About the Deck Calculator
A deck is a small building in its own right. Under the boards you walk on is a frame of timber that has to carry people, furniture, barbecues, hot tubs and snow, and that frame is where most of the cost, and all of the structural risk, lies. Before ordering anything it helps to have a clear list of the framing parts and how many of each are needed.
A typical attached deck has a ledger bolted to the house, joists running out from it at a regular spacing, a rim joist across their far ends, and a beam underneath near the outer edge, sitting on posts set on concrete footings. Metal joist hangers fix the joists to the ledger and rim.
This deck calculator works out the number of joists and hangers, the lengths of the ledger, rim and beam, the number of posts and footings, and the concrete to fill the footing holes.
How to Use the Deck Calculator
Choose imperial or metric.
Enter the deck length along the house and its width out from the house.
Enter the joist spacing, centre to centre — 16 in (400 mm) is usual.
Enter the maximum post spacing along the beam — often 6 to 8 ft (1.8 to 2.4 m).
Enter the footing diameter and depth, and the actual post width, so the concrete can be worked out.
How the Framing Is Worked Out
joists = length ÷ joist spacing, rounded up, + 1
hangers = 2 × (joists − 2) (both ends of each inner joist)
posts = length ÷ post spacing, rounded up, + 1
concrete = posts × (π × r² × depth − post² × depth)Step-by-Step Example
A 16 × 12 ft deck, joists at 16 in, posts up to 8 ft apart, 12 in footings 36 in deep, 6×6 posts (5½ in).
Joists: 16 ft ÷ 16 in = 12 spaces → 13 joists of 12 ft
Hangers: 2 × (13 − 2) = 22
Ledger, rim, beam: 16 ft each
Posts: 16 ÷ 8 = 2 spaces → 3 posts and footings
Per footing: π × 0.5² × 3 − 0.458² × 3 = 1.73 ft³
Total: 3 × 1.73 = 5.2 ft³ → 9 × 80 lb bagsA 4.8 × 3.6 m deck in metric, with joists at 400 mm, also needs 13 joists.
The Parts of a Deck Frame
Ledger — a board of joist size, bolted or lag-screwed through the house's rim board, with flashing above it to keep water out of the wall. Ledger failures are the most common cause of deck collapses, so the fixings and flashing matter more than anything else in the frame.
Joists — the parallel members that carry the decking. Their size depends on how far they span between ledger and beam.
Rim joist — the board across the outer ends of the joists, which ties them together and carries the fascia.
Beam — two or three joist-size boards nailed or bolted together, or a solid or engineered timber, sitting on the posts. The joists either sit on top of it or hang from it.
Posts and footings — posts, usually 6×6 (140 mm square), transfer the load to concrete footings that must reach below the frost line and bear on firm ground.
Sizing Is a Separate Step
This calculator counts the parts; it does not size them. Joist, beam and post sizes depend on the spans, the timber species and grade, the joist spacing and the design loads, and must come from span tables — in the US the International Residential Code's deck provisions and the American Wood Council's DCA 6 guide, elsewhere the national code or a structural engineer. As a rough idea, 2×8 joists at 16 in centres typically span roughly 11 to 13 ft in common species, and a hot tub needs its own engineered support.
Footings and Frost
Footings must sit below the frost line — the depth to which the ground freezes in winter — or frost heave can lift and crack them. That can be 12 in in mild climates and 4 ft or more in cold ones. The calculator takes whatever depth you enter. Wide footings spread the load on soft ground; many builders set a wider concrete pad at the bottom of the hole, or use precast bases.
Joist Hangers, Bolts and Connectors
Modern decks rely on metal connectors as much as on timber. Joist hangers hold each inner joist to the ledger and rim; the two end joists are usually nailed or screwed through the ledger and rim instead. Post bases or post caps tie the posts to the footings and beam, and hurricane ties or lateral load connectors may be required to stop the joists lifting or the deck pulling away from the house. Use connectors rated for treated lumber — usually hot-dip galvanised or stainless steel — with the nails or screws the manufacturer specifies; ordinary nails and screws corrode quickly in treated timber.
Building Permits
Most jurisdictions require a permit for a deck attached to a house or raised above a certain height, often 30 in (760 mm). Inspectors typically check the footing holes before concrete is poured, the ledger attachment and the framing before decking goes on, so plan the order of work around the inspections.
Understanding Your Result
Joists gives the number and length of the joists.
Area gives the deck area.
Ledger, rim and beam gives their lengths and the number of hangers.
Posts gives the number of posts and footings.
Footing concrete gives the volume and the bags of concrete.
Worth knowing covers sizing and the frost line.
When Should You Use This Calculator?
Planning a deck, to produce a materials list.
Comparing layouts, such as joist spacing or post spacing.
Ordering concrete for the footings.
With the deck board calculator, for the decking itself.
Common Mistakes
Forgetting the closing joist. It is spaces plus one.
Weak ledger fixings. Use bolts or structural screws and flashing.
Guessing member sizes. Use span tables.
Shallow footings. Go below the frost line.
Leaving out hangers. Every inner joist needs one at each end.