How to work out gravel volume, tonnes and bags

By Measure & Make · Updated 15 October 2026

Measure the area in metres

For a rectangle, multiply length by width. For a circle, measure the full diameter, passing through its centre. The planner calculates circular area using π × (diameter ÷ 2)². For an irregular border, split the area into simple shapes and sketch what you counted so that sections are not counted twice.

Enter the depth of the gravel layer

Depth is entered in millimetres. Measure or choose the layer of decorative gravel itself, excluding membranes or sub-base. Use the supplier’s guidance for the selected material and its intended use. A gravel layer specified for one size of stone or one application may be unsuitable for another.

Volume is not the same as weight

Area multiplied by depth in metres gives cubic metres. To estimate kilograms, multiply that volume by the loose bulk density. Bulk density describes the aggregate and the spaces between the pieces. It is different from solid-rock density and can vary with the product, grading, moisture and how it is handled. The starting 1,600 kg/m³ is an example; replace it with the supplier’s figure where available.

Worked example: a 5 m by 3 m area

At 50 mm deep, 15 m² needs 0.75 m³ of gravel. With 10% extra that becomes 0.825 m³. Using the example density of 1,600 kg/m³ gives 1,320 kg, or 1.32 tonnes. This can be ordered as two 850 kg bulk bags, or fifty-three 25 kg bags. These are alternative purchase options, not quantities to add together.

Use the right buying unit

Choose bulk bags, small bags or loose tonnes in the budget section, then enter the price for that exact unit. Changing the buying option changes the quantity used in the cost calculation. The quantity estimate rounds bags up; loose-tonne pricing uses the estimated weight, so check the supplier’s minimum order or delivery increments.

Check the listing

• “Bulk bag” does not automatically mean one tonne.
• Ask whether the stated density is a loose bulk value.
• Keep membrane, edging, sub-base and delivery separate from the gravel quantity.
• Allow for uneven ground using measured sections rather than an unexplained extra depth.

As an example of varying package sizes, this supplier listing offers more than one bag size. Confirm the listing for the product you actually choose.