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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 yielding 0.38 ft³. Here it is next to our math, before rounding:

Hole depthSakrete tableOurs, 3.5 in square postOurs, hole filled solid
12 in1.25 bags1.21 bags1.44 bags
18 in2 bags1.82 bags2.15 bags
24 in2.5 bags2.42 bags2.87 bags
30 in3 bags3.03 bags3.59 bags

Sakrete doesn't say what post its table assumes. With a 3.5 in square post (a typical 4x4) subtracted, our numbers sit close to theirs; filling the hole solid comes out noticeably higher. That gap is the post, and it is why this calculator asks for one.

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.
    “A 40 lb (18.1 kg) bag yields approximately 0.30 ft3 (8.5 L), a 50 lb (22.7 kg) bag yields approximately 0.375 ft3 (10.6 L), a 60 lb (27.2 kg) bag yields approximately 0.45 ft3 (12.7 L), an 80 lb (36.3 kg) bag yields approximately 0.60 ft3 (17 L) and a 90 lb (40.8 kg) bag yields approximately 0.675 ft3 (19.1 L).”
  2. QUIKRETE, QUIKRETE® 5000 High Early Strength Concrete Mix (No. 1007) — Product Data Sheet. Checked 2026-09-24.
    “YIELD • A 50 lb (22.6 kg) bag yields approximately 0.375 ft3 (10.6 L) • A 60 lb (27.2 kg) bag yields approximately 0.45 ft3 (12.7 L) • An 80 lb (36.2 kg) bag yields approximately 0.60 ft3 (17 L)”
  3. QUIKRETE, QUIKRETE® Fast-Setting Concrete Mix (No. 1004) — Product Data Sheet (revised November 2025). Checked 2026-09-24.
    “YIELD • Each 50 lb (22.6 kg) bag yields approximately 0.375 cu ft (10.6 L) • Each 60 lb (27.2 kg) bag yields approximately 0.45 cu ft (12.7 L) … Start by digging the post hole about 3 times the diameter of the post. Hole depth should be 1/3 the overall post height.”
  4. Sakrete, Sakrete 5000 Plus High Early Strength Concrete Mix — Technical Data Sheet. Checked 2026-09-24.
    “PACKAGE SIZE YIELD WATER REQUIRED SLUMP RANGE 50 lb 22.7 kg 0.38 ft3 0.011 m3 2.1 qts 2.0 L 2 – 3 in 50 – 75 mm 60 lb 27.2 kg 0.47 ft3 0.013 m3 2.5 qts 2.4 L 2 – 3 in 50 – 75 mm 80 lb 36.3 kg 0.6 ft3 0.017 m3 3.5 qts 3.3 L 2 – 3 in 50 – 75 mm … A general rule of thumb is that the depth should be 1/3 the length of the post or pole and the diameter should be 3 times that of the pole or post”
  5. Sakrete, Sakrete Fast Setting Concrete Mix — Technical Data Sheet. Checked 2026-09-24.
    “Post Setting - Based on Average 10” Diameter Hole: … Bag Size Yield Water 50 lb (22.6 kg) 0.38 ft3 (.01 m3) 2.5 to 3 qts (2.4 - 2.9 L) 12” (305 mm) deep 18” (457 mm) deep 1.25 bags 2 bags 24” (610 mm) deep 30” (762 mm) deep 2.5 bags 3 bags”
  6. QUIKRETE, QUIKRETE® Quantity Calculator. Checked 2026-09-24.
    “All yields are approximate and do not include allowance for uneven subgrade, waste, etc.”
  7. Sakrete, Sakrete Calculators. Checked 2026-09-24.
    “PRO TIP: Remember to add 10% to your estimates for safety!”
  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)”