How much ballast do I need for my model railway?

Of course, none of this helps a modeller in a shop trying to decide whether to buy one tub or two for their layout, and shops will never be able to predict an individual's ballasting techniques or design requirements, but maybe you can scale down the problem. Instead of trying to visualise or work out how many tubs or bags will cover an entire layout, consider that a regular coffee mug full of ballast will cover around 3-metres of OO gauge track or 6-metres of N gauge track with a moderate shoulder. Granted, that still doesn't take into account modelling techniques or larger shoulders, but at least you have a chance of visualising how many coffee cups of ballast there are in a bag or tub, which is a lot easier than guessing how many bags or tubs will cover a loft!