What size joists should I use for decking?

47 x 150mm C16 treated softwood is the standard for most residential decks with spans up to 2.4m. For longer spans or heavy loads, use 47 x 200mm. For very short spans (under 1.2m) or lightweight use, 47 x 100mm may suffice. Always use pressure-treated timber rated for ground contact (UC4).

What spacing should deck joists be?

400mm centres is the standard for most decking boards. This provides adequate support and prevents board bounce. Composite decking may require 350-400mm centres depending on the manufacturer. For diagonal board layouts, reduce spacing to 350mm.

Do I need bearers under deck joists?

Yes, bearers (also called beams) run perpendicular to the joists and transfer loads to the posts or pads. Space bearers at 1200mm centres maximum. Bearers should be the same size or larger than the joists. They sit on concrete pads, pier blocks, or screw piles.

What are joist hangers and do I need them?

Joist hangers are galvanised metal brackets that connect joists to bearers or ledger boards. They provide a strong, reliable connection. Use them where joists meet bearers or attach to a wall ledger. Always use the correct nails (not screws) specified for the hanger.

WoodCalcs

Deck Joist Calculator — Subframe Sizing & Quantities

Calculate joist sizes, spacing, bearer quantities, and hardware for a deck subframe.

Joist Size

Joist Spacing

Deck Length

Direction joists are spaced along

Deck Width

Direction joists span across

Price per Joist (optional)

Enter your supplier price for a cost estimate

How We Calculate This

This calculator determines the number and size of joists needed for your deck subframe.

Joist count formula

Number of joists = ceil(Deck length / Joist spacing) + 1

The +1 accounts for the first joist at the starting edge. Joists span the deck width and are supported by bearers running perpendicular at 1200mm maximum centres.

Bearer calculation

Bearers = ceil(Deck width / 1200) + 1

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.