Mulch, compost or topsoil for beds and borders — volume, litres and bag count.
Multiply bed length × width × depth to get volume in cubic metres, convert to litres (×1000), then divide by your bag size. A 75L bag of mulch or compost is common, but topsoil and bark come in various sizes.
50–75mm for weed suppression and moisture retention. Any thinner and weeds push through; much deeper can suffocate plant roots.
Mulch (bark, woodchip) sits on the surface to suppress weeds; compost is dug in to improve soil. Topsoil is for raising levels.
At 50mm depth you need about 50 litres per square metre.