Quikrete Post Hole Calculator

Setting a fence post, mailbox post, or deck footing in concrete starts with a simple question: how many bags of Quikrete do you actually need? Buy too few and you are making a second trip to the store with wet concrete waiting at home. Buy too many and you are stuck with heavy bags cluttering the garage. The Quikrete Post Hole Calculator answers that question with real geometry instead of guesswork, turning your hole dimensions into an exact bag count, total volume, and estimated cost.

The calculator uses the volume of your post hole, subtracts the space the post itself occupies, and divides the remaining volume by the published yield of each Quikrete bag size. It then adds a small waste allowance and rounds up to whole bags, because the store does not sell fractions of a bag. You get six clear results: concrete per hole, bags per hole, total bags to buy, total concrete in cubic yards, total weight to haul, and the estimated cost.

This guide is useful for homeowners, fence installers, deck builders, and anyone pouring footings for pergolas, signs, or basketball hoops. Whether you are setting four fence posts on a Saturday or digging twenty footings for a deck, the same math applies, and this page walks through it step by step with fully worked examples.

What Is a Post Hole Concrete Calculation?

A post hole concrete calculation is the process of finding the volume of concrete needed to fill the space around a post set in the ground. A post hole is a cylinder dug into the earth, and the post sits inside it. The concrete fills only the ring-shaped space between the post and the soil, not the entire hole. The basic formula is the volume of a cylinder, V = pi x r squared x h, computed once for the hole and once for the post, with the post volume subtracted from the hole volume.

The result is expressed in cubic feet, the unit bagged concrete is rated in. Every Quikrete bag has a published yield: a 50 lb bag of Fast-Setting mix yields about 0.375 cubic feet, a 60 lb bag yields about 0.45 cubic feet, and an 80 lb bag yields about 0.60 cubic feet. Dividing your net hole volume by the bag yield tells you how many bags each hole requires. A simple illustration: a hole that needs 2.1 cubic feet of concrete divided by the 0.60 cubic foot yield of an 80 lb bag means about 3.5 bags per hole, so you plan for 4.

Getting this right matters because concrete is heavy, perishable once mixed, and sold in fixed bag sizes. A careful calculation done before you dig saves money, time, and sore muscles on project day.

Why Getting the Bag Count Right Matters

Running short of concrete mid-project is one of the most common frustrations in DIY post setting. Fast-Setting Quikrete begins to firm up within 20 to 40 minutes, so you cannot simply leave a half-filled hole and finish it tomorrow. A second partial batch poured onto a hardened first layer creates a cold joint, a weak seam where the two pours never fully bond. That seam can let water in and weaken the footing over time, especially in freeze-thaw climates.

Overbuying has its own costs. Concrete bags are heavy — an 80 lb bag is no joke to load, unload, and carry — and every extra bag is money sitting unused. Bagged concrete also has a limited shelf life; once the bag absorbs moisture from humid air, the powder can partially harden into useless lumps. Accurate estimating means you buy what you need plus a small, deliberate waste allowance, and nothing more.

How to Use the Quikrete Post Hole Calculator

Step 1: Measure your hole diameter in inches and enter it in the Hole Diameter field. For example, enter 12 for a hole dug with a 12-inch auger. Step 2: Enter the Hole Depth in inches, for example 36 for a three-foot-deep hole. Measure from the bottom of the hole to ground level. Step 3: Enter the Post Diameter in inches. For a 4x4 wood post, enter 4. For a round steel pipe, enter its outside diameter. Step 4: Enter the Number of Holes, for example 4 for a four-post fence section. Step 5: Choose your Quikrete Bag Size from the dropdown: 50 lb, 60 lb, or 80 lb. The 80 lb bag is selected by default. Step 6: Enter the Price Per Bag in dollars, for example 7.50, using your local store price. Step 7: Click Calculate to see all six results. Click Reset to start over with new numbers.

Worked Example 1: Four Fence Posts

Suppose you are setting four fence posts. Each hole is 12 inches in diameter and 36 inches deep, and each post is a 4x4 (treated as 4 inches in diameter). You plan to use 80 lb bags of Quikrete Fast-Setting mix at 7.50 dollars per bag.

First, compute the hole volume as a cylinder: radius 6 inches, so volume = pi x 36 x 36 = 4,071.5 cubic inches. Divide by 1,728 to convert to cubic feet: 2.36 cubic feet. Next, compute the post volume: radius 2 inches, so volume = pi x 4 x 36 = 452.4 cubic inches, or 0.26 cubic feet. Subtract: 2.36 - 0.26 = 2.09 cubic feet of concrete per hole.

