Key Takeaways
- For paths and patios, lay gravel at roughly twice the stone size. On planted borders, lay it 5 to 7.5cm deep. On a driveway it wants roughly twice the stone size, so 40mm for 20mm gravel, and only ever over a properly compacted sub-base, or the stone ruts under the wheels in short order.
- A bulk bag of gravel weighs around 850kg and covers about 7.5 square metres at 7.5cm deep.
- Add a minimum 10% on top for compaction and wastage to save shortfalls later, or 15-20% for driveways where the stone gets compacted hard.
- Gravel comes either as an 850kg bulk bag or as 20kg mini bags in packs of 20 or 40. A 40-bag pack is 800kg, so it covers about the same as a bulk bag does. Pick where you need convenience and transportability: mini bags can move easily on a wheel barrow and store neatly.
- Depth is what makes a gravel job look right or cheap. Too shallow and the membrane shows through quickly, and no amount of raking hides it.
Gravel Calculator
When quoting on jobs, I get asked how much gravel someone needs more than almost anything else, and the question behind the question is nearly always the same: they have already had a quote, or already half-ordered, and they want to know if they have got it right. Usually they have measured the area fine. What they have got wrong is usually the depth or under or over-order due to shoddy calculation.
The calculator above handles the arithmetic and will give you the most accurate sense of how much over or under the closest match will be. What it cannot do is tell you what depth you should be aiming for, and that is the number that decides whether the job looks like a proper job. So that is where the rest of this page goes.
How Deep Should Gravel Be?
On planted beds and borders, decorative gravel should be 5 to 7.5cm deep. That is enough to cover the weed membrane properly and give you even coverage without gaps, and shallow enough that the stone stays where you put it. Driveways want roughly twice the stone size, so about 40mm for 20mm gravel, because a deeper layer shifts and ruts under a car. The other thing that moves the number is the size of the stone itself. 20mm stone chippings simply cannot sit in a 30mm layer without the membrane showing through the gaps.
Depth by Project
| Project | Depth I work to |
Notes |
|---|---|---|
| Decorative border or bed | 5 to 7.5cm | Deep enough to keep light off weed seed |
| Garden path (foot traffic) | Twice the stone size: 20mm for 10mm gravel, 40mm for 20mm | Any deeper and feet sink in |
| Patio or seating area | Twice the stone size: 40mm for 20mm gravel | Furniture legs sink in anything deeper |
| Driveway | Twice the stone size: 40mm for 20mm gravel | Over a compacted sub-base, never straight onto soil |
| Shed base or bin store | Twice the stone size: 40mm for 20mm gravel | Over a compacted sub-base |
Depth by Stone Size
| Stone size | Depth | On a driveway |
|---|---|---|
| 10mm gravel | 20mm | 20mm |
| 20mm gravel | 40mm | 40mm |
| Pebbles up to 50mm | 70mm | Not suitable |
| Cobbles up to 120mm | 100mm | Not suitable |
A larger aggregate settles and binds together better than a small one, which is why 20mm is my default for a drive, as opposed to a 10mm crushed gravel. Pebbles and cobbles are a different job again: they are rounded, so they roll rather than lock, and they need the extra depth just to sit properly, although I never use them without setting them in mortar.
How Much Does a Bulk Bag of Gravel Cover?
A bulk bag holds around 850kg, which works out at about 0.57 cubic metres of gravel. At 7.5cm on a planted bed that covers about 7.5 square metres. We also sell the same gravel as 20kg mini bags in packs of 20 or 40, and a 40-bag pack is 800kg, so it covers very nearly the same ground as a bulk bag. Here is the full range:
| Depth | Bulk bag |
40-bag pack |
20-bag pack |
|---|---|---|---|
| 30mm | ~19 m² | ~17.9 m² | ~8.9 m² |
| 40mm | ~14 m² | ~13.4 m² | ~6.7 m² |
| 50mm | ~11.4 m² | ~10.7 m² | ~5.4 m² |
| 60mm | ~9.5 m² | ~8.9 m² | ~4.5 m² |
| 75mm | ~7.5 m² | ~7.2 m² | ~3.6 m² |
| 100mm | ~5.7 m² | ~5.4 m² | ~2.7 m² |
Because the mini bags come in 400kg steps rather than 850kg ones, they often land closer to what you actually need. A 4m by 3m patio at 40mm works out at about 790kg with the compaction added: that is one bulk bag with about 60kg left over, or one 40 bag pack with about 10kg spare. The calculator works both out for you.
How Much Does Gravel Weigh?
Around 1,490kg per cubic metre, or 1.5 tonnes. That number catches people out more than any other on this page, because it is roughly five times the weight of the same volume of bark or wood chip.
