Concrete footings· 3 min read
How Many Bags of Concrete for Fence Posts? Round, Square and Tapered Footings
A fence with 10 posts: how many bags of concrete for square and round footings with straight or tapered sides, and how much a 2° or 4° slope saves you.
For a fence with ten posts you buy the concrete once — and either three bags are left over or the last hole stays empty. How many bags you need depends on the shape of the hole: square or round, with straight or sloping sides. The difference is surprisingly large.
Example: 10 footings, 300 mm at the bottom, 800 mm deep
Every variant is 300 mm at the bottom (side of the square or diameter of the circle) and 800 mm deep. We add a 10% reserve and use 25 kg bags yielding 12.5 l each (Baumit Beton B20, per the manufacturer's price list). The size and depth are only an example; what your fence needs depends on the fence system's instructions, local rules and frost depth, or a structural engineer.
Straight sides: square vs round
- Square hole 300 × 300 × 800 mm: 0.3 × 0.3 × 0.8 = 72 litres.
- Round hole 300 mm across: π/4 × 0.3² × 0.8 = 56.5 litres.
A circle always has π/4, or 78.5%, of the area of a square with the same side. A round hole is also what a hand or powered auger digs, with straight walls.
Tapered sides: what the slope does
A footing that narrows towards the top has a wider base and only as much at the top as the post needs. The slope is measured from vertical: each side moves in by depth × tan(angle). At 800 mm and 2° that is 27.9 mm per side, leaving 244 mm at the top. At 4° it is 55.9 mm, leaving 188 mm.
| Shape | Slope | Top | One footing | 10 footings + 10% | 25 kg bags |
|---|---|---|---|---|---|
| Square | 0° | 300 mm | 72.0 l | 0.792 m³ | 64 |
| Square | 2° | 244 mm | 59.4 l | 0.654 m³ | 53 |
| Square | 4° | 188 mm | 48.5 l | 0.533 m³ | 43 |
| Round | 0° | ⌀ 300 mm | 56.5 l | 0.622 m³ | 50 |
| Round | 2° | ⌀ 244 mm | 46.7 l | 0.513 m³ | 42 |
| Round | 4° | ⌀ 188 mm | 38.1 l | 0.419 m³ | 34 |
A slope of just 4° cuts almost a third off the square footing: 43 bags instead of 64. A round footing at 4° takes 34 bags — 30 fewer than a straight square hole. The ratio of round to square stays at 78.5% with sloping sides too.
The volume of a truncated pyramid or cone is not the average of the top and bottom times the depth. The calculator uses the exact formulas: V = h/6 × (A₁ + A₂ + 4 × Aₘ) for a square and V = π × h/3 × (R² + R × r + r²) for a circle.
Concrete footingsOpen the fence footings in the calculatorShould you subtract the post?
The post takes up a little room in the footing. A 60 × 60 mm post set 600 mm into concrete is 0.06 × 0.06 × 0.6 = 2.16 litres, about 4.5% of the square footing at 4°. The calculator doesn't subtract it — with slim posts it is less than the reserve you add anyway.
Fast-setting post mix
There are fast-setting mixes made for fence posts. Baumit FixBeton (class C16/20), for example, is intended by the manufacturer for setting fence posts, can be poured dry into the hole and then watered, and yields about 13 l per 25 kg bag. For the square footings at 4° that is 533.3 l ÷ 13 l, or 42 bags. Follow the instructions on the bag and enter the yield of the product you actually buy.
The calculator works out concrete and digging quantities, not structural design. For a tall fence, solid infill panels or gates that catch the wind, have the footings sized according to the fence manufacturer's instructions or by a structural engineer.