An 80 lb bag yields 0.60 cubic feet, so bags per hole = 2.09 / 0.60 = 3.49 bags. For four holes, that is 13.96 bags; adding a 5 percent waste allowance gives 14.66, which rounds up to 15 bags. Total concrete is 2.09 x 4 / 27 = 0.31 cubic yards. Total weight is 15 x 80 = 1,200 lb, and total cost is 15 x 7.50 = $112.50.

Worked Example 2: Six Deck Footings

Now consider six deck footings. Each hole is 16 inches in diameter and 42 inches deep, with a 6-inch round steel post. You choose 60 lb bags at 6.25 dollars each.

Hole volume: radius 8 inches, so pi x 64 x 42 = 8,443.4 cubic inches, or 4.89 cubic feet. Post volume: radius 3 inches, so pi x 9 x 42 = 1,187.5 cubic inches, or 0.69 cubic feet. Net concrete per hole: 4.89 - 0.69 = 4.20 cubic feet.

A 60 lb bag yields 0.45 cubic feet, so bags per hole = 4.20 / 0.45 = 9.33. Six holes need 55.98 bags; with 5 percent waste that is 58.78, rounded up to 59 bags. Total concrete = 4.20 x 6 / 27 = 0.93 cubic yards. Total weight = 59 x 60 = 3,540 lb, and cost = 59 x 6.25 = $368.75. At this scale, comparing the bag total against a ready-mix delivery quote starts to make sense.

Understanding Quikrete Bag Yields

Bag yield is the volume of mixed, hardened concrete one bag produces, and it is the single most important number in any bag estimate. Quikrete publishes yields for its mixes: Fast-Setting concrete in the 50 lb bag yields about 0.375 cubic feet, the 60 lb bag about 0.45 cubic feet, and the 80 lb bag about 0.60 cubic feet. Regular Quikrete concrete mix has slightly different yields, so always check the bag you are actually buying rather than assuming.

Yield is measured under controlled conditions, which is why experienced builders add a waste allowance. Real holes are rarely perfect cylinders: augers wobble, shovels undercut the sides, and soil crumbles into bell-shaped bottoms. Each imperfection adds volume the math did not predict. The calculator's built-in 5 percent allowance covers typical over-digging; if your soil is loose or your holes are hand-dug and irregular, mentally add another bag or two to the total.

Factors That Change How Much Concrete You Need

Hole shape is the biggest variable. A post-hole digger or auger makes a fairly true cylinder, but hand-dug holes tend to be wider at the top and irregular along the sides. Bell-bottom holes — wider at the base for extra stability — intentionally hold much more concrete than a straight cylinder of the same depth. When in doubt, measure the diameter at two or three depths and use the largest, or add extra bags.

Soil conditions matter too. In sandy or gravelly soil, the hole walls can cave slightly during the pour, stealing concrete. In rocky soil, you may have to over-dig to remove stones, enlarging the hole. Wet soil at the bottom of the hole displaces mix and can weaken the concrete if water pools there — pump or bail standing water before pouring.

Tips for Setting Posts in Concrete

1. Dig below the frost line in cold climates; concrete footings that sit above it will heave when the ground freezes.
2. Make the hole diameter about three times the post width for good lateral support.
3. Pour 6 inches of gravel at the bottom of each hole for drainage before setting the post.
4. Brace the post plumb in both directions before any concrete goes in, and recheck with a level after pouring.
5. With Fast-Setting mix, you can pour the dry powder around the post and add water on top — no wheelbarrow mixing needed.
6. Crown the concrete slightly above ground level and slope it away from the post so water drains off instead of pooling.
7. In hot weather, dampen the hole before pouring so dry soil does not suck water out of the mix.
8. Do not disturb or load the post until the concrete has set; Fast-Setting mix typically firms in 20 to 40 minutes.
9. Keep a spare bag on hand beyond the calculated total; it is cheap insurance against an oversized hole.

Frequently Asked Questions

1. How many 80 lb bags of Quikrete do I need per post hole?

It depends on your hole size, but a typical 12-inch diameter by 36-inch deep hole with a 4x4 post needs about 3.5 bags, so plan on 4 per hole. Larger holes, such as 16-inch diameter footings for a deck, can need 9 or more 80 lb bags each. Enter your exact dimensions in the calculator above for a precise count.

2. What is the yield of a 50 lb bag of Quikrete?

A 50 lb bag of Quikrete Fast-Setting concrete yields approximately 0.375 cubic feet of mixed concrete. The 60 lb bag yields about 0.45 cubic feet, and the 80 lb bag yields about 0.60 cubic feet. These figures come from the manufacturer's published data and assume the mix is prepared as directed.

