Practical guide
Concrete Bags per m³ and Fence Post Volumes
Calculate bags per cubic metre, convert area using depth and account for fence post displacement before whole-bag rounding.
Updated 16 October 2026
Reviewed by the 99Tools team. Editorial and corrections policy.
Bags per cubic metre
One cubic metre is 1000 L. Divide by the selected bag’s declared finished litres to obtain a nominal bags-per-m³ ratio. The ratio can be fractional, while a purchase needs whole bags. At a hypothetical 12 L per bag, the ratio is 83.333 bags/m³ and a one-m³ requirement rounds to 84 bags before extra allowance. A 40 kg or 20 kg label alone does not establish this yield.
Apply purchasing rules to the actual volume
Use the combined project requirement rather than rounding every small section independently. Add one extra allowance only when it is not already included, then round whole bags and full purchase packs. A quoted pack size can increase supplied volume beyond the selected allowance. Do not use nominal bags-per-m³ as an instruction to round every cubic metre separately and then repeat the result across a project.
Fence post hole volume
For a round hole with a concentric round post, concrete-filled volume is π × (hole diameter² − post diameter²) ÷ 4 × filled depth. Use metres in this formula. For a rectangular hole and rectangular post, subtract their cross-sectional areas before multiplying filled depth. Measure the actual filled zone and all holes. Post footing geometry and concrete specification must come from the design; the calculator does not choose these dimensions.
A fence post arithmetic example
Four hypothetical round holes with 300 mm hole diameter, 100 mm post diameter and 600 mm filled depth contain about 0.150796 m³ after displacement. A chosen 5% allowance gives about 0.158336 m³. At an illustrative 12 L finished yield, the combined order rounds to 14 bags. These are geometry fixtures, not fence footing recommendations or verified product yields. Different post shapes, sloped holes and unfilled zones require the actual take-off.
Bags for 80 square feet
Convert area to m² before applying the specified depth. Eighty square feet is 7.4322432 m². At a hypothetical 100 mm depth, geometric concrete volume is 0.74322432 m³ before allowance. Then use the declared bag yield. Neither 80 square feet nor one square metre has a universal bag count without depth. The depth in this example is arithmetic only and is not a slab design.
Strength labels and cement-only recipes
A 25 MPa or high-strength label does not replace declared yield, mixing instructions or the required application. M15 and 3000 psi searches likewise do not establish one cement bag count. Cement-only batching requires confirmed cement, sand, aggregate and actual batch yield; use the recipe option in the main Concrete Calculator. A material quantity result does not approve a retaining wall assembled from unopened bags or resolve drainage and stability.
Use the related calculators
Apply the explanation above with your own figures. Open a calculator below, check every input and review the result assumptions before making a decision.
Frequently asked questions
Why subtract a fence post from the hole?
The buried post displaces concrete in the filled zone. Use the actual hole and post geometry.
Can I use 40 kg to infer litres?
No. Enter the product’s declared finished yield.
Should I round each hole to bags?
For one combined pour/order, add the concrete volumes before whole-bag rounding.
Does the calculator design a bag retaining wall?
No. A retaining structure requires a suitable engineered system and site assessment.