How It Is Calculated
This calculator estimates the total volume of concrete needed for a rectangular slab and optionally for concrete around fence posts. The calculation is done in cubic feet, then converted to cubic yards and to the number of pre-mixed bags.
Slab Volume
If you provide a slab length L (ft), width W (ft), and thickness T (in), the slab volume in cubic feet is:
Slab volume (ft³) = L × W × (T / 12)
Post Hole Concrete Volume
For each post, we calculate the volume of the cylindrical hole and subtract the volume of the square post. The hole diameter D (in) and depth H (in) give the hole volume. The post width P (in) gives the post volume. Concrete per post is the difference (if negative, it is set to zero).
Hole volume (ft³) = π × (D / 24)² × (H / 12)
Post volume (ft³) = (P / 12)² × (H / 12)
Concrete per post (ft³) = max(0, hole volume – post volume)
Total post concrete = concrete per post × number of posts N.
Total Volume and Waste
Net volume = slab volume + total post concrete. Then add a waste factor F (%).
Total volume (ft³) = net volume × (1 + F / 100)
Convert to cubic yards by dividing by 27.
Number of Bags
Standard bag yields are used: 40 lb bag = 0.30 ft³, 60 lb bag = 0.45 ft³, 80 lb bag = 0.60 ft³. The number of bags is rounded up to the nearest whole bag.
Bags = ceil( total volume (ft³) / yield per bag )
Assumptions
- Concrete mix yields: 40 lb bag covers 0.30 ft³, 60 lb bag covers 0.45 ft³, 80 lb bag covers 0.60 ft³.
- Posts are square and centered in cylindrical holes.
- Slab thickness is uniform.
- Waste factor accounts for spillage, over-excavation, and uneven subgrade.
- All dimensions are in US customary units.
Worked Example
Inputs: Slab length = 10 ft, Slab width = 10 ft, Slab thickness = 4 in, Number of posts = 0, Hole diameter = 8 in, Hole depth = 24 in, Post width = 4 in, Waste factor = 10%.
Step 1 – Slab volume: 10 × 10 × (4/12) = 10 × 10 × 0.3333 = 33.33 ft³.
Step 2 – Post concrete: Since number of posts = 0, total post concrete = 0 ft³.
Step 3 – Net volume: 33.33 + 0 = 33.33 ft³.
Step 4 – Waste: 33.33 × (1 + 10/100) = 33.33 × 1.10 = 36.67 ft³.
Step 5 – Cubic yards: 36.67 / 27 = 1.36 yd³.
Step 6 – Bags: 40 lb: ceil(36.67 / 0.30) = ceil(122.23) = 123 bags. 60 lb: ceil(36.67 / 0.45) = ceil(81.49) = 82 bags. 80 lb: ceil(36.67 / 0.60) = ceil(61.12) = 62 bags.
Results: Total volume = 36.67 ft³, Total volume = 1.36 yd³, 40 lb bags = 123, 60 lb bags = 82, 80 lb bags = 62.
Practical Tips
- Always order a little extra concrete to account for uneven ground or spillage – the waste factor helps with that.
- For large slabs, consider ordering ready-mix concrete by the cubic yard instead of bags.
- If you are also building a deck, you might need to calculate the concrete for footings separately – see our Decking Calculator for deck post requirements.
- For a patio or walkway, you can use the Paint Calculator to estimate paint for the surface after the concrete cures.
- Mix concrete in a wheelbarrow or mixer; add water gradually to achieve a workable consistency.