How It Is Calculated
This calculator determines the volume of concrete needed to fill the hole around a fence post. The calculation subtracts the volume of the post from the volume of the hole, then adds a waste factor to account for spillage or over-excavation.
First, convert all dimensions to inches (they are already in inches). For each post, calculate the hole volume and post volume:
Hole volume = π × (d / 2)² × h
where d = hole diameter (in), h = post depth (in)
If post is round: Post volume = π × (p / 2)² × h
If post is square: Post volume = p² × h
where p = post diameter or width (in)
Concrete volume per post = hole volume − post volume (cubic inches). Multiply by the number of posts to get total before waste. Then apply the waste factor:
Total with waste = Total before waste × (1 + w / 100)
where w = waste factor (%)
Convert to cubic feet by dividing by 1728, and to cubic yards by dividing by 27. Finally, divide by the bag yield (0.33 ft³ for 40 lb, 0.45 ft³ for 60 lb, 0.6 ft³ for 80 lb) and round up to the nearest whole bag.
Assumptions
- The hole is cylindrical and vertical.
- The post is centered in the hole.
- The post shape is either round or square as selected.
- The depth entered is the full depth of the hole below ground.
- The waste factor accounts for spillage and over-excavation.
- Bag yields are approximate: 40 lb bag yields 0.33 ft³, 60 lb yields 0.45 ft³, 80 lb yields 0.6 ft³.
- The calculator does not include concrete for a base or collar above ground.
Worked Example
For 1 post, depth 24 in, hole diameter 8 in, round post with diameter 4 in, waste factor 10%, and 60 lb bags:
Hole volume = π × (8/2)² × 24 = π × 16 × 24 = 1206.37 in³
Post volume = π × (4/2)² × 24 = π × 4 × 24 = 301.59 in³
Concrete per post = 1206.37 − 301.59 = 904.78 in³
Total before waste = 904.78 in³
With waste = 904.78 × 1.10 = 995.26 in³
In cubic feet = 995.26 / 1728 = 0.58 ft³
In cubic yards = 0.58 / 27 = 0.021 yd³
Bag yield for 60 lb = 0.45 ft³
Number of bags = ceil(0.58 / 0.45) = ceil(1.29) = 2 bags
Practical Tips
- Always dig the hole slightly wider than the post to allow for concrete flow and proper setting.
- Use a waste factor of 10% to 15% to account for spillage and irregular hole shapes.
- For heavy gates or tall fences, consider using a larger hole diameter for extra stability.
- Check local building codes for minimum post depth and hole size requirements.
- If you are also planning a concrete slab or driveway, use the Concrete slab calculator or Concrete driveway cost calculator for those projects.
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Calculators that go with this one:
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- Fence post calculator — Calculate the number of fence posts and concrete needed for any straight fence
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- Concrete calculator — Estimate the amount of concrete needed for a slab and/or fence posts
- Concrete foundation calculator — Estimate the concrete volume, weight, and material cost for a rectangular slab foundation
- Deck board calculator — Calculate the number of deck boards needed for your deck project