What this calculator does
Footings form a grid, so the count multiplies in both directions. A 6 by 4 metre deck at 2.4 metre maximum spacing needs 4 footings along each of 3 lines, which is 12 in total.
Because it is a grid, tightening the spacing is expensive in both directions at once. Going from 3 metres to 1.8 takes the count from 9 to 20, more than doubling, even though the spacing changed by only 40%. Each footing is a hole, a bag of concrete and a post base.
The formula
Footing lines across the width are the width divided by the spacing, rounded up, plus one. Footings per line follow the same rule along the length. Multiplying gives the total, and the actual spacing is reported since it is usually tighter than the maximum entered once the count is rounded up.
| Term | Meaning |
|---|---|
| Footing | A concrete pad or pier carrying a deck post down to bearing soil. |
| Beam span | Distance between posts along a beam, which is what the spacing really limits. |
| Joist span | Distance between beams, governed by the joist size rather than the footing spacing. |
| Bearing capacity | What the soil can carry, which determines the footing size rather than the count. |
The inputs explained
| Field | What to enter |
|---|---|
| Deck length (m) | Deck length in metres. |
| Deck width (m) | Deck width. |
| Maximum footing spacing (m) | Maximum footing spacing. This should come from the beam span table for your timber size, not be chosen freely. |
When to use it
Planning a deck substructure
The footing count drives the excavation, concrete and hardware quantities.
Comparing beam sizes
A deeper beam spans further and needs fewer footings, which can be cheaper overall.
Estimating the concrete
Each footing needs its own volume, and the count multiplies it.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does spacing change the footing count?
The same deck at a range of maximum footing spacings.
| Maximum footing spacing | Footings needed | Footing lines across the width | Actual spacing along the length |
|---|---|---|---|
| 1.8 m | 20 | 4 | 1.50 m |
| 2.4 m | 12 | 3 | 2.00 m |
| 3 m | 9 | 3 | 3.00 m |
Questions
How far apart should deck footings be?
Determined by the beam size and the load, not chosen freely. Span tables for your timber section give the maximum, commonly 1.8 to 3 metres for domestic decks. Using a deeper beam lets you span further and use fewer footings.
How big should each footing be?
That depends on the soil bearing capacity and the load each carries, which is a separate calculation from the count. Common domestic footings are 300 to 450 mm diameter. Poor or made ground needs larger footings or a different foundation entirely.
Why is the actual spacing less than my maximum?
Because the count rounds up to a whole number of bays. A 6 metre length at 2.4 m maximum needs 3 bays, which come out at 2.00 m each rather than 2.4. The maximum is a limit, and the actual spacing usually sits below it.
Do footings need to go below the frost line?
In freezing climates, yes, for the same reason as fence posts: frost heave will lift a shallow footing and rack the deck. Local codes specify the depth. In non-freezing climates the depth is governed by reaching adequate bearing soil.
For the concrete per hole, see the post hole concrete calculator. For decking boards above, see the fence and decking calculator.