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Fence Post Concrete Calculator

Most post-hole calculators fill the whole hole. A post takes up room, so this one subtracts it, then shows the difference between sharing bags across holes and mixing whole bags hole by hole.

Calculator

Anything under the concrete that isn't concrete. Leave 0 if none.
Post
Measure the real post. A “4x4” is not 4 in across.
Your call — we add exactly what you enter. QUIKRETE says its yields include no allowance for waste; Sakrete suggests adding 10%.
Cubic feet
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Cubic yards
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Bags, shared
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Bags, whole bags per hole
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Bag yield: QUIKRETE Concrete Mix data sheet, checked 2026-09-24.

How the number is worked out

concrete depth = hole depth − gravel at bottom
hole = π × (diameter ÷ 2)² × concrete depth
post (square) = width² × concrete depth
post (round) = π × (diameter ÷ 2)² × concrete depth
concrete per hole = hole − post

All inches are converted to feet before multiplying, so the result comes out in cubic feet. The post is assumed to stand on the gravel, running through the full depth of the concrete.

Shared bags or whole bags per hole?

The calculator gives two bag counts because there are two ways to do the job:

  • Shared — you mix in a wheelbarrow and spread the batch across several holes. You need the total volume divided by the bag yield, rounded up once.
  • Whole bags per hole — you pour dry mix straight into each hole, or mix one hole at a time and don't carry leftovers over. Each hole is rounded up to whole bags on its own, which adds up fast.

Worked example

Ten posts, 10 in holes dug 36 in deep, no gravel, posts measuring 3.5 in × 3.5 in, 80 lb bags:

  • Hole: π × (5 ÷ 12)² × 3 = 1.636 ft³
  • Post: (3.5 ÷ 12)² × 3 = 0.255 ft³
  • Concrete per hole: 1.381 ft³
  • Ten holes: 13.81 ft³ ÷ 0.60 = 23.0 → 24 bags shared
  • Per hole: 1.381 ÷ 0.60 = 2.3 → 3 bags × 10 = 30 bags

Same fence, six bags apart, purely from how you mix. Filling the hole without subtracting the post would have given 16.36 ft³, or 28 bags shared.

Checked against a manufacturer's own post table

Sakrete prints a post-setting table on its Fast Setting Concrete Mix data sheet, for a 10 in hole and 50 lb bags. We ran every depth in it through this calculator, with a 3.5 in square post (a typical 4x4) subtracted and no rounding: 4 of 4 come out within 0.2 bags of Sakrete's figure. Filling the hole solid instead comes out noticeably higher; that gap is the post, and it is why this calculator asks for one. We don't reprint the table here; the link above goes to Sakrete's own sheet.

What this calculator is not for

It does not tell you how deep or how wide to dig. QUIKRETE and Sakrete both print the same rule of thumb on their data sheets — a hole about three times the post's width, dug to about a third of the post's length (QUIKRETE, Sakrete) — but post height, wind load, soil and your local frost depth and code can all call for more. Check your building department. It also assumes a straight-sided round hole; a hole that bells out at the bottom takes more.

Sources

  1. QUIKRETE, QUIKRETE® Concrete Mix (No. 1101) — Product Data Sheet. Checked 2026-09-24.
  2. QUIKRETE, QUIKRETE® 5000 High Early Strength Concrete Mix (No. 1007) — Product Data Sheet. Checked 2026-09-24.
  3. QUIKRETE, QUIKRETE® Fast-Setting Concrete Mix (No. 1004) — Product Data Sheet (revised November 2025). Checked 2026-09-24.
  4. Sakrete, Sakrete 5000 Plus High Early Strength Concrete Mix — Technical Data Sheet. Checked 2026-09-24.
  5. Sakrete, Sakrete Fast Setting Concrete Mix — Technical Data Sheet. Checked 2026-09-24.
  6. QUIKRETE, QUIKRETE® Quantity Calculator. Checked 2026-09-24.
  7. Sakrete, Sakrete Calculators. Checked 2026-09-24.
  8. NIST, NIST Handbook 44 (2026), Appendix C — General Tables of Units of Measurement. Checked 2026-09-24.
    “1 728 cubic inches (in3) = 1 cubic foot (ft3) 27 cubic feet = 1 cubic yard (yd3)”