Posted By Dave Merchant The density of compost varies, so it's sold by the litre (as users expect a certain "amount" means a certain number of plant pots full). Bags should however have something akin to "warning - heavy - take care when lifting" stamped on them. Bulk aggregate etc. is by the tonne, but only because it's simpler to make up the delivery on a weighbridge (and nobody expects anyone to try and lift it). The problem with labelling something variable by mass is that it opens the supplier up to complaints if the bag is too light, and equally if it's fractionally HEAVIER than the label and someone puts their back out lifting it. "Approx 25kg" can work for gravel (and hence those bags often say that), but compost can double if it gets wet.

Posted By Ron Hunter The supplier is also under pressure to comply with the Weights and Measures Act. Volumetric qty makes compliance with that Legislation easier too.

Posted By Alan Nicholls Another reason for garden compost to be labeled with the volume rather than weight; the bags are packed in a moist state, the bags have perforations in them a stacked pallet will loose a percentage of the weight over time. Some can loose up to 10% of filled weight. That's why volume is used. Same for peat.