Deck Joist Count Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The number of joists is calculated by dividing the deck's width by the chosen spacing, then adding 1 for the starting joist. For a 4 m wide deck with 40 cm spacing, 11 joists are needed.
Explanation
Joists form the secondary frame of a wood deck: laid directly on the ground (on adjustable pedestals or masonry supports) perpendicular to the direction of the visible deck boards, they're what actually supports the decking rather than the bare ground. Unlike our deck board calculator, which counts the visible surface covering, this calculator covers the supporting structure underneath, invisible once the deck is finished but just as essential to its long-term behavior. As with roof battens (see our roof tile count calculator for this structurally identical problem applied to roofing), the number of joists isn't simply the width divided by the spacing: you always need one more joist than the number of intervals, to support both edges of the deck. The recommended spacing depends on the wood species and thickness of the boards laid on top: the thinner the boards or the softer the species, the tighter the spacing must be to avoid any sagging under weight, generally between 40 and 50 cm for common use — a value always to be confirmed against the installation instructions of the chosen board manufacturer, which take precedence over this generic estimate. This calculator also gives the total joist length to order, since each joist runs the full length of the deck.
Example: a 4 m wide, 5 m long deck, 40 cm spacing
Inputs
Width: 4 m. Length: 5 m. Spacing: 40 cm.
Calculation
Number of intervals = 400 ÷ 40 = 10 exactly. Number of joists = 10 + 1 = 11. Total length = 11 × 5 = 55 m.
Result
11 joists are needed, for a total of 55 m to order.
Frequently asked questions
Why does joist spacing depend on board thickness?
Because a thinner board sags more under weight between two support points: tighter spacing limits each board's unsupported span and avoids an unpleasant sense of flex, or even a risk of breakage over the long term. Board manufacturers generally state a maximum spacing not to exceed based on their product's thickness and species.
Should joists be laid directly on the ground?
No, generally not: joists most often rest on adjustable pedestals (which let you adjust the deck's height and levelness independently of ground irregularities) or on spaced masonry supports, never in direct continuous contact with ground that would trap moisture and accelerate wood decay.
Does this calculation also give the number of pedestals needed?
No, this calculator covers only the joists themselves. The number of pedestals depends both on the joist spacing calculated here and a separate spacing along each joist (generally 40 to 60 cm), an additional figure to combine with the deck's length to get the total number of support points.