Gravel Calculator UK

Many gravel calculator pages stop at a weight. Our calculator helps you with any shape, adds the extra you choose and then goes on to give you the bags and volume, showing you the cheapest and closest fit options in bulk and mini bags. It also conveniently allows you to add the result you choose, to our basket, so you can easily complete your purchase. Just choose your gravel, choose the shape, type the measurements and tap the project you are doing, and it fills in the depth for you. What it cannot do is walk your garden and measure, so the rest of this page covers the depth, the weight, the extra you should be ordering and why.

How Deep Should Gravel Be?

Gravel depth depends on the job: 5 to 7.5cm on a planted bed, and twice the stone size on anything walked or driven on. The two numbers do different jobs, so they are not in conflict.

On a bed the gravel is a mulch. It has to be deep enough to cover the membrane and keep light off weed seed, and nobody is standing on it. On a path, patio or drive the gravel is a surface, and loose stone laid too deep moves under your weight. 10mm gravel wants about 20mm, and 20mm gravel wants about 40mm. Go deeper and you get the same effect as the gravel traps beside a race track: feet sink in, wheels dig in, and the drive ruts.

For a path I recommend something in the 6 to 14mm range, and 20mm is fine on a path that gets heavier use. For a drive, 20mm is my default. Angular stone locks together far better than rounded stone, and a larger grade settles and binds better than a small one. 40mm slate is a border grade: the plates tip and slide underfoot, so I would not put it anywhere you walk. Pebbles on a driveway are laid loose, never set in concrete, and they keep to the 10 to 20mm drive sizes.

In England a gravel drive needs no planning permission, because gravel lets rain soak through, as the Planning Portal's guidance on paving front gardens explains.

How Much Does a Bulk Bag, a Tonne and a 20kg Bag of Gravel Cover?

A bulk bag of gravel covers about 14m² at 40mm deep and about 7.5m² at 7.5cm deep. That is because an 850kg bag holds about 0.57m³, and Outdoor Aggregates measured its own bags to get that figure.

We 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. Because the packs come in 400kg steps, they often land closer to what you actually need. The calculator works through every mix of bulk bags and packs and shows you the cheapest and the closest fit side by side.

How Do I Calculate How Much Gravel I Need?

To calculate how much gravel you need, multiply the area by the depth in metres, then multiply by 1,490 to get the weight in kilos. The area is length times width, and the extra allowance goes on last.

Here is a 4m by 3m patio in 20mm gravel, laid at 40mm:

1. Area: 4m × 3m = 12m².
2. Volume: 12m² × 0.04m = 0.48m³.
3. Weight: 0.48 × 1,490 = about 715kg.
4. With 10% extra: about 787kg.

That is one bulk bag with about 63kg left over, or one 40-bag pack of mini bags with about 13kg spare. Either works. I would go with whichever is easier for you to get to the patio.

Measure in metres with a tape, never in strides. For an L-shaped area, split it into two rectangles, measure each one and add them together, which is what the L shape option in the calculator does for you. For a circle, measure straight across the middle. For anything properly irregular, like a curved bed or a wandering path, square it off and measure the smallest rectangle that holds the whole shape. You will order a little over, and that is the right way round to be wrong. Coming up short means a second delivery charge and a real chance the second batch does not quite match the first, because 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.

How Much Does Gravel Weigh?

Gravel weighs about 1,490kg per m³, close to 1.5 tonnes, taken from our own 850kg bags.

That number catches people out, because a bulk bag is most of a tonne sitting at the kerb. Unless you have ordered a loose artic wheeler, 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 side gate. If the job is round the back and the jack cannot reach, every kilo of that bag goes through on a barrow, one load at a time. A standard 85L builder's barrow filled level holds about 125kg of gravel, which is right at the limit most barrows are rated for, so half to two-thirds full, about 60 to 85kg, is a load you can actually push. That is 10 to 14 runs for one bulk bag. That's where our mini bags could be just the ticket.

I advised some less-able clients to walk the route from the kerb to the job before ordering, barrow in hand if they had one. If they struggled, or the answer was "through the house" or "up six steps", I told them to 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 stacks wherever you like. That is a fair trade for your back, and any bags left over keep for the next job. Untick Bulk Bags in the calculator and it works the order out in packs of mini bags only.

How Much Extra Gravel Should You Order?

