Calcmeister

Fence post calculator

Measure the total length of your fence line and the desired spacing between posts. Enter the hole dimensions and post size to get the required number of posts and concrete bags.

ft
ft

Desired center-to-center spacing between posts.

in

Diameter of the hole dug for each post.

in

Depth of the hole from ground surface to bottom.

%

Extra concrete for spillage and uneven holes.

$

Cost per 80 lb bag of concrete mix (e.g., Quikrete).

Number of posts
– posts
Total concrete volume
– ft³
80 lb bags needed
– bags
Concrete material cost
– $

    How It Is Calculated

    This calculator helps you determine the number of fence posts needed and the amount of concrete required to set them. The calculations follow these steps:

    1. Number of posts: Divide the total fence length by the desired post spacing, round down to the nearest whole number, then add 1 for the starting post.
    2. Concrete volume per hole: Compute the volume of the cylindrical hole, subtract the volume of the post that displaces concrete, and convert from cubic inches to cubic feet.
    3. Total concrete volume: Multiply the volume per hole by the number of posts, then add a waste factor to account for spillage and uneven holes.
    4. Bags and cost: Divide the total concrete volume by the yield of one 80 lb bag (0.6 ft³), round up to the nearest whole bag, and multiply by the price per bag.

    N = floor(L / S) + 1

    Vhole = (π × (D / 2)² × H − W² × H) / 1728

    Vtotal = Vhole × N × (1 + F / 100)

    B = ceil(Vtotal / 0.6)

    C = B × P

    Where:

    Assumptions

    Worked Example

    Suppose you have a fence length of 100 ft, post spacing of 8 ft, post hole diameter of 10 in, post hole depth of 30 in, post size 3.5 in (4x4), concrete waste factor of 10%, and price per 80 lb bag of $6.50.

    1. Number of posts: floor(100 / 8) + 1 = floor(12.5) + 1 = 12 + 1 = 13 posts.
    2. Volume per hole: (π × (10/2)² × 30 − 3.5² × 30) / 1728 = (π × 25 × 30 − 12.25 × 30) / 1728 = (2356.194 − 367.5) / 1728 = 1988.694 / 1728 ≈ 1.151 ft³.
    3. Total concrete volume before waste: 1.151 × 13 = 14.963 ft³.
    4. With 10% waste: 14.963 × 1.10 = 16.46 ft³.
    5. 80 lb bags needed: ceil(16.46 / 0.6) = ceil(27.43) = 28 bags.
    6. Concrete material cost: 28 × 6.50 = $182.00.

    Practical Tips

    Questions and answers

    Why do I need to add 1 to the number of posts?

    Adding 1 accounts for the starting post at the beginning of the fence line. For example, a 100 ft fence with 8 ft spacing requires 12 intervals, but you need 13 posts to support both ends.

    What if my fence has corners or gates?

    This calculator assumes a straight fence line. For corners or gates, you may need additional posts or different spacing. Adjust the fence length and spacing manually to approximate, or consult a professional.

    Can I use a different bag size for concrete?

    Yes, but this calculator is designed for 80 lb bags. If you use 60 lb bags (yield 0.45 ft³) or 90 lb bags (yield 0.75 ft³), you can adjust the bag count by dividing the total concrete volume by the appropriate yield.

    Why is the waste factor important?

    The waste factor accounts for concrete lost to spillage, uneven hole shapes, or extra used for leveling posts. A typical waste factor of 10% ensures you have enough concrete without running short.

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    Last reviewed: 2026-10-15. Results are estimates for planning. Check them before you rely on them for design, purchase or safety decisions.