Pricewise

Decide which item to buy by comparing the price per unit of the items.

Will it be a 1x 2000 ml water at $3 or 4x 500 ml water at $1 each?
$3 / 2000 ml = $0.0015 / ml
$1 / 500 ml = $0.002 / ml

Pricewise will help you decide.