What that means in practice: a single bulk bag is most of a tonne sitting on your driveway. It arrives on a pallet, and the driver wheels it into place with a trolley jack, so it needs firm, level ground he can push that jack across — a driveway or hard standing, not up a kerb or through a gate. If your garden is round the back and the jack can’t reach, every kilo of that bag is going through on a barrow, and a builder's barrow holds about 80kg of gravel before it becomes genuinely unpleasant to push. That is ten or eleven runs per bulk bag.
I advise clients to walk the route from the kerb to the job before ordering, barrow in hand if they have one. If the answer is “through the house” or “up six steps”, order the 20kg mini bags instead. They cost a bit more per kilo, but a 20kg bag goes through a side gate in one hand and you can stack them wherever you like. That is a fair trade for your back. They also store well for the future.
How Much Extra Should You Order for Compaction?
10-20% on a normal garden job. Gravel beds in as it settles and as it gets walked on, and you will always lose a bit to the edges, to the barrow, and to the corner of the bed you measured slightly optimistically. On a driveway I go to 20%, because the stone gets rolled or driven in properly rather than just settling under its own weight. The compaction is the point on a drive, not a side effect.
The calculator has this built in with a 0, 10, 15 and 20% option, and it defaults to 10%. Whatever you pick gets applied before it works out the bag count, so the number you see is the number to order.
How to Measure Your Area
Work in metres and use a tape, not strides. For a rectangle it is length times width. For an L-shaped area, split it into two rectangles, measure each separately and add them together. For a circle, measure across the widest point, halve it for the radius, then 3.14 times radius times radius.
For anything genuinely irregular - a curved bed, a wandering path - square it off. Measure the largest rectangle that contains the shape and work from that. You will order a bit over, and that is the right way round to be wrong. Coming up short means a second delivery charge, and a real risk that the second batch does not quite match the first. Stone is a natural product and batches vary.
One habit worth stealing: measure the width in three places and use the largest. Garden beds are never as square as they look from the kitchen window.
What Goes Under the Gravel?
On a decorative bed or a light-use path, a weed membrane over cleared, level ground. On anything that takes weight - a driveway, a parking area, a shed base - you need a compacted sub-base underneath, typically MOT Type 1, and the gravel depth in the tables above sits on top of that rather than replacing it.
This is the step people skip, and it is the reason most failed gravel drives fail. There is a full guide on the site covering what to put under gravel for each type of job, and it is worth reading before you order anything.
FAQs
How much gravel do I need for a 10m² area?
At 40mm on a path or patio, 10m² needs 0.4m³, which is about 600kg. That is one bulk bag with stone to spare. At 7.5cm on a planted bed, 10m² needs 0.75m³, which is about 1,120kg, so order two bulk bags.
How many bulk bags of gravel do I need for a driveway?
A single driveway of around 5m by 3m at 40mm deep needs about 0.6m³, or roughly 900kg before compaction. Add the 20% I would use on a drive and you are at about 1,075kg, which is two bulk bags with around 625kg spare. The same job in minis is one 40 bag pack plus a 20 bag pack, and that leaves you only about 125kg over. On driveway-sized jobs the minis usually waste far less.
Can I use this calculator for slate chippings?
Yes, though slate is lighter than gravel, so the weight it gives you will be slightly on the generous side. For slate specifically I would treat the volume figure as the reliable number and check the bag weight with your supplier before ordering a large load.
What depth of gravel stops weeds coming through?
Depth alone will not stop weeds. Seed blows in and germinates in the dust that collects between the stones regardless of how deep you go. A membrane underneath stops anything coming up from below, and gravel laid to the right depth on top of it keeps the membrane covered and out of the light, but you will still get the occasional surface weed. Those pull out with two fingers.
Why does my gravel look thinner than the depth I ordered?
Because it has compacted, which is normal within the first few weeks, and because gravel spreads towards the edges wherever there is nothing to stop it. Proper edging solves the second problem permanently and is worth doing at the same time as the gravel rather than afterwards.
Can you use a gravel calculator for pebbledash or resin-bound gravel?
No. Pebbledash is sold by wall coverage rather than volume, and resin-bound systems have their own mix ratios that depend on the kit. For either of those, speak to the supplier directly.
Next Steps
Once you have your number, the ordering is the easy bit. We carry the full range of decorative gravel, chippings and slate as bulk bags or as 20kg mini bags in packs of 20 and 40, along with the MOT Type 1 for the sub-base and the membrane to go under it, so the whole job can come on one delivery. If you are not sure which stone suits the job, or your access is tight and you want a second opinion on which way to order it, get in touch and we will work it out with you.
If you would rather check your gravel figures with a person, ring Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.