How it is worked out
Volume is length × width × depth. Depth is entered in millimetres and divided by 1,000, so 75 mm is 0.075 m.
One cubic metre is 1,000 litres. The bag count is the litres ÷ the bag size, rounded up.
Tonnes are the volume × the weight of one cubic metre. That weight changes a lot between materials and with how wet they are, which is why this page asks for it and does not guess.
Worked example
The numbers this page opens with: length 6 m, width 2 m, depth 75 mm, extra for settling 0 %, bag size 50 litres, weight per cubic metre 0 tonnes.
The answer: area 12.00 m²; volume 0.90 m³; volume in litres 900 litres; 50 litre bags 18 bags.
Questions
Bags or a bulk delivery?
Compare the bag count with the cubic metre figure. Landscape yards sell loose by the cubic metre or part of one, and the bag count tells you how many trips to the car that would replace.
My garden bed is not a rectangle. What then?
Work out the area with the square metre calculator, then enter it here as that length × a width of 1.