Order 10% extra gravel on a normal garden job, and 20% on a driveway. The calculator starts on 10%, and you can choose 0, 5, 10, 15 or 20%. The extra covers the dips in the ground you did not measure, the stone pushed into the soil or membrane as it beds in, and the corner of the bed you measured slightly optimistically. On a drive the tyres drive the stone down into the sub-base, and that is the point of a drive rather than a side effect, so it needs more. Whatever you pick is added before the bags are counted, so the number you see is the number to order.

How Much Gravel Do You Need to Top Up?

Top up a gravel bed or path once the depth drops under about 30mm or the membrane starts to show. On a driveway, allow 10 to 20% of the gravel you first laid every two to three years.

Check the depth once a year with a ruler pushed down to the membrane in a few places. To size a top-up, put the area into the calculator and type in only the depth you are adding, not the full depth. A path laid at 40mm that has thinned to 25mm needs about 15mm. Rake it level first, because gravel drifts to the edges long before it disappears.

What Goes Under the Gravel?

A planted bed needs a weed membrane under the gravel, and a path, patio or driveway needs a compacted sub-base as well. On a path the membrane goes over the compacted sub-base. On a driveway a heavier geotextile goes under it.

Under a path or patio that means 100mm of compacted MOT Type 1, and under a driveway 150mm. The gravel depths in the tables above sit on top of that and do not replace it. This is the step people skip, and it is the reason most failed gravel drives fail. The sub-base calculator works out the MOT Type 1, and What Should You Put Under Gravel? covers the membrane for each kind of job.

A membrane under gravel is at its best, because nothing rots down into it from above. It will cut the weeding for several years. It will not end it, because seed blows in and germinates in the dust that collects between the stones. Proper edging matters as much as the membrane, because gravel spreads to the edges wherever there is nothing to stop it, and Edging a Garden covers the options.

Next Steps

Once you have your number, the ordering is the easy bit. Outdoor Aggregates carries the full range of decorative gravel and slate chippings as bulk bags or 20kg mini bags, along with the MOT Type 1 for the sub-base and the heavy-duty weed membrane to go under it, so the whole job can come on one delivery. For the build itself, the gravel path, gravel patio and gravel driveway guides take you through it step by step. If you want a surface that stays put rather than loose stone, the Self-Binding Gravel Range compares the grades we sell.

If you would rather talk your gravel depth and bag order through with a person, ring Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.

Have you ever ordered gravel for a path or a drive? We would love to hear in the comments what depth you laid it at and whether the bags covered it.

FAQs

How much gravel do I need for 100 square metres?

A 100m² area of gravel at 40mm deep needs about 4m³, which is about 5,960kg. With 10% extra that comes to about 6,560kg, or eight 850kg bulk bags with a little to spare.

How much does 20kg of gravel cover?

A 20kg bag of gravel covers about a third of a square metre at 40mm deep, and about 0.18m² at 7.5cm. That is why I would buy mini bags in packs of 20 or 40 rather than one at a time.

How many bulk bags of gravel do I need for a driveway?

A single 5m by 3m driveway in 20mm gravel at 40mm deep needs about 894kg, or about 1,073kg with 20% extra. That is two bulk bags with plenty spare, or one 40-bag pack plus one 20-bag pack with about 127kg over. On driveway-sized jobs the packs often waste far less.

Can I use this calculator for slate chippings?

Slate chippings work in this calculator just as gravel does, and they have their own entries in the product list. Outdoor Aggregates' measured bag holds the same 0.57m³ of slate as of gravel, so the weights come out the same. Lay 20mm slate at 4 to 5cm on a path and keep 40mm slate for beds and borders.

How heavy is a bag of gravel?

A bulk bag of gravel weighs 850kg, and a mini bag weighs 20kg. At Outdoor Aggregates the mini bags come in packs of 20 (400kg) and 40 (800kg), and Horticultural Grit also comes in packs of 10. The calculator lists the bag sizes each product comes in.

Why does my gravel look thinner than the depth I ordered?

Gravel looks thinner after a few weeks because it settles into the membrane and drifts towards the edges, and both are normal. A rake puts the settling right. Proper edging stops the drift for good, and it is worth doing at the same time as the gravel rather than afterwards.

Can you use a gravel calculator for self-binding or resin-bound gravel?

A loose gravel calculator gives the wrong answer for self-binding and resin-bound gravel, because both are worked to a compacted or mixed depth. Self-binding gravel is laid at about 75mm loose to finish at 50mm, and a resin-bound kit has its own mix ratio. For either, speak to us or the kit maker before ordering.