Price per unit length

Hi,



Is there a way of using cs cart to price per unit length?



For instance, say I sell rope, I wan the customer to choose the length of rope required, This can’t be the quantity field as they may want one or more lengths of the rope.



For instance say a customer want 55metres of green rope, is there a way to get cs cart to adjust the price automatically based on the cost per metre?



Thanks



Tim