Calculated Field

I have a form with 3 fields:

<form>
Purchase Price: <input type="text" name="purchasePrice" size="10">

Sale Price: <input type="text" name="salePrice" size="10">

Profit: <input type="text" name="profit" size="10">
</form>

I'd like the profit field to be automatically filled in once the salePrice field loses focus.

Any help on this would be greatly appreciated.

This will do it

function profit(){
pp = document.form[0].purchasePrice.value;
sp = document.form[0].salePrice.value;
profit = pp - sp;
document.form[0].profit.value = profit;
}