Decking Calculator

How to use this calculator

1. 1Measure the deck's length and width in metres.
2. 2Decide which way the boards run — along the length gives the fewest cuts.
3. 3Read the board count with the 10% cutting allowance already included, priced across softwood and composite.
4. 4Add to your project board and print the list.

Worked example: 20m² deck

A 5m × 4m deck is 20m²; with 10% wastage you'll need boards covering 22m². The calculator prices the exact count and shows the total, so there are no surprises at the merchant.

Practical guidance

With standard 145mm boards and a 5mm gap, each row covers 150mm of deck width — roughly 6.7 board-metres per square metre. Add 10% for cuts, and more if you're laying diagonally or around obstacles. Joists go at 400mm centres for timber boards and 300–350mm for most composites — check the maker's spec, because spanning too far is what gives that trampoline bounce. Leave 5–8mm between boards and 10mm against walls for drainage and movement.

Composite costs about twice softwood up front but never needs staining and carries 10–25 year guarantees; softwood is fine on a budget if you oil it every year or two. Lay a weed membrane under any low deck, and never rest joists straight on soil — use pedestals, concrete pads or a slabbed base. Order boards in lengths that minimise joints across the deck's long dimension, and buy all your boards in one batch so colour and grooving match.