Quikrete Post Calculator: How Much Concrete

The Quikrete post calculator works out how many bags of concrete mix you need to set a fence, deck, or mailbox post. You enter the post dimensions, hole depth, and hole diameter, and it returns the concrete volume in cubic feet and the number of 40, 50, or 80-pound bags. This guide explains the math behind it, how to measure your hole correctly, and common mistakes that lead to wasted mix or a wobbly post.

Formula

The calculator finds the volume of concrete needed by subtracting the volume of the post from the volume of the hole. Both are treated as cylinders:

V = π × (D/2)² × h − π × (d/2)² × h

• V = concrete volume (cubic feet or cubic yards)
• D = hole diameter (feet)
• d = post width (feet)
• h = hole depth (feet)

Since the hole and post share the same depth, you can simplify to V = h × π × [(D/2)² − (d/2)²]. The calculator above uses the full form so you can see each part. All measurements are converted to feet first: hole diameter in inches is divided by 12, post width in inches is divided by 12.

For bag count, the calculator uses the industry-standard coverage: one 80-pound bag of Quikrete concrete mix yields about 0.6 cubic feet of set concrete. A 60-pound bag yields about 0.45 cubic feet, and a 40-pound bag yields about 0.3 cubic feet. The bag count is rounded up because you cannot buy a fraction of a bag.

Worked Example

Using the default values from the calculator: an 8-foot post, a 2-foot hole depth, a 10-inch hole diameter, and a 4-inch post width.

1. Convert hole diameter to feet: 10 in ÷ 12 = 0.833 ft. Radius = 0.4165 ft.
2. Convert post width to feet: 4 in ÷ 12 = 0.333 ft. Radius = 0.1665 ft.
3. Hole volume: π × (0.4165)² × 2 = π × 0.1735 × 2 = 1.090 ft³.
4. Post volume: π × (0.1665)² × 2 = π × 0.0277 × 2 = 0.174 ft³.
5. Concrete volume: 1.090 − 0.174 = 0.916 ft³.
6. Convert to cubic yards: 0.916 ÷ 27 = 0.034 yd³.
7. 80-lb bags needed: 0.916 ÷ 0.6 = 1.53, round up to 2 bags.

The calculator shows 0.03 yd³, 0.9 ft³, and 2 bags for the defaults. That matches the hand calculation.

How to Measure Your Hole Correctly

The accuracy of the result depends entirely on your measurements. Here are the key points:

• Hole depth is measured from the ground surface to the bottom of the hole, not from the top of the post. For most fence posts, the hole should be about one-third to one-half the length of the post above ground. A common rule is 2 feet deep for a 6-foot fence, 3 feet for an 8-foot fence.
• Hole diameter is the width of the hole at its widest point. Auger holes are usually fairly uniform, but hand-dug holes may flare at the bottom. Measure the average diameter, not the narrowest part.
• Post width is the actual dimension of the wood or metal post. A nominal 4×4 post is actually about 3.5 inches wide, so use 3.5 if you want precision. The calculator defaults to 4 inches, which slightly overestimates the post volume and underestimates concrete needed—safe for planning.
• If you set the post on a gravel base for drainage, add that gravel depth to the total hole depth, but do not subtract it from concrete volume—the concrete goes on top of the gravel.

When in doubt, round your hole measurements up. Extra concrete is cheaper than a second trip to the store.

Choosing the Right Bag Size

Quikrete concrete mix comes in 40, 60, and 80-pound bags. The calculator gives the count for 80-pound bags, but you can convert:

• 80-lb bag = 0.6 ft³
• 60-lb bag = 0.45 ft³
• 40-lb bag = 0.3 ft³

For a small job like a mailbox post, 40 or 60-pound bags are easier to handle. For a fence line with many posts, 80-pound bags reduce the number of bags you need to mix. The total weight matters: 10 posts needing 2 bags each at 80 pounds is 1,600 pounds of mix—plan your vehicle and wheelbarrow accordingly.

Also consider the type of mix: Quikrete Fast-Setting Concrete sets in about 20-40 minutes, while standard mix takes longer. The calculator works for both because the volume per bag is the same. Fast-setting is convenient for posts because you do not have to brace them as long, but you must work quickly.

Common Mistakes and Misconceptions

One frequent error is confusing hole diameter with post width. The hole must be at least three times the post width for proper support—a 4×4 post needs a 12-inch hole, not a 4-inch hole. The calculator will show a very small concrete volume if you enter a 4-inch hole, which should signal a problem.

Another mistake is using the post length instead of the hole depth. An 8-foot post set 2 feet deep means 6 feet above ground, but the concrete only goes in the 2-foot hole. Entering 8 feet for depth would give you four times the concrete you need.

A common misconception is that you should fill the entire hole with concrete. In most cases, you want concrete only around the post below ground, not mounded above ground. The calculator assumes the concrete fills the hole exactly to ground level. If you plan to mound it up for drainage, add a few extra inches to the depth.

Finally, remember that the bag coverage (0.6 ft³ per 80-lb bag) is for the dry mix volume after it sets. Some people think a bag of dry mix equals a bag of wet concrete—it does not. The dry mix contains aggregates and cement that shrink slightly when water is added, but the 0.6 ft³ figure is the standard industry estimate for set volume.

When to Use This Calculator vs. a General Concrete Calculator

A general concrete calculator (like one for a slab or footing) gives you cubic yards or cubic feet, but not bag count. The post calculator adds the bag count because posts are typically set with bagged mix, not ready-mix trucks. For a single post, buying a truck of concrete is impractical.

If you are setting many posts, you might consider ordering ready-mix concrete by the cubic yard. The calculator’s first output (cubic yards) tells you how much to order. However, ready-mix has a minimum order (often 1 cubic yard), so for fewer than about 20 posts, bagged mix is usually more economical.

This calculator also differs from a simple volume calculator because it subtracts the post volume. For a 4×4 post in a 10-inch hole, the post takes up about 19% of the hole volume. Ignoring that would mean buying 20% more concrete than needed.

Safety and Practical Tips

Mixing concrete involves dust and caustic ingredients. Wear a dust mask, safety glasses, and gloves. Mix in a wheelbarrow or mixing tub with water, following the bag instructions for water amount—usually about 3 quarts per 80-lb bag. Too much water weakens the concrete; too little makes it hard to work.

Set the post plumb (vertically level) before pouring concrete. Use temporary braces to hold it in place. For fast-setting mix, you have only a few minutes to adjust. For standard mix, you can take more time.

After pouring, slope the top of the concrete away from the post so water does not pool around the wood. This helps prevent rot. Allow the concrete to cure for at least 24-48 hours before attaching anything heavy to the post.

If you are setting posts in cold weather, use a mix rated for low temperatures or wait for warmer conditions. Concrete that freezes before curing loses strength.

Frequently Asked Questions

How many bags of Quikrete do I need for a 4×4 post? For a typical 4×4 post (actual 3.5 inches) set 2 feet deep in a 10-inch hole, you need about 0.9 cubic feet of concrete, which is 2 bags of 80-pound mix, 2 bags of 60-pound, or 3 bags of 40-pound. If your hole is 12 inches wide, you will need about 1.4 cubic feet—3 bags of 80-pound.

Can I use the calculator for round posts? Yes, but enter the post diameter as the post width. The formula uses the same calculation for a cylinder, so a round post works fine. Just measure the diameter at the widest point.

What if my hole is not a perfect cylinder? The calculator assumes a straight-sided cylinder. If your hole is wider at the bottom (common with hand digging), you will need more concrete. Estimate the average diameter and add 10-20% to the bag count to be safe.

Is fast-setting concrete stronger than standard mix? No, both reach similar compressive strengths after full cure. Fast-setting just hardens quicker, which is convenient for posts. Follow the curing time on the bag—usually 24-48 hours before loading.

How deep should a fence post be? A common rule is one-third to one-half of the above-ground height. For a 6-foot fence, set the post 2 to 3 feet deep. For an 8-foot fence, 2.5 to 4 feet deep. Check local building codes for frost depth in your area—posts must go below the frost line to prevent heaving.

Can I use this calculator for deck posts? Yes, but deck posts often require larger holes and sometimes concrete footings. The calculator gives the concrete volume for the post hole, but if you are pouring a separate footing, add that volume separately. Always consult your local building code for deck post requirements.

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in l | Post length | ft | 8 | 0 | 20
in d | Hole depth | ft | 2 | 0 | 10
in w | Hole diameter | in | 10 | 4 | 36
in p | Post width | in | 4 | 2 | 12
out yd | Concrete volume | yd³ | (pi * ((w/12)/2)^2 * d - pi * ((p/12)/2)^2 * d) / 27 | 2
out ft3 | Concrete volume | ft³ | pi * ((w/12)/2)^2 * d - pi * ((p/12)/2)^2 * d | 1
out bags | 80-lb bags | bags | ceil((pi * ((w/12)/2)^2 * d - pi * ((p/12)/2)^2 * d) / 0.6) | 0