How Many Bags of Gravel Do I Need?

To find the number of bags, work out the weight of gravel first, then divide it by the weight of one bag. A tonne is 50 bags of 20 kg or 40 bags of 25 kg, so a job that needs 2.5 tonnes takes about 125 of the smaller bags. The rest of this guide shows where the weight comes from.

The calculation in four steps

1. Finished volume = area × depth. Use metres for everything, so the answer is in cubic metres.
2. Add an allowance for waste (spillage and uneven depth): multiply by 1 + waste ÷ 100. The gravel calculator starts with 5%.
3. Weight = volume × density. Loose gravel is about 1.6 tonnes per cubic metre, but the real figure depends on the stone and how wet it is, so use your supplier's figure when you have it.
4. Bags = weight in kilograms ÷ the weight of one bag, rounded up.

Worked example: a driveway 10 m by 3 m, 50 mm deep

Decorative gravel at 1.6 t/m³, loose and only raked level (so no compaction allowance), with 5% waste. These are the numbers the gravel calculator gives for the same inputs.

Rounding up matters: 100.8 bags means 101, because 100 would leave the last stretch uncovered. Buying whole bulk bags can leave a good deal over, so for a large area it is worth comparing the cost of bulk bags with the cost of loose delivery.

Which bag size is which?

Gravel is sold in small bags, in bulk bags and loose by the tonne. The calculator offers 20 kg and 25 kg bags and an 850 kg bulk bag as examples. Bag sizes differ between sellers, so use the weight printed on the bag you will buy, and check whether the label gives a weight or a volume.

If the bags are sold by volume

In US units, bags are often sold by volume, such as 0.5 or 1 cubic foot, and the number of bags is the volume divided by the bag volume. For example, a patio 20 ft by 10 ft with 3 in of stone is 20 × 10 × 0.25 = 50 ft³. With 5% waste that is 52.5 ft³, which is 105 bags of 0.5 ft³ or 53 bags of 1 ft³.

Things that change the answer

• Depth. Doubling the depth doubles the number of bags. A path that is 25 mm deep needs half as many as the same path at 50 mm.
• Compaction. Gravel that is only raked level needs no extra. Sub-base that is rolled or plated is usually allowed 10 to 15% extra, which the calculator expresses as a factor of 1.10 to 1.15.
• Density. A heavier stone, or wet stone, weighs more per cubic metre, so the same job needs more weight and more bags.
• Tonnes and tons. A tonne is 1000 kg. A US ton is 2000 lb, which is about 907 kg, so do not mix them up when comparing quotes.

Checking the result

The gravel calculator does the four steps for any area, depth, density and bag size, and shows the working. Its results are estimates, so check them against your supplier's quantities and coverage figures before ordering.