Need to be able to purchase a fraction of a product - eg fabric .2 of a metre

Hi there

I need customers to be able to buy in fractions.

The businesses on my site need to add their fabric by the metre - so they have 20 metres at $15 a metre.
The shopper needs to be able to select that they want to buy 0.1 of a metre or 0.25 of a metre.
The business needs to be able to select that they want to sell in increments of say .1 of a metre or .25 etc.

The price should automatically work itself out in the shopping cart.

How do I do this?

cheers

Stef