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Concrete Slab and Footing Calculator Guide (Yardage)

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Estimating concrete for fence post holes requires two key measurements: the hole's diameter (or width and depth if square) and how deep the post will be buried. This guide walks you through the calculation method and shows you how to convert cubic feet to cubic yards—the standard unit for ready-mix concrete delivery.

Why Concrete Volume Matters for Fence Posts

Setting fence posts directly into concrete ensures they stay plumb and resist heaving from freeze-thaw cycles. Unlike dry-setting a post, concrete-set posts remain stable for decades. Knowing how much concrete you need avoids both expensive overage and mid-project shortages. Most fence projects use standard concrete bags (60 lb or 80 lb) or ready-mix truck delivery, so calculating cubic yards helps you choose the right purchase option.

Standard Hole Dimensions

According to industry guidance, fence post holes should be about three times as large in diameter as the post itself. For a 4×4 post, that's roughly a 12-inch diameter hole. Hole depth depends on both fence height and frost line in your region. As a general rule, fence posts should be buried to a depth equal to one-third of their above-ground height, plus 6 inches. For a 6-foot fence, this translates to approximately 24–30 inches deep, depending on your local frost line and soil conditions.

A typical concrete footprint for a single fence post hole looks like this: a 12-inch diameter hole at 24–30 inches deep requires roughly 0.06–0.07 cubic yards of concrete—equivalent to about 3–4 standard 60 lb bags of concrete mix (not cement). Larger posts (like 6×6) in deeper holes (30–36 inches) can need 0.1 cubic yards or more.

The Cubic Yards Conversion

Concrete is sold and delivered in cubic yards. One cubic yard equals 27 cubic feet. To convert your hole dimensions to cubic yards:

  1. Calculate the volume in cubic feet using the cylinder formula (π × radius² × depth) for round holes, or length × width × depth for square holes.
  2. Divide the cubic feet result by 27 to get cubic yards.

For example, a 12-inch diameter hole (0.5-foot radius) at 24 inches deep is π × (0.5)² × 2 feet = 1.57 cubic feet ÷ 27 = 0.058 cubic yards. This method scales reliably whether you're setting one post or a dozen.

Slab vs. Spot Footings

Some fence projects involve a concrete slab or wider footing pad rather than individual post holes. This is common when building on sloped terrain or creating a more permanent structure. For a slab, measure its length, width, and thickness (usually 4–6 inches for fence applications), then calculate cubic yards using the same conversion: (length × width × thickness) ÷ 27. Adding a 6-inch gravel base under the concrete improves drainage and frost protection, though gravel is measured separately from concrete volume.

Planning for Waste and Weather

In practice, add 10–15% extra to your calculated concrete volume to account for spillage, settling, and uneven terrain. If you're mixing on-site or ordering bags, this buffer prevents unexpected shortages. Weather also matters: concrete sets slower in cold temperatures and faster in heat, so plan delivery timing with your local forecast in mind.

Estimating Your Total Concrete

Once you know your post hole diameter, depth, and the number of posts, multiply the single-hole volume by the total post count and convert to cubic yards. Add 10–15% for waste and uneven holes. If you're unsure about your frost line depth or want to compare depths, run the cylinder formula above for each depth scenario to see the material difference.

Related guides

Use the formulas above to estimate your yardage, or see Concrete Per Fence Post for a hole-by-hole material guide →