The calculation is Quantity of Sales divided by Quantity of visitors x 100 So 1,000 uniques & 12 sales would be a (12/1,000)x100= 1.2% conversion rate.
Grant is right. Â If you wanted to recalculate your thing you would have sales / visitors * $ per sale, which would tell you the average dollars you get from each visitor to your site. Â That could be an important figure too, if you think about how much it costs to get each visitor there.
By the way. I've had 3 ecommerce sites and my most recent one I thought was the best but was the worst to convert. I think people are now a lot more "internet savvy" they know they can probably compare products in Google and get it cheaper, where as before they might have just paid for it instantly!