Concrete footings· 3 min read
How to Calculate Concrete for Pergola Footings: Volume, Bags and Digging
A 4 × 3 m pergola on 6 footings tapered from 400 to 300 mm: how many m³ of concrete, how many 25 kg bags, how much soil to dig and when ready-mix pays off.
A timber pergola stands on posts, and the posts stand on concrete footings. Before you build, you need to know how much concrete to buy: a few bags from the DIY store, a full pallet or a ready-mix truck. Here is a worked example, and you can recheck every number in the calculator.
Example: a 4 × 3 m pergola on 6 footings
- 6 posts: 3 on each long side (one every 2 m along 4 m) in two rows 3 m apart.
- Tapered footings: 400 × 400 mm at the bottom of the hole, 300 × 300 mm at ground level. The sides slope about 3.6° from vertical.
- 800 mm deep below ground.
- A 100 mm collar above ground, 300 × 300 mm — the timber post then stays out of the grass and damp soil.
We take the number of posts and the footing size and depth as given. The calculator works out quantities and geometry, not structural design — footing size depends on the load, the soil and the frost depth where you build. Check local building rules, the kit instructions or a structural engineer.
How to work out the volume of a tapered footing
A footing with straight but sloping sides is a truncated pyramid — a prismoid. Its volume is V = h/6 × (A₁ + A₂ + 4 × Aₘ), where A₁ is the bottom area, A₂ the top area and Aₘ the area at half depth. For our footing: 0.8/6 × (0.16 + 0.09 + 4 × 0.35 × 0.35) = 0.0987 m³, or 98.7 litres. Simply averaging the top and bottom areas gives 100 litres — a small error at a gentle slope, a bigger one at a steep slope.
| Item | Volume |
|---|---|
| One footing below ground | 98.7 l |
| Collar 300 × 300 × 100 mm | 9 l |
| One footing in total | 107.7 l |
| 6 footings | 0.646 m³ |
| 6 footings + 10% reserve | 0.711 m³ |
| Digging (below-ground part only) | 0.592 m³ |
The 10% reserve covers uneven hole walls, spillage and what is left in the mixer.
Concrete footingsOpen the pergola footings in the calculatorHow many 25 kg bags do I need?
It depends on the yield printed on the bag — how many litres of fresh concrete one bag makes. For its dry mix Beton B20 (strength class C16/20, suitable for foundations according to the manufacturer), Baumit Slovakia gives about 12.5 l per 25 kg bag. That means:
- 710.6 l ÷ 12.5 l = 56.85, so 57 bags.
- That is 1,425 kg of dry mix — a little more than one pallet, which Baumit lists as 54 bags.
- Other products yield less: Sakret gives 0.46–0.47 l/kg for its Premium-Beton, about 11.5 l from 25 kg. At 11.5 l per bag you would need 62 bags.
Digging: how much soil comes out
Only the part below ground counts: 6 × 98.7 l = 0.592 m³. The collar above ground needs no digging. The figure assumes you dig the hole to the exact shape of the footing; anything you dig wider either takes extra concrete or needs formwork.
For comparison: a straight 400 × 400 mm hole 800 mm deep with a 400 × 400 mm collar means 0.768 m³ of digging and, with the same reserve, 77 bags instead of 57. Tapering the sides saves 20 bags — half a tonne less to carry and mix.
Ready-mix or bags?
| 25 kg bags | Ready-mix | |
|---|---|---|
| Quantity | 57 bags (1,425 kg) | 0.75 m³ when ordered in 0.25 m³ steps |
| Work | mixed batch by batch in a mixer | straight from the truck, all at once |
| Keep in mind | you have to haul and carry the bags | truck access, minimum order and delivery |
For six footings, bags are often the practical choice: you pour one hole at a time and don't need access for a truck. Ready-mix suits you when you want to pour everything at once or have more footings. Order steps, minimum quantities and delivery charges vary by plant, so ask beforehand.
Set post anchors or bases into the fresh concrete as their manufacturer instructs, and cure the concrete as the bag says. For a taller or heavier pergola, for example one with a roof, have the footings sized by a structural engineer.