Last checked
May 12, 2026
We checked the page logic, support notes, and related links on this page.
Drain gravel tool
French Drain Gravel Calculator
Estimate french drain gravel volume, tonnes, bulk bags, and practical trench-fill buying totals before you order aggregate for the run.
Volume + tonnes + unitsWaste-aware resultBuying checks
How to use it
Planning before buying
Use this calculator for a planning check, then confirm the final order or quote against live product data and site conditions.
Why trust it
See how the site is maintained
Read the calculator methodology and editorial policy for the standards behind these pages.
Planning summary
Quick answer
Best for converting dimensions and depth into a delivered quantity before you choose bagged, bulk, or loose supply.
Planning summary
Watch most
The common misses are using the neat trench width only, forgetting outlets or widened sections, and mixing the gravel envelope with separate bedding or topsoil reinstatement allowances.
Planning summary
Best next move
Measure the full trench envelope first, then decide whether the quantity should cover the gravel surround only or the wider aggregate build-up around outlets and inspection points.
Recently used calculators
Pick up from the calculators you used recently on this device.
Quote-ready brief
Use these actions to turn the live calculator result into a cleaner request for builders, suppliers, or merchants.
Run the calculator, then use these actions to prepare the estimate for a real quote request.
Need help deciding what to ask for? Read the quote checklist or contact the team at hello@buildcostlab.com.
Practical checks before you buy
These notes are where BuildCostLab goes beyond a generic calculator result by surfacing the assumptions, buying traps, and next decisions that usually move the real order.
What this estimate includes
The measured volume, waste-adjusted buying quantity, density or unit-size conversion, and a rough material spend when a price is entered.
What it may not include
Unexpected excavation differences, compaction behaviour, haulage constraints, and local delivery charges unless you add them separately.
Key assumptions
French drain gravel estimates work best when the trench run, gravel depth, trench width, outlet detail, and the chosen washed aggregate route are all clear before ordering.
Worked example
Example: a 12m trench at 350mm width and 450mm gravel depth gives 1.89m3 before waste. Add 10 percent and the planning quantity becomes 2.079m3. At roughly 1.6 tonnes per m3, that is about 3.33 tonnes, so four 0.85-tonne bulk bags is the safer planning order.
How this estimate is worked out
We multiply length by width by depth, add the waste allowance, then convert the adjusted volume into tonnes or whole buying units using the stated density and delivery format.
What assumptions sit underneath it
French drain gravel estimates work best when the trench run, gravel depth, trench width, outlet detail, and the chosen washed aggregate route are all clear before ordering.
How rounding is handled
Because bulk materials are bought by bag, bulk bag, tonne, or loose load, the final answer rounds to a real buying quantity rather than stopping at the theoretical trench or base volume.
What changes the result most
Trench width, gravel depth, outlet details, widened sections, and whether the supplier prices by bulk bag or loose tonne usually move the order fastest.
Where people under-order
French drains often rise above the neat trench total once corners, outlets, soakaway ties, and overbreak are added back into the real gravel envelope.
Practical buying checks
Check whether the gravel order covers the washed aggregate only, then confirm membrane wrap, pipe bedding, and reinstatement materials separately.
Quote-ready checklist
Use these prompts when you want to turn the estimate into a clearer builder, installer, or merchant request.
- State the measured area, target depth, and whether the depth is compacted or loose-delivered.
- Ask how the material will be supplied: bags, bulk bags, loose load, or ready-mix route where relevant.
- Flag any access, storage, delivery, or waste-removal limits before the first quote is treated as final.
Explore this project hub
Open the full Drainage Estimating project hub to move from quick estimate to deeper guidance.
Related calculators for the same drainage trench job
Use these linked tools when the gravel order also depends on bedding, membrane coverage, pipe lengths, or the compacted base layers around the drainage line.
Pipe Bedding Calculator
Estimate pipe bedding volume, tonnes, and bulk-bag buying quantities for drainage trenches before you order sand or gravel.
Geotextile Membrane Calculator
Estimate geotextile membrane rolls, overlaps, and covered area for driveways, french drains, trenches, and separating aggregate layers.
Drainage Pipe Calculator
Estimate drainage pipe lengths, stock pieces, waste allowance, and practical buying totals for trench runs before you order fittings and chambers.
Sub-Base Calculator
Estimate sub-base volume, tonnes, and delivered quantity for paving, patios, paths, and driveway foundations before you order.
Quick answers
These answers are designed to resolve the last practical buying questions people usually have after running the calculator.
How do I use the French Drain Gravel Calculator?
Enter the trench length, width, gravel depth, and a realistic waste allowance, then compare the result as cubic metres, tonnes, and buying units before you order.
What changes the French Drain Gravel Calculator estimate most?
The biggest drivers are trench width, gravel depth, widened sections around outlets or corners, and whether the supplier prices by bag, bulk bag, or loose tonne.
Should I round the result up?
Usually yes. Overbreak, widened trench sections, and delivery minimums often justify a modest overage rather than landing exactly on the theoretical trench volume.
Use this estimate in a quote request
Copy the estimate, add your own notes, and send the same scope to each builder or supplier so the quotes are easier to compare.
- Confirm what the quote should include: materials only, labour only, or both.
- State access, finish level, timing, and any unknowns clearly.
- Ask each supplier or installer to price the same scope and exclusions.
You can also open the wider Drainage Estimating project hub if the quote depends on more than one material.