Drainage Gravel Calculator
How much drainage gravel do I need?
Clean 20 mm drainage gravel fills French drains, soakaways and the bottom of planters, where you may want a layer of 100 mm or more. Enter the area's length and width and the depth you want to lay, and the calculator works out the volume in cubic metres, the weight to order in tonnes, and the number of bulk bags. It uses a density of about 1.5 tonnes per cubic metre for drainage gravel, which is the usual UK trade figure, so as a rough guide one tonne covers roughly 13 square metres at 50 mm deep.
Order a little over rather than under: it is far cheaper than paying a second delivery charge to finish the job, and loose aggregate settles and compacts. The suggested cost is only a guide, so change the price per tonne to match your own supplier's quote.
Frequently asked questions
Is the drainage gravel calculator free?
Yes. It is completely free, with no sign-up and nothing to install. It runs entirely in your browser.
How accurate is it?
It gives a reliable estimate for ordering based on standard UK figures, but real quantities vary with ground conditions, wastage and how materials settle. Always add a margin and confirm with your supplier before buying.