Timber sizes are written as width and thickness in millimetres: for example 135 × 19 mm is a common decking profile. Quoted dimensions are the market-standard (nominal) size; actual machined dimensions can vary slightly by product and are stated on each product page.
Boards are supplied in set lengths (commonly 1.8 m to 6.0 m depending on the product) or as random-length packs for hardwood decking. Ordering to suit your joist spacing and deck dimensions reduces wastage; the calculator can estimate board counts by length.