Bulk bags vs small bags: which is cheaper?
Bulk bags are almost always cheaper per tonne — often less than half the price. The catch is delivery, storage and the fact you cannot send back what you do not use.
The price-per-tonne test
An 850kg bulk bag of 20mm gravel typically costs £70–£95 delivered, which is roughly £85–£110 per tonne. A 20kg handy bag at £5 works out at £250 per tonne — nearly three times as much.
The break-even point is usually around 300–400kg. Below that, small bags win on convenience. Above it, take the bulk bag.
When small bags still win
You only need a top-up, you have no space for a bulk bag, the lorry cannot reach the property, you are working alone with a wheelbarrow, or you need the material spread over several weekends and want it kept dry. Cement in particular should always be bought in 25kg bags and kept off the floor — a part-used bulk bag of powder is a wasted bulk bag.
Hidden costs to include
Delivery charges, whether the lorry has a crane offload, whether you need to be home, and whether the bag can be placed where you want it. A bulk bag dropped at the kerb 30 metres from the back garden is two hours of barrowing.