3. Should I subtract the post volume from the hole volume?

Yes. The post occupies space inside the hole, and concrete only fills the annular space between the post and the soil. Ignoring the post overstates the concrete needed, especially with large posts or piers. The calculator on this page performs this subtraction automatically.

4. How deep should a post hole be?

A common rule is one-third of the post's above-ground height plus 6 inches of gravel, with a minimum of 24 inches for fence posts in mild climates. In regions with frost, the footing must extend below the local frost line, which can be 36 to 48 inches or more. Always check local building codes for decks and structural posts.

5. Can I pour dry Quikrete into the hole and add water?

Yes, that is the intended method for Quikrete Fast-Setting concrete. Pour the dry mix evenly around the post, then add the recommended amount of water on top — about a gallon per 50 lb bag. The water percolates down and hydrates the mix. Do not use this pour-dry method with standard concrete mix, which must be mechanically mixed first.

6. How much water does an 80 lb bag of Quikrete need?

For Fast-Setting mix poured dry into the hole, use about 1.5 gallons of water per 80 lb bag. If you are mixing in a wheelbarrow first, start with about 1 gallon and add small amounts until the mix reaches a thick, workable consistency. Too much water weakens the finished concrete, so add it gradually.

7. How long does Quikrete take to set around a post?

Fast-Setting Quikrete typically begins to harden in 20 to 40 minutes and can support the post within a few hours. Standard concrete mix takes much longer, usually 24 to 48 hours before it bears weight. Full strength for any concrete develops over about 28 days, so avoid heavy loads on structural footings for the first week.

8. Do I need gravel at the bottom of a post hole?

Gravel is strongly recommended. A 6-inch bed of compacted gravel improves drainage, keeps the post end out of standing water, and gives the concrete a stable base. For wood posts especially, drainage at the bottom significantly extends the post's life by reducing rot.

9. How many bags fit in a cubic yard?

A cubic yard contains 27 cubic feet. At 0.60 cubic feet per 80 lb bag, one cubic yard requires 45 bags of 80 lb Quikrete. With 60 lb bags at 0.45 cubic feet each, it takes 60 bags. Once your project approaches a full cubic yard, price out a ready-mix delivery, which is often cheaper than bagged concrete at that volume.

10. Is it cheaper to use bags or order ready-mix concrete?

For small jobs under about half a cubic yard, bags are usually cheaper and far more convenient. Between half and one cubic yard it is close, and most contractors switch to ready-mix delivery around one cubic yard because the per-yard price drops sharply. Get a delivered quote and compare it against your bag total before deciding.

11. Can I set a post in concrete without mixing?

With Fast-Setting mix, yes: pour the dry powder into the hole around the post and soak it with water. The chemical reaction proceeds without mechanical mixing. This method works well for fence and mailbox posts but is not recommended for structural footings, where a fully mixed, uniform batch gives more reliable strength.

12. What happens if I run short of concrete in a hole?

If the first batch is still wet and plastic, you can mix more and continue pouring with minimal harm. If it has already begun to set, the new concrete will not bond properly and you get a cold joint — a weak horizontal seam. To avoid this, always calculate first, buy a spare bag, and mix enough for each hole in one continuous pour.

13. How do I estimate concrete for an irregular hand-dug hole?

Measure the diameter at the top, middle, and bottom of the hole and use the largest measurement in the calculation. Hand-dug holes are almost always bigger than they look. Alternatively, add 10 to 15 percent extra to the calculated bag count instead of the standard 5 percent waste allowance.

14. Does the calculator include a waste allowance?

Yes. The total bag count includes a 5 percent allowance for over-digging, spillage, and uneven hole walls, and the result is always rounded up to whole bags. For very rocky soil or hand-dug holes, consider buying one or two bags beyond the calculated total.

15. Can I use this calculator for deck footings and piers?

Absolutely. Enter the footing diameter and depth as the hole dimensions and the pier or post diameter as the post size. For sonotube footings, use the tube's inside diameter as the hole diameter and enter 0.1 as a nominal post size if there is no post, so the calculator treats nearly the full volume as concrete.

CONCLUSION

The Quikrete Post Hole Calculator turns a guessing game into a five-minute measurement task. By combining the true geometry of your holes with the published yield of each bag size, it gives you a bag count you can trust, a weight total for planning the haul, and a cost estimate for the budget. The worked examples show the method is simple: cylinder volume minus post volume, divided by bag yield, plus a small waste margin.

The single most important takeaway is this: always calculate before you dig, and always buy the spare bag. