Post Hole Concrete Calculator
Concrete to set fence, deck and mailbox posts. The hole should be three times the post width and a third of the post’s length deep (or below frost). The post displaces concrete, so the calculator subtracts it — a 4×4 in a 10″ × 30″ hole is about two 80 lb bags.
Buy 17 × 80 lb bags — under 1 yd³ bagged mix beats a short load.
- Net volume (9.2 ft³)
- 0.34 yd³
- Waste 10%
- + 0.03 yd³
- Ready-mix ticket
- 0.50 yd³
- Approx. weight
- 1,381 lb
- 80 lb bags
- 17
- 60 lb bags
- 23
- 40 lb bags
- 34
- Materials + delivery$6–$9 / bag · Sep 2026
- $102 – $153
- Installed (with labor)typ. $363
- $252 – $528
Quantities follow the ticket. Links may earn us a commission at no cost to you; they never change a number.
How to calculate post hole by hand
Per hole (ft³) = π × (Hole dia ÷ 24)² × Depth (ft) − (Post width ÷ 12)² × Depth (ft) · bags = ft³ ÷ 0.60
Worked example
- Hole: π × (10 in ÷ 24)² × 2.5 ft = 1.36 ft³.
- Post: (3.5 in ÷ 12)² × 2.5 ft = 0.21 ft³.
- Concrete per hole: 1.36 − 0.21 = 1.15 ft³ → 1.9 × 80 lb bags, call it 2.
- Eight posts: 9.2 ft³ + 10% = 10.1 ft³ → 17 × 80 lb bags (0.37 yd³).
- 4×4 post, 10″ × 30″ hole
- ≈ 2 × 80 lb bags
- 4×4 post, 12″ × 36″ hole
- ≈ 3 × 80 lb bags
- 6×6 post, 12″ × 36″ hole
- ≈ 2.5 × 80 lb bags
- 6×6 post, 18″ × 42″ hole
- ≈ 7 × 80 lb bags
Questions crews ask
How many bags of concrete do I need per fence post?
For a 4×4 post in a 10″ wide, 30″ deep hole, about 1.15 ft³ of concrete — two 80 lb bags, or three 60 lb, or four 40 lb. A 6×6 in a 12″ × 36″ hole needs about 2.5 × 80 lb bags. Round up per hole, not just in total.
How big should a post hole be?
Three times the post width in diameter (10–12″ for a 4×4, 16–18″ for a 6×6) and one-third of the post’s total length deep — 24–30″ for a 6 ft fence, 36″+ for gate posts — and always below the frost line. Flare or bell the bottom for uplift resistance.
Do I mix the concrete or pour it dry?
Fast-setting mix (Quikrete Fast-Setting, Sakrete Fast Set) is designed to be poured dry around the post and watered in — about a gallon of water per 50 lb bag — and sets in 20–40 minutes. Standard mix should be mixed first. Either way, set the post on 6″ of gravel so the end drains.
How much concrete for a 6x6 post?
In a 12″ × 36″ hole: hole 2.36 ft³ minus post 0.63 ft³ = 1.73 ft³ → 3 × 80 lb bags. In an 18″ × 42″ hole for a gate or deck post: 6.2 − 0.74 = 5.4 ft³ → 9 × 80 lb bags. Above about 30 holes it is a short-load ready-mix job.
Should the concrete come above grade?
Yes, crown it an inch above grade and slope it away from the post so water sheds. Concrete stopped below grade forms a cup that holds water against the wood and rots the post at the ground line.