Post-Hole Concrete Calculator

Setting a post in concrete is one of those deceptively simple jobs that separates a fence that stands proud for 30 years from one that leans, rots, and falls over by year five. The difference is rarely the post itself — it's the hole geometry, the concrete volume, the drainage at the base, and the cure time before lateral loads hit. Whether you're installing a single mailbox post on a Saturday morning or a 200-post privacy fence around a backyard, the math is identical: figure the cubic feet of concrete needed to fill the void between the post and the soil, then divide by the cubic-foot yield of your bag. This calculator does that math across 40, 50, 60, and 80 lb bags, with frost-line guidance, waste factor, fast-setting recommendations, and total project cost.

The standard residential rule is that one-third of the post's length should be in the ground — a 6-foot fence with an 8-foot 4x4 means a 24-30 inch hole minimum. Northern climates push that deeper: 36 inches in the upper Midwest, 48+ in northern New England, 60+ in Canada or Alaska. Frost heave lifts shallow posts an inch or two every winter until they pop out. The fix is simple: go below the frost line, use a gravel base for drainage, and crown the concrete so water sheds away from the post. Hole diameter follows the 3x rule — a 4x4 (3.5" actual) needs a 10-12" hole, a 6x6 (5.5") needs 14-16", a chain-link line post (2-3/8") needs 8-10". Tall fences, gates, and structural posts lean to the high end.

The Formula Explained

Every post-hole concrete calculation comes down to volume subtraction. The hole is a cylinder. The post takes up some of that cylinder. The concrete fills the rest.

• Bag Yields (manufacturer typical): 40-lb = 0.30 cu ft, 50-lb = 0.375 cu ft, 60-lb = 0.45 cu ft, 80-lb = 0.60 cu ft. These are after-water-added volumes and represent industry averages across major brands.
• Round Up Always: You can't buy 1.4 bags of concrete. The calculator rounds bags-per-hole up to the next whole number, then multiplies by post count, then adds your waste factor.
• Waste Factor: 10% is the standard for medium jobs (10-50 posts). Use 15% for tiny jobs (1-5 posts) where rounding errors hurt more, and 5-7% for large jobs (50+ posts) where errors average out.

Frost-Line Reference Table

Frost line is the maximum depth at which soil freezes in a typical winter for your region. Any post that needs to resist vertical load (deck columns, gate posts, signs) or that you want to last decades without re-setting needs to extend below the frost line.

5-Step How-To: Setting a Post in Concrete

1. 1. Dig the hole. Use a clamshell post-hole digger for 1-5 posts, a manual auger for 5-20 posts, or a one-person/two-person gas auger for larger jobs. Make the hole cylindrical with vertical walls, three times the post width in diameter, and at least to the frost line depth for your region. Add 4-6 inches at the bottom for a gravel base.
2. 2. Add gravel and brace. Drop 4-6 inches of 3/4-inch crushed stone into the bottom and tamp firm with a 2x4. Set the post on the gravel, check plumb on two faces with a level, and brace with two diagonal 2x4s screwed loosely to the post and staked to the ground. Plumb again — the post must be dead vertical before the concrete goes in.
3. 3. Pour the concrete. For wet-mix, mix concrete and water in a wheelbarrow per bag instructions, then shovel around the post in 6-8 inch lifts, consolidating with a 2x4 or rebar to remove air voids. For fast-setting dry pour, dump dry bags into the hole around the post to within 3-4 inches of grade, then pour roughly one gallon of water per 50-lb bag on top and let it soak in.
4. 4. Level and crown. Re-check plumb one more time before the concrete sets. Trowel the top into a slight dome (crown) that slopes down and away from the post on all sides, so rainwater sheds off instead of pooling at the post-concrete interface. The crown should rise about 1 inch above the surrounding grade at the post and taper down to grade at the hole edge.
5. 5. Cure and back-fill. Leave braces in place for at least 40 minutes (fast-setting) or 24 hours (standard mix) before removing. Wait a full 24 hours (fast-set) or 7 days (standard mix) before attaching heavy hardware like gate hinges. Back-fill any soil disturbed around the concrete collar and tamp firmly to shed water.

Common Use Cases

Pro Tips from Working Crews

• • Always use a gravel base. 4-6 inches of compacted 3/4-inch crushed stone at the bottom of every hole. It drains water away from the post (huge for wood post longevity), provides a load-bearing pad, and resists settling. Skipping the gravel is the #1 cause of premature post rot.
• • Crown the top of the concrete. Trowel a slight dome so water sheds away from the post on all sides. A flat or dished top traps water at the post-concrete interface, which freeze-thaws into cracks within a few seasons.
• • Dry-pour vs wet-mix: Dry-pour fast-setting is fine for fence posts and mailboxes. Always wet-mix for deck posts, gate posts, structural posts, and anything load-bearing. Wet-mixed concrete is 10-20% stronger.
• • Brace until cured. Two diagonal 2x4s, screwed to the post and staked to the ground at roughly 45 degrees. Leave braces on for the full cure time before removing. A leaning post is forever — you cannot re-plumb concreted-in posts without digging out.
• • Buy one extra bag. Even with a 10% waste factor, ordering one extra bag per 10 posts saves the second trip to the home center when one hole turns out 2 inches deeper than planned.
• • Cold-weather slowdown: Below 50°F, fast-setting concrete can take 60-90 minutes to set initially and full strength can take double the normal time. Below 32°F, don't pour at all — water freezes before it can hydrate the cement.
• • Pre-soak wood posts: Soaking the bottom 2 feet of pressure-treated wood posts in a bucket of water for 30 minutes before setting reduces wicking and prevents premature concrete dry-out on hot, windy days.
• • Skip the "trick" bell: Belling the hole bottom wider than the top is great in theory for frost resistance, but it's much harder to dig and only modestly more effective than just going below the frost line. Pick depth over belling.

Bag Size Quick Reference

Each bag size has trade-offs in cost per cubic foot, ease of lifting, and trips to/from the truck. Here's the practical guide:

• • 40 lb: Lightest, easiest to lift. Best for single mailbox or sign installs where you can carry one bag at a time. Highest cost per cubic foot.
• • 50 lb: The Quikrete Fast-Setting standard. Good balance of yield and liftability for solo work. Most common bag at Home Depot and Lowe's for fence installs.
• • 60 lb: The sweet spot for fence-post installs. One bag per typical 4x4 hole means clean math and easy pickup planning. Two-person teams handle these all day.
• • 80 lb: Lowest cost per cubic foot. Best for deck/gate/pergola installs where you need 3+ bags per hole and can use a wheelbarrow. Heavy — uses good lifting technique to avoid back injury.

Whether you're a weekend warrior installing a single mailbox or a fence crew quoting a 300-post commercial job, this calculator gives you defensible bag counts, frost-line aware depth guidance, fast-set vs standard recommendations, and total project cost — all in one screen. Bookmark it, share it with your crew, and never make a second trip to the home center again.