French Drain Calculator – Aggregate, Pipe, Fabric & Cost
THE HANDYMAN HUB · FREE CALCULATOR
French Drain Materials Calculator
Estimate trench excavation, drainage aggregate, pipe and geotextile, with optional whole purchases and job pricing.
YOUR ESTIMATE
Your French drain estimate
Trench quantities, material purchases and optional job price.
Enter trench dimensions, then calculate.
Inputs changed. Calculate again to update your estimate.
OPTIONAL BUSINESS TOOL
Turn your drainage estimate into a professional quote
Use the Handyman Hub Quote & Invoice Generator to create customer-ready quotes and invoices. Add your checked materials, labour and business details, then print or save your finished document.
- Professional quotes and invoices
- Your business and customer details
- Clear labour and material items
- Print or save as PDF
$4.99 one-time payment
Lifetime access · No subscription
Secure purchase through Gumroad. This separate product is optional. The calculator and its estimate printout remain free, without an account or email signup.
Keep drainage fill, excavation and surface reinstatement separate
Use this tool for a specified French drain trench, pipe and geotextile schedule. The aggregate envelope, pipe displacement and trench excavation represent different volumes; keep their dimensions visible when comparing a supplier quote.
The aggregate calculator can check a separate supplier weight or bag conversion. Use the selected drainage aggregate’s density and do not apply another purchase uplift to a quantity that already includes one here. The excavation calculator covers separately measured spoil removal.
For reinstating a lawn above or beside the work, estimate the specified soil and turf separately with the topsoil calculator and turf calculator. Surface channel sections belong in the channel drainage calculator; that material list is not interchangeable with a buried drain.
French drain materials: from measured trench to shopping list
This free French drain calculator estimates the materials for a drainage layout you have already specified. It separates excavation, aggregate fill, pipe displacement and fabric wrapping so you can see how each quantity was obtained. Start with one trench or add several sections with different widths and depths. Identical copies and bulk entry make larger measured jobs quicker to enter.
The basic result needs no prices. It shows excavated volume, aggregate volume and loose spoil using the dimensions you provide. Optional sections convert aggregate into an order, count pipe stock lengths and plan geotextile rolls. Tradespeople can add labour and other costs, then markup and tax. Print or save the result without registering or entering an email address.
Measure the trench and aggregate envelope separately
Enter the measured route length, trench width, full excavation depth and the height occupied by drainage aggregate. If a layer of soil or another finish sits above the aggregate, the fill height is smaller than the excavation depth. The calculator keeps those dimensions separate so soil cover does not accidentally become an aggregate order.
All dimensions describe a rectangular cross-section. Divide a route into separate entries when its width, depth or aggregate height changes materially. Average dimensions can be useful for a measured estimate, but a rectangular model does not reproduce sloping sides or irregular excavations exactly. At intersecting trenches, avoid entering the same physical volume twice. Extra pits, chambers and irregular details need their own assessed allowance.
Metric entry uses metres for route lengths and millimetres for widths and depths. Imperial entry uses feet and inches. Changing the measurement system converts existing values, including optional settings and relevant unit prices. Currency selection changes presentation; it does not perform an exchange-rate conversion.
Why pipe outside diameter matters
When the optional pipe section is enabled, one pipe is assumed to run along each entered trench. The calculator subtracts its external cylindrical volume from the aggregate envelope. This uses outside diameter because aggregate cannot occupy the physical space enclosed by the pipe, including its hollow interior. Internal bore is relevant to water volume, but is the wrong measurement for aggregate displacement.
Pipe purchasing is a separate calculation. Each trench length divides by the effective installed stock length or coil length and rounds up. Copies are applied afterwards. Spare lengths are added before rounding into purchase packs and deducting matching whole packs already owned. The estimate does not move pipe offcuts between trenches or infer compatible bends, junctions and outlets. Enter their assessed cost under other costs, and check the actual connection arrangement before ordering.
Convert aggregate volume into an actual order
The net geometric aggregate volume is calculated first. Your reserve percentage then allows for measured uncertainty or handling losses. It starts at zero so the result does not hide an automatic allowance. Choose volume-only if you simply want a quantity to discuss with a supplier.
Loose volume orders round up to the order increment you enter. Weight orders require the supplier's bulk density and a weight increment. Bag purchases use that same bulk density and the stated mass of each bag, rounding to whole bags before deducting usable whole bags already owned. A bag described commercially as a bulk bag does not necessarily contain one tonne; use its actual stated contents.
Bulk density includes the spaces between aggregate particles. Solid mineral density would substantially misrepresent this conversion. Moisture, grading and packing can alter supplied bulk density, so there is no universal density preset here. The calculator does not select an aggregate grading or filter design. Use the product and quantity basis appropriate to the specified drainage system.
Geotextile wrap width, joins and rolls
The optional fabric calculation models a full wrap around the aggregate envelope. Required width is twice the trench width plus twice the aggregate height, plus your top closure overlap. It includes the bottom, both side faces and the top. It does not use the full excavation depth when soil cover lies above the aggregate.
Rolls run lengthwise along the trench. If a roll is narrower than the required wrap, the calculation adds parallel strips using your between-strip overlap. If the trench is longer than a roll, it adds pieces using your roll-end overlap. The extra length at each trench end is applied separately. This lets you include an assessed end detail, but it does not design closed end faces, corners or junctions.
The cut plan places longer full-width pieces first and reuses available length offcuts for shorter pieces. It does not reuse narrow strips trimmed from roll edges, and it is not a guaranteed mathematical minimum. Reported roll counts are therefore a practical purchasing plan for the entered configuration. Confirm that your specified geotextile and installation permit the proposed strip joins, overlaps and orientation.
A simple worked example
A trench 10 m long, 300 mm wide and 500 mm deep has an excavation volume of 1.5 m³. If aggregate occupies 400 mm of its depth, the aggregate envelope is 1.2 m³ before pipe displacement. With a 100 mm outside-diameter pipe along the full route, displacement is approximately 0.07854 m³, leaving 1.12146 m³ of aggregate. Adding 10% reserve gives approximately 1.23361 m³.
For that same aggregate envelope, a full fabric wrap needs 1.4 m before top overlap. A 200 mm closure overlap makes it 1.6 m wide. A 2 m wide roll can cover that with one strip. This example explains arithmetic only; its dimensions and overlaps are not an installation recommendation.
Excavation, spoil and job pricing
Excavated volume uses the full trench dimensions. Loose spoil then adds your bulking percentage. Pipe and aggregate are not subtracted from excavation: they are installed after material is removed. The result does not decide how much spoil can remain onsite, classify waste or select a disposal container.
Optional labour can use measured trench length, hours or days. Select one method to avoid double counting. Other whole-job costs can cover fittings, additional outlet pipe, plant, haulage and disposal. Keep material prices consistently before tax. Markup is added to total cost, and entered tax applies to the resulting selling price. Closing a pricing section retains its values; switching pipe or fabric off excludes those materials and their costs.
Product guidance and scope
Supplier guidance should determine the selected materials and installation detail. NDS drainage guidance provides background on drainage systems. TERRAM filtration and separation guidance explains the role of permeable geotextiles in separating materials while permitting water movement. Consult the exact product installation instructions for overlaps and suitability.
This tool estimates quantities for a supplied layout. It does not establish drainage capacity, infiltration suitability, the position of buried services, safe excavation support, discharge permission or appropriate falls. Proprietary gravel-free drainage bundles are a different purchasing system and should use their supplier's quantities rather than this aggregate-envelope model.
French drain calculator FAQs
Can I use it without a pipe?
Yes. Leave the pipe checkbox off to estimate the full aggregate envelope. Whether a pipe-free arrangement suits your site must be determined separately.
Does aggregate height mean total trench depth?
Only when aggregate fills the complete depth. Exclude soil cover or other layers from aggregate height while retaining the full excavation depth.
What density should I enter?
Use bulk density supplied for your chosen drainage aggregate. The calculator deliberately leaves it blank because grading and condition change the value.
Why can fabric need more than area divided by roll area?
Wrapping requires sufficient width, while overlaps and long runs consume additional length. An area-only division ignores those geometry constraints.
Can I paste multiple trenches?
Yes. Bulk entry uses six columns: name, length, width, full depth, aggregate height and copies. The same engine calculates card and bulk inputs.
Does the calculator choose the pipe size or slope?
No. It uses your specified outside diameter and route measurements. Hydraulic design and suitable levels must be established separately.
Can I print the estimate for free?
Yes. Calculate, choose Print / Save PDF, then use the preview's print button. The output includes quantities and costs without site navigation or promotional content.