Measure the hole and gravel
Enter the total hole depth; a separate gravel layer reduces the depth filled with concrete. Do not subtract gravel twice.
Measure it this way Not to scale · Measure the actual space to be filled, not a guessed outside dimension.
Post hole concrete estimate
Choose a fence, gate, deck, mailbox or custom post application. Only the concrete volume is calculated—structural hole sizing is not.
The calculation method
How much concrete goes around a fence post?
Subtract the physical volume occupied by a buried post from the portion of the hole actually filled with concrete.
Measure the actual post
A nominal 4×4 piece of dimensional lumber is often approximately 3.5×3.5 inches in cross section. Use actual measurements for the post you have.
Count material two ways
A shared mixing job can pool leftover mix across holes, while separately opening complete bags for each hole gives a potentially larger whole-bag total.
Worked example
Worked example: ten holes around 4×4 posts
Consider ten round 12-inch holes, 2 feet of concrete fill, and actual 3.5-inch square posts embedded through the full concrete height. The concrete is the difference between the cylinder volume and the volume occupied by the post.
Check the arithmetic
- Per-hole net volume: π × (0.5 ft)² × 2 ft − (3.5/12 ft)² × 2 ft ≈ 1.400657 ft³
- Ten holes: ≈ 14.00657 ft³ before allowance
- 0.60 ft³ per bag, no allowance: 24 bags when pooled; 3 × 10 = 30 bags when rounded by hole
Review the assumptions. These are mathematical examples, not live material orders, current supplier quotes or structural design recommendations.
Buying and measurement guidance
Fence, gate, deck and mailbox post scenarios
The same geometry can be useful for multiple post-hole projects, but the required structural dimensions are not interchangeable.
Round or square hole
Choose the actual shape of the excavated space. A square post inside a circular hole requires the square’s diagonal to fit.
Gravel does not become concrete
If the hole has 4 inches of gravel below the concrete, enter it separately; gravel itself is not included in poured concrete volume.
Local requirements still apply
Frost-line depth, local soil conditions, fence loads and deck support specifications need locally applicable guidance and often professional review.
Wood fence post examples · Quantity only
How much concrete around nominal 4×4 and 6×6 posts?
These are illustrative hole sizes, not structural recommendations. Each example assumes a 36-inch-deep round hole with a separate 6-inch gravel layer, leaving 30 inches filled with concrete around an embedded square wood post.
| Nominal wood post | Actual post width | Example round hole | Net concrete / hole | 80 lb bags / hole | 80 lb bags / 10 holes* |
|---|---|---|---|---|---|
| 4×4 | 3.5 in | 12 in × 36 in deep | 1.75 ft³ | 3 | 30 |
| 6×6 | 5.5 in | 18 in × 36 in deep | 3.89 ft³ | 7 | 65 |
*Ten-hole bag totals round the combined net volume once and assume a common batch of mix. If mixing separately and rounding each hole, total bags may be higher. The 80 lb bag yield is approximately 0.60 ft³ for the named QUIKRETE Concrete Mix No. 1101. Measure actual post dimensions: nominal wood sizes are not always actual sizes. Enter your project's correct geometry, gravel and quantity in the fence post calculator. Follow local requirements for depth, load and frost conditions.
Next project decision
Check what actually takes up space in each post hole
A post and any separate gravel base take up volume that should not be charged as concrete; hole shape also changes the calculation.
- Record round-hole diameter or square-hole side length accurately, including the planned gravel base depth.
- Subtract the embedded post portion, and compare buying bags for the full job with rounding separately per hole.
- Depth and footing design are subject to local conditions and requirements; this is quantity planning only.
Frequently asked questions
Fence Post Concrete Calculator FAQs
Answers to real measurement and purchase questions. Product data and field conditions can change actual requirements.
Do I subtract the volume of my wooden post?
Yes, but only for the part of the post that physically occupies the concrete-filled area of the hole.
Why are pooled bags different from bags per hole?
The pooled estimate assumes unused mix can be shared across holes; per-hole rounding assumes each hole uses only complete bags allocated to that hole.
Can I account for gravel in the bottom?
Yes. Enter total hole depth and gravel depth separately; the concrete fill height is their difference.
Does this tool design a code-compliant fence or deck footing?
No. It estimates quantities only; applicable design requirements depend on the post use, soil and jurisdiction.
Accuracy checklist
Before you order materials
These checks apply to the calculator above and help prevent expensive mistakes.
Units are labeled
ft / in / ft³ / yd³ and product bag coverage all mean different things. Change a value only when you know the unit.
Net, waste and purchase are separate
The geometric amount, adjusted quantity and supplier purchase increments should not be mistaken for one another.
Sources must fit the chosen material
Official product data is a starting point for planning. Check installation specifications and local requirements before use.
Continue your project
Related concrete calculators
Choose the next task rather than making one calculator answer every question.
General Concrete CalculatorEstimate cubic yards for common shapes →Slab Volume CalculatorPlan slab volume and sections →Slab Cost CalculatorEstimate materials and installation →Concrete Bag CalculatorChoose real package yields →Sonotube CalculatorVolume for round concrete forms →Self-Leveling CalculatorFloor-leveler bags by product →
Methods and references
Source material and limitations
See the sources behind named product assumptions. These are not endorsements or live quotes.
Transparency: Calculators provide mathematical planning estimates. Product availability, real mixed yield, structural design, code requirements, and on-site conditions need independent verification. Always confirm current product specifications, installation instructions and local supplier requirements before purchasing.