Sizu Home › Calculators › Post Holes and Concrete
Opens on the book's own fence. Change any number to your own and press Calculate.
Holes, group 1 (e.g. the gate posts)
Holes, group 2 (e.g. the line posts)
Bags
Enter the holes and press Calculate.
The formula
- Hole depth = post in the ground + gravel. Post length = in the ground + above
- Concrete per hole = π × (d ÷ 2)² × depth in the ground − post² × depth in the ground
- Bags per hole = concrete (ft³) ÷ bag yield, rounded up per hole; then add one spare for the job
- Gravel = π × (d ÷ 2)² × gravel depth, all holes together ÷ bag size, rounded up. 1 m³ = 35.315 ft³
The book's worked example, Chapter 3. Route A: two gate holes of 300 × 900 mm take 6 bags each and 3 line holes of 300 × 665 mm take 4: 12 + 12 + 1 spare = 25 bags of Fast-Setting (1,250 lb), and 4 bags of gravel. Route B, all 900 mm deep with a 6x6 hinge post in a 400 mm hole: 9 + 5 × 6 + 1 = 40 bags, a ton of concrete (2,000 lb), and 6 bags of gravel.
Fast-Setting mix sets in 20 to 40 minutes; leave it about 4 hours before you hang rails on it, and pour it in lifts no deeper than the bag says. Depth rules disagree: half the post's height above ground, a third of its length, your frost line and your city's minimum. Use the deepest that applies. Prices are illustrative US home-center prices from October 2026, shown without a currency sign; put in your own. Everything stays in your browser: no sign-up, nothing stored.