How the concrete column calculator works
A round column is a cylinder, so its volume is the area of the circle times the height. The circle area is π × radius², and the radius is half the diameter printed on the tube form. Because the radius is squared, a small change in diameter makes a big difference: a 30 cm tube holds more than twice as much as a 20 cm tube of the same height.
Volume = π × (diameter ÷ 2)² × height × number of columns × 1.10The 10% allowance covers the tube swelling slightly when full, the bell of concrete that spreads at the bottom of a hole that was dug wider than the tube, and spillage when filling through a narrow opening. Bags are counted from the usual yield of dry premix: about 2,130 kg per cubic metre, so a 25 kg bag gives roughly 11.7 litres and an 80 lb bag about 0.60 ft³.
Worked example
Six deck piers use 30 cm tubes cut to 1.2 m. One tube holds π × 0.15² × 1.2 = 0.0848 m³, which is 84.8 litres. Six of them hold 0.509 m³, or 0.560 m³ with the allowance. At 2,130 kg per cubic metre that is about 1,190 kg of premix: 48 bags of 25 kg, roughly eight per hole. The imperial version, six 12 in tubes 4 ft tall, holds 6 × 3.14 = 18.85 ft³, or 20.7 ft³ (0.77 yd³) with the margin, which is 35 bags of 80 lb.
Volume of standard tube forms
| Tube diameter | Litres per metre | ft³ per foot | 80 lb bags per 4 ft tube |
|---|---|---|---|
| 6 in (15.2 cm) | 18.2 | 0.196 | 1.3 |
| 8 in (20.3 cm) | 32.4 | 0.349 | 2.3 |
| 10 in (25.4 cm) | 50.7 | 0.545 | 3.6 |
| 12 in (30.5 cm) | 73.0 | 0.785 | 5.2 |
| 16 in (40.6 cm) | 129.7 | 1.396 | 9.3 |
| 18 in (45.7 cm) | 164.2 | 1.767 | 11.8 |
| 24 in (61.0 cm) | 291.9 | 3.142 | 20.9 |
Bag figures in the table exclude the 10% margin. Tubes are commonly sold in lengths of 4 ft and 12 ft in North America and in 1 m to 3 m lengths in Europe; cut them with a fine saw and keep the offcuts for short piers.
Tips for pouring piers and columns
- Count the buried part. A pier usually runs from below the frost line to a little above ground, so the height to enter is the whole tube, not the visible stub.
- Add the footing pad. If each tube sits on a wider cast base or a plastic bell footing, work that volume out separately and add it; a large bell can hold a big share of the total.
- Fill in lifts. Pour in layers of about 30–50 cm (12–18 in) and rod or tap each layer to release trapped air, then set post anchors while the top is still workable.
- Brace the tubes. Backfill and tamp soil around the tube before pouring, and brace tall columns on two sides so they stay plumb when full and heavy.
- Get sizing checked. Diameter, depth, reinforcement and spacing of piers depend on the load, the soil and the frost depth where you live. Use the figures in your local code or from an engineer; this calculator only tells you how much concrete a chosen size needs.
Frequently asked questions
How many bags of concrete for a 12 inch tube?
A 12 in tube holds 0.785 ft³ per foot of height, so a 4 ft tube holds 3.14 ft³. That is about 5.2 bags of 80 lb, or 6 bags once you allow for spillage.
How many 80 lb bags of concrete for an 8 inch tube form?
An 8 in tube holds 0.349 ft³ per foot. A 4 ft length takes 1.40 ft³, which is 2.3 bags of 80 lb; buy 3 per hole or pool the leftovers across several piers.
How much concrete is in a 30 cm column 1 m tall?
π × 0.15² × 1 = 0.0707 m³, or about 71 litres. That is six bags of 25 kg or four bags of 40 kg per metre of column, before any allowance.
What size tube form do I need for a deck footing?
Small decks often use 8 to 12 in (20–30 cm) tubes, but the right size depends on the load per post and the soil bearing capacity. Check the span tables in your local code or ask the building office.
Should I use bags or ready-mix for piers?
Bags suit most deck and fence jobs because each hole takes only a few bags. Above roughly half a cubic metre (two-thirds of a yard) spread over many holes, a mixer or a small ready-mix load becomes easier.
Last reviewed: 2026-09-27 · How we calculate