calculating number of decking joists

I'm planning of laying a deck of 21ft x 18.5ft.

The boards I'm looking at are 3.6m x 95mm x 28mm

The joists I've looked at are 2.4m x 95mm x 45mm - as it's coming out from the house I can't go much higher due to obstructions.

I've done some online calculators which have given me different amounts.... the most being 118 joists!!!!

I worked it out to be 113 joists.

I've just gone through something similar but for a garage conversion and those joists aren't big enough for the span. You need at least 120x45mm for a span of 2.4m, and they need to be spaced 400mm apart. You can have a larger spacing for the joists but they'd need to be thicker. The link below should give you the info you need

http://www.home-extension.co.uk/tech2.html