Example: six forms, 300 mm inside diameter and 1.2 m fill
One form = π × 0.15² × 1.2 = 0.084823 m³. Six forms = 0.508938 m³. An illustrative 10% allowance gives 0.559832 m³. If the supplier accepts 0.1 m³ order increments, round to 0.6 m³. That increment is an ordering assumption, not a truck's capacity.
Measure the space that concrete will occupy
Use the form's inside diameter and actual concrete fill height. Tube outside diameter includes the form material and is not the same measurement. If the tube extends above the finished concrete, do not include that empty length. Record the diameter and height together with how many forms share those dimensions.
A solid tube differs from a post hole containing an embedded post. Where another object occupies space, its displaced volume must be deducted if that model is appropriate. The post-hole tool handles a defined annular fill; the solid-tube tool deliberately does not deduct a post.
Square the radius, not the diameter
Convert 300 mm diameter to 0.300 m, then divide by two for a 0.150 m radius. A common error is using the whole diameter as the radius, which multiplies the volume by four. Convert the height into metres before multiplying so the result is cubic metres.
Doubling the diameter also multiplies the required volume by four at the same height. Doubling the height doubles the volume. This is why a modest diameter correction matters more than an equally small-looking change to the height.
Keep the base and exposed shaft distinct
The straight-cylinder formula does not include a flared footing, bell, enlarged base or rectangular cap. Measure each additional shape separately, taking care not to count an overlapping portion twice. A form set inside an excavated hole may also have additional concrete outside the tube if the detail requires it.
Count reinforcement, form length, ties, supports and hardware as separate items. Their design is not inferred from the calculated concrete quantity. Use the current drawing and approved installation detail to define the shape you are measuring.
Turn the combined volume into an order
Add the unrounded volume of all tubes before applying your documented loss allowance. If different tubes use the same mix during one pour, purchase rounding belongs on the combined order. Rounding each small tube to a truck increment can substantially overstate the amount.
For bags, divide the allowance quantity by the selected complete concrete-mix bag's published yield and round up to a whole bag. A cement bag is not a complete concrete bag. Compare the workable mixing and placement plan as well as material price.
Do it in order
- Record inside diameter, fill height and count for every form size.
- Calculate the combined net cylindrical volume and add separately measured bases.
- Apply one chosen allowance, then supplier increments or whole-bag rounding.
Planning boundary
This quantity method does not select pier diameter, foundation depth, reinforcement or load capacity.
Sources checked
Do I enter tube height or hole depth?
Enter the concrete-filled height of the solid tube. Any different hole or footing geometry must be measured separately.
Can I use the outside diameter?
Use the internal diameter that encloses the concrete. The outside diameter includes the form wall.