Fence posts are usually set in holes three times the post width and one-third of the post length deep, with a few inches of gravel under the post for drainage. Fast-setting concrete is popular because you can pour it dry into the hole and add water, but it yields slightly less per bag than regular premix.
How to use the Fence Post Concrete Calculator
- Number of posts
- Hole diameter
- Hole depth
- Post size
- Gravel base depth
- Waste
- Premix bag size
- Price per bag
- Every result field is editable: type into Total concrete and Hole 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
Enter the number of posts, the hole diameter and depth, the post size, the gravel base depth, bag size and price. The calculator subtracts the post from the hole, multiplies by the concrete depth and divides by the bag yield.
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
- Take measurements at floor level and at the ceiling – walls and rooms are often slightly out of square.
- Use the manufacturer’s coverage, spacing and installation data when it differs from the typical values here.
- Keep a little extra of every finish material for touch-ups and repairs later.
- Check local building codes and permits before structural, electrical, plumbing or gas work.
Common mistakes to avoid
- Sizing heating or cooling equipment by area alone when insulation, windows and climate make a big difference.
- Buying too little paint, stain or flooring and ending up with mismatched batches.
- Overlooking the extra material for steps, railings, gables and trim.
Inputs explained
| Field | What to enter |
|---|---|
| Number of posts | A number |
| Hole diameter | Units: in, cm, mm, ft, m |
| Hole depth | Units: in, cm, mm, ft, m |
| Post size | Choose one: 4×4 (3½ in), 6×6 (5½ in), 4×6 (3½ × 5½ in), 2⅜ in round steel … |
| Gravel base depth | Units: in, cm, mm, ft, m |
| Waste | In % |
| 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
- Concrete per post = (pi÷4×[Hole diameter]^2 – [Post size]^2)×max([Hole depth] – [Gravel base depth], 0)
- Total concrete = [Concrete per post]×[Number of posts]×(1 + [Waste]÷100)
- Bags per post = [Concrete per post]÷[Premix bag size]
- Total bags = ceil([Total concrete]÷[Premix bag size])
- Cost = [Total bags]×[Price per bag]
Formula
Concrete per post = (π/4 × hole ز − post area) × (hole depth − gravel)Bags = total concrete ÷ bag yieldWorked example
Inputs — Number of posts: 20; Hole diameter: 10 in; Hole depth: 30 in; Post size: 4×4 (3½ in); Gravel base depth: 4 in; Waste: 10 %; Premix bag size: 80 lb bag (0.60 ft³); Price per bag: $6.5 /piece.
Results — Concrete per post = 0.997 ft³; Total concrete = 21.94 ft³; Bags per post = 1.66; Total bags = 37; Cost = $240.50.
Frequently asked questions
How many bags of concrete per fence post?
For a 4×4 post in a 10 in hole 30 in deep (4 in gravel), about 1.2 ft³ – two 80 lb bags or three 50 lb bags.
How deep should fence posts be?
One-third to one-half of the above-ground height, and below the frost line in cold climates.
Should I use fast-setting concrete?
It is convenient for posts; regular premix is cheaper and slightly stronger.
What formula does the Fence Post Concrete Calculator use?
Concrete per post = (π/4 × hole ز − post area) × (hole depth − gravel).