How it works
For each hole the calculator takes the area of the hole, subtracts the post, multiplies by the depth above any gravel base and then by the number of holes.
Concrete height = Hole depth − Gravel base
Per hole = (π × (Hole diameter ÷ 2)² − Post cross-section) × Concrete height
Concrete = Per hole × Number of holes × (1 + Extra ÷ 100)
Bags = ⌈ Concrete ÷ Yield per bag ⌉
Gravel base = π × (Hole diameter ÷ 2)² × Gravel base × Number of holes
⌈ ⌉ means rounding up to the next whole bag. The volumes themselves are not rounded.
The post cross-section is side × side for a square post, or π × (diameter ÷ 2)² for a round one. The default post of 3½ in is the dressed size of a nominal 4 × 4 (AWC NDS Supplement Table 1A), and the default yield of 0.60 cu ft is QUIKRETE’s figure for an 80 lb bag – change both to match what you use. The default hole size is only a starting value for the form, not a recommendation: hole diameter, depth and footing come from your local code and the post or fence maker.
Worked example
Inputs
- Hole diameter
- 10 in
- Hole depth
- 3 ft
- Post shape
- Square
- Post size
- 3 1/2 in
- Gravel base
- 0 in
- Number of holes
- 4
- Yield per bag
- 0.6 cu ft
- Extra concrete
- 0 %
Result
- Concrete
- 5.52 cu ft (0.205 cu yd)
- Concrete per hole
- 1.38 cu ft
- Bags
- 10
- Gravel base
- 0 cu ft (0 cu yd)
Common mistakes
- Entering the nominal post size. A “4 × 4” is 3½ × 3½ in when dressed (AWC NDS Supplement Table 1A); measure larger posts, because timbers are dressed differently.
- Taking a hole depth from a rule of thumb. Depth depends on frost depth, soil, post height and load – follow your local code and the post or fence maker.
- Forgetting that dug holes flare and wander. A hole that ends up wider than planned takes noticeably more concrete; add an extra allowance.
- Assuming all bags of the same weight yield the same volume. Read the yield on the bag you are buying.
Assumptions and limits
- Each hole is a straight cylinder of the diameter you enter, and every hole is the same size.
- The post stands on the gravel base (or the bottom of the hole) and its full cross-section is subtracted over the concrete height.
- Concrete fills the hole from the top of the gravel base to the top of the hole. Gravel is the full hole cross-section × base depth.
- Bags are rounded up to whole bags after the extra allowance. This is a material quantity only, not a structural design.
Frequently asked questions
How deep should a post hole be?
It depends on your local code, frost depth, the soil and what the post carries. This calculator doesn’t give depths – ask your building department and follow the fence or deck maker’s instructions, then enter that depth here.
How many bags of concrete per post?
Run the calculator with your hole size and post size. In the example of four 10 in holes 3 ft deep with 3½ in square posts, each hole takes about 1.38 cu ft; with QUIKRETE’s 0.60 cu ft yield for an 80 lb bag that is 10 bags for the four holes.
Why does the post size matter?
The post takes up part of the hole, so less concrete goes in. The calculator subtracts the post’s actual cross-section, which is why you should enter the dressed size rather than the nominal one.
Does the gravel base count toward the hole depth?
Yes. Enter the total hole depth and the gravel depth separately; concrete is calculated only above the gravel, and the gravel volume is shown on its own line.
Sources
- QUIKRETE Concrete Mix (No. 1101) Product Data Sheet — The QUIKRETE Companies. Yield per bag: 40 lb ≈ 0.30 ft³, 50 lb ≈ 0.375 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³, 90 lb ≈ 0.675 ft³.
- NDS Supplement (2018), Table 1A – Nominal and Minimum Dressed Sizes of Sawn Lumber — American Wood Council. Dry dressed sizes: nominal 1 in → 3/4 in, 2 in → 1-1/2 in; widths 4 → 3-1/2 in, 6 → 5-1/2 in.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 2, 2026