Article — Post Hole Concrete Calculator
Post Hole Concrete Calculator: Bags per Fence Post
For a typical 4×4 wood fence post in a 10-inch diameter × 30-inch deep round hole, you need about 1.26 ft³ of concrete after accounting for post displacement and 10% waste, or 4 × 50-lb bags, or 3 × 60-lb bags, or 3 × 80-lb bags. The math: hole volume = π × (5/12)² × (30/12) = 1.36 ft³. Bag yields per QUIKRETE and Sakrete data sheets: 50-lb = 0.375 ft³, 60-lb = 0.45 ft³, 80-lb = 0.60 ft³. NOAA frost-depth maps drive minimum hole depth — 30 inches in the upper Midwest, 12 inches in the southern US.
This calculator handles round and square holes in inches. Enter hole dimensions, depth, post size, number of posts, and waste factor (default 10 percent). It outputs concrete volume in ft³, bag counts for 50-lb, 60-lb, and 80-lb sizes, and totals when you set quantity above 1.
Post hole concrete fundamentals
Set posts in concrete to resist three forces: vertical (weight of fence and any climbing children), lateral (wind), and frost heave (groundwater freezing and expanding 9% as it turns to ice). Concrete's mass and bond to the post handle all three, provided the hole is deep enough and wide enough. The standard guideline: hole diameter ≥ 3× post width, hole depth ≥ 1/3 of post height plus 6 inches of gravel at the bottom.
A 4×4 post (actually 3.5 × 3.5 in) in a 10-inch diameter hole has a 3.25-inch concrete collar around the post. That collar carries lateral load from wind to the surrounding soil. Narrow collars (under 2 inches) crack under wind; wide collars (over 6 inches) waste concrete without adding strength. Three times the post width is the proven compromise.
Bags per post hole math
Volume = π × r² × depth for round holes, w × l × depth for square. All in feet (or convert inches to feet by dividing by 12). For a 10-inch diameter × 30-inch deep round hole: π × (5/12)² × (30/12) = 1.36 ft³. Subtract the post displacement (3.5/12 × 3.5/12 × 30/12 = 0.213 ft³ for a 4×4 going to full depth): net concrete needed = 1.15 ft³.
Add the waste factor (10 percent default for spills and irregular hole walls): 1.15 × 1.10 = 1.26 ft³ total. Bag counts (rounding up): 50-lb bags = ceil(1.26 / 0.375) = 4 bags; 60-lb = ceil(1.26 / 0.45) = 3 bags; 80-lb = ceil(1.26 / 0.60) = 3 bags. Either 3 × 80-lb or 4 × 50-lb gets the job done with a small excess.
The 0.60 ft³ yield of an 80-lb bag of QUIKRETE Concrete Mix means 80 lb of dry mix plus about 6 quarts of water produces concrete that fills 60 percent of a cubic foot. Per bag pricing in 2026 averages $6-$7 at home centers; in cubic yard terms that is $270-$315/yd³, versus $140-$200/yd³ for ready-mix delivered. For 5+ post holes, ready-mix usually wins; for 1-4, bagged is more convenient.
Post hole depth and frost line
Post bottoms must sit below the local frost line. NOAA publishes frost-depth maps for the US: 0-6 inches in southern Florida and Texas Gulf, 12-18 inches in central states, 30-36 inches in the Midwest (Illinois, Ohio), 42-48 inches in the upper Midwest (Minnesota, Wisconsin), and 60+ inches in Maine and North Dakota. Always confirm local code; some municipalities specify deeper than NOAA's predicted frost line.
Frost heave is the slow failure mode that destroys shallow-set posts. Water in the soil freezes from the surface down. Each ice layer pushes up against the post in winter; spring thaw creates gaps; the next winter freezes water in the gaps and pushes again. Posts set at 18 inches in Minnesota emerge from the ground 2-3 inches per year. After 5-7 years the fence is unusable. The fix: set posts deeper than the frost line so the bottom never freezes.
QUIKRETE vs Sakrete bag yields
Both major brands publish yields in their bag data sheets. The standard yields: 50-lb bag = 0.375 ft³, 60-lb bag = 0.45 ft³, 80-lb bag = 0.60 ft³, 90-lb bag = 0.675 ft³. These yields assume proper mixing with the recommended water — typically 6 quarts per 80-lb bag. Adding too much water doesn't increase yield; it just weakens the concrete.
QUIKRETE Concrete Mix and Sakrete Concrete Mix are nominally identical: portland cement, sand, and gravel premixed to ASTM C387 standards. The technical difference is regional sourcing of aggregates — QUIKRETE plants on the East Coast use different sand than QUIKRETE plants in California. For post-hole work, either brand performs identically.
40-lb bag 0.30 ft³50-lb bag 0.375 ft³60-lb bag 0.45 ft³80-lb bag 0.60 ft³90-lb bag 0.675 ft³1 cubic yard 27 ft³ = 45 × 80-lb bagsFast-setting vs standard concrete
QUIKRETE Fast-Setting and Sakrete Fast-Setting are formulated for post-hole work specifically. Pour the dry mix into the hole around the post, then add water on top — no mixing required. The mix sets in 30-40 minutes and reaches 90 percent strength in 4 hours. Standard concrete mix requires hand mixing in a wheelbarrow or mixer and takes 24 hours to set, 7 days to reach usable strength, and 28 days for full cure.
Fast-setting is the easier choice for DIY: no mixing, no wheelbarrow, no waiting overnight before continuing to the next post. The cost premium is about 30 percent per bag, but the labor savings dwarf the materials cost for jobs under 20 posts. For larger jobs (50+ post fences), standard mix delivered by ready-mix truck is far cheaper.
Gravel base and drainage
Drop 4-6 inches of compacted gravel at the bottom of every post hole before setting the post. The gravel drains water away from the post bottom (preventing wood rot) and gives the concrete a stable bearing surface. Use 3/4-inch crushed stone or pea gravel; both work. Quantity needed: roughly 0.1-0.2 ft³ per 10-inch diameter hole, or half a 50-lb gravel bag per post.
Skipping gravel is the most common shortcut in DIY fence builds. The result, 5-10 years later: rotted post bottoms where moisture pooled, plus accelerated frost heave because water collected at the post-concrete interface. The gravel layer costs $2 per post and prevents a $50-$100 replacement post job a decade out.
Common post hole concrete mistakes
The first mistake is mixing too much water. The data sheet specifies water-to-mix ratio for a reason — extra water weakens the cured concrete. Symptoms: a thin slurry that doesn't fully set after 24 hours, low compressive strength, posts that wobble after a year. Stick to the bag's water spec.
Below 40°F, water in the concrete mix can freeze before the cement hydrates. The result is weak, crumbly concrete that fails within months. Wait for warmer weather (50°F+) or use cold-weather concrete admixtures (calcium chloride accelerator or specialized cold-weather mixes like Sakrete Fast-Setting with cold-weather rating).
The second mistake is filling the hole all the way to the surface with concrete. Concrete above grade can wick water into the post's base, causing rot. Best practice: dome the concrete slightly below ground level (1-2 inches), then back-fill with soil to surface grade. Water runs off rather than pooling. Sloped concrete tops with smooth troweling shed water effectively.
- Round hole volume = π × r² × depth (in ft³ when dims in ft)
- Recommended hole diameter = 3× post width minimum
- Frost line depth (MN/WI) = 42-48 inches
- Frost line depth (FL/TX) = 0-6 inches
- 50-lb bag yield = 0.375 ft³
- 80-lb bag yield = 0.60 ft³ (most common DIY choice)
- Waste factor = 10% default (15% for first-timer)
- Gravel base = 4-6 inches, 3/4-in crushed stone