digging. Follow local utility marking requirements and construction rules. If the drain is near a foundation, carries significant runoff, connects to an existing system, or may affect neighboring property, get qualified site-specific advice. Use the calculator as a starting material estimate and verify quantities with the plans and supplier before ordering.