Before pouring footings or setting fence and deck posts you need to know how much soil comes out of each hole and how much concrete goes back in. If the post sits in the hole, its volume is subtracted; a few inches of gravel at the bottom helps drainage.
How to use the Hole Volume Calculator
- Hole shape
- Diameter / length
- Width
- Depth
- Number of holes
- Post in hole
- Gravel base depth
- Premix bag size
- Price per bag
- Every result field is editable: type into Total hole volume and Depth plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose round or rectangular holes, enter the size, depth and number of holes, the post size if one is set in the concrete, the gravel base depth and bag size.
Premixed bag yields
| Bag | Yield | Bags per yd³ |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
| 25 kg | ≈ 0.0117 m³ | ≈ 85 per m³ |
Tips for an accurate estimate
- Measure the formwork, not the drawing – forms bulge and subgrades are rarely perfectly flat, so add 5–10%.
- Below about 1 cubic yard, premixed bags are usually cheaper; above that, ready-mix saves time and gives a more consistent pour.
- Order ready-mix in quarter- or half-yard steps and ask about short-load fees for small deliveries.
- Keep fresh concrete damp for at least a week – curing matters as much as the mix for final strength.
Common mistakes to avoid
- Running short mid-pour, which leaves a weak cold joint.
- Adding extra water on site to make concrete easier to place – it lowers strength and increases cracking.
- Ignoring the volume of rebar, posts or pipes in the pour, or the gravel base under it.
Inputs explained
| Field | What to enter |
|---|---|
| Hole shape | Choose one: Round, Rectangular |
| Diameter / length | Units: in, cm, mm, ft, m |
| Width | Units: in, cm, mm, ft, m |
| Depth | Units: in, cm, mm, ft, m |
| Number of holes | A number |
| Post in hole | Choose one: None, 4×4 post (3½ in), 6×6 post (5½ in), Round post 4 in … |
| Gravel base depth | Units: in, cm, mm, ft, m |
| Premix bag size | Choose one: 40 lb bag (0.30 ft³), 50 lb bag (0.375 ft³), 60 lb bag (0.45 ft³), 80 lb bag (0.60 ft³) … |
| Price per bag | Units: /piece |
What each result means
- Total hole volume = [Hole area]×[Depth]×[Number of holes]
- Soil removed (≈ 25% swell) = [Total hole volume]×1.25
- Concrete needed = max([Hole area] – [Post in hole]^2, 0)×max([Depth] – [Gravel base depth], 0)×[Number of holes]
- Premix bags = ceil([Concrete needed]÷[Premix bag size])
- Concrete cost = [Premix bags]×[Price per bag]
Formula
Round hole: V = π/4 × D² × depth; rectangular: V = L × W × depthConcrete = (hole − post − gravel base) × holesWorked example
Inputs — Hole shape: Round; Diameter / length: 10 in; Width: 10 in; Depth: 36 in; Number of holes: 10; Post in hole: 4×4 post (3½ in); Gravel base depth: 6 in; Premix bag size: 80 lb bag (0.60 ft³); Price per bag: $6.5 /piece.
Results — Hole area = 78.5 in²; Total hole volume = 0.606 yd³; Soil removed (≈ 25% swell) = 0.758 yd³; Concrete needed = 11.51 ft³; Premix bags = 20; Concrete cost = $130.00.
Frequently asked questions
How big should a post hole be?
About three times the post width in diameter (10–12 in for a 4×4) and one-third of the post length deep, below the frost line.
How much does soil expand when dug?
Loose excavated soil takes up 20–30% more volume than in the ground (swell).
Why put gravel in the bottom?
A 4–6 in gravel base lets water drain away from the end of the post.
What formula does the Hole Volume Calculator use?
Round hole: V = π/4 × D² × depth; rectangular: V = L × W × depth.