How to calculate floor load capacity

Can anyone explain how to calculate floor load capacity made of wood (joist)? I found this website: How to calculate floor load capacity | eHow UK (http://www.ehow.co.uk/how_7485788_calculate-floor-load-capacity.html) however when it comes to step 4, i cant seem to get a similar answer to the example. where does his joist space of 40cm come into the formula and what 'F' value he used. Any advice will be very helpful. Thank you. Ron.