Although most fermentation vessels come with printed markings to show the volume of the liquid inside, these should be used as a guide only, as the are not always accurate. Kits and recipes are designed for very specific volumes, so inaccuracies can really affect the end product. In many cases, you may wish to brew your beer, wine or cider "short" (i.e make less of it) and this is where calibration is key.
You may also wish to calibrate any measuring jugs you use, as these mass-produced items are notoriously inaccurate and if you are relying on them to calibrate a bigger vessel, you could well be making the situation worse. The easiest way to calibrate your jug, demijohn or fermentation vessel is by weight. 1 litre of water weighs 1kg, so put your jug on scale, zero the scale, then add 1kg of water and mark off one litre. Do this for as many litres as your jug will hold. Now that your jug is calibrated, you can simply use this to add water to your demijohn or fermentation vessel, and mark off in one or two litre increments: