How much concrete goes around a fence post?
Subtract the area occupied by the post from the area of the round hole. Multiply by the depth filled with concrete. A square post uses width squared. A round post uses π × radius². Use the measured post width and the depth filled with concrete above any gravel.
Worked example for four 12-inch holes
A 12 in diameter hole filled 24 in deep around a 3.5 in square post takes about 1.40 ft³. Four holes take about 5.60 ft³ before allowance and 6.16 ft³ with 10% extra. At 0.60 ft³ per bag, round up to 11 bags for the combined pour. If you buy full bags separately for each hole, allow three bags per hole.
Worked metric example · six 300 mm holes
Each hole is 300 mm wide and filled 600 mm deep around a 90 mm square post. The empty hole holds about 42.41 L. The post displaces 4.86 L, leaving about 37.55 L of concrete per hole before allowance. With 10% extra, six holes need about 248 L.
| How you use the mix | How to round | Bags to buy |
|---|---|---|
| Share bags across holes | Round 248 ÷ 10 up once | 25 |
| Use whole bags at each hole | Round about 41.3 ÷ 10 up for each hole | 30 |
Which depth and post width should I enter?
Measure the depth that will actually contain concrete. For a 700 mm hole with 100 mm of gravel at the bottom and concrete to grade, enter 600 mm. Enter the actual post width or diameter. The corners of a square post must also fit inside the round hole.
The calculator assumes the post occupies the full filled depth. Use the hole dimensions, post embedment, and mix specified for your project.
Presets use approximate mixed yields from QUIKRETE, Australian Builders, and SAKRETE Canada. For other products, use the mixed yield on the label.
For a round pier without a post, enter zero for the post width. If you already know the total volume, use the concrete bag calculator. For other shapes, use the concrete volume calculator.