Concrete Pier Calculator — m, cm to m³, bags, cost

How to Calculate Concrete for Piers and Sonotubes (Metric)

Deck footings, fence posts, porch piers and pole-frame columns are poured into cardboard tube forms or square box forms. The shaft volume of a round form is a plain cylinder, so a small over-order per hole multiplies quickly across a footing layout. Set the shape, enter the form diameter (or square side) and the hole depth in m, cm or mm, and the tool returns cubic metres, 25 kg and 50 kg bag counts, weight in kg and cost in your selected currency using the same takeoff a foundation crew uses on site.

Depth must reach below the local frost line — most of Central and Northern Europe needs 80 cm to 120 cm, and your local building authority publishes the exact figure. A 5–10% waste allowance covers form over-fill, spillage and slightly oversized augered holes; raise it toward 15% for hand-dug holes. For continuous footings use the strip footing calculator or the spread footing calculator; for connecting beams across a pier grid, see the grade beam calculator.

Formulas

Round pier: V = π × (d/2)² × h. Square pier: V = s² × h. Bell footing (frustum): V = (π × h / 3) × (R² + R·r + r²) − π·r²·h, where R = flare radius and r = pier radius (the overlapping shaft is subtracted to avoid double-counting). A 50 kg bag of dry mix yields about 0.024 m³ and a 25 kg bag about 0.012 m³ of placed concrete. Concrete density is taken as 2,400 kg/m³.

Reinforcement per EN 1992-1-1 (Eurocode 2)

Pick vertical bars Ø8 through Ø20 (B500B) with 2–6 bars per pier plus horizontal ties. Nominal cover is set per exposure class (XC1–XC4, XD/XS per EN 1992-1-1 §4.4.1) and bars run the full hole depth plus a 150 mm embedment; lap splices follow roughly 40·ø. Standard bar stock length is 6 m. For reinforced caps over pile groups cross-check the pile cap calculator, compare bag-versus-truck pricing with the concrete bag calculator, and size flatwork tied to these piers with the concrete slab calculator.

Frequently Asked Questions

How deep should piers be? Below the frost line. In Central Europe common depths run 80 cm to 120 cm; milder coastal zones allow shallower. Always confirm the code-mandated frost depth with your local building authority.

When should I add a bell footing? When soil bearing capacity is low or column loads are high, a flared (belled) base spreads the load over more soil. Typical bell diameters are 1.5× to 2× the shaft diameter; the calculator subtracts the overlapping shaft so the frustum volume is not double-counted.

Bags or ready-mix? Bagged concrete is practical up to roughly 1 m³ total; beyond that, ready-mix delivery is usually cheaper per cubic metre and far less labour. Toggle the cost section between 25/50 kg bags and ready-mix to compare.

On pour day

Three things crews forget when pricing piers. Minimum-load fees — most ready-mix plants charge €60–120 surcharge when you order under ~2 m³, so a deck with 6 piers at Ø250 mm × 1.0 m (≈0.3 m³) almost always costs less in 25/50 kg bags. Bell over-dig — augered flares dig wider than the form, so use 10–15% waste, not 5%, on any hole with a belled base. Vertical bar projection — verticals must extend above grade enough to lap into the post base, column or beam above (typically 150–300 mm plus a 40·ø splice per EN 1992-1-1 §8.7), so order bars at depth + projection, not just the hole depth. Typical 2025 EU prices: €6–9 per 50 kg bag, €110–160 per m³ delivered (min-load surcharge below ~2 m³), €2.30–3.20 per linear metre of Ø12 B500B rebar. Save runs to History and export with the diagram as text, CSV, A4 JPG, or PDF — estimates are planning numbers, not a substitute for a structural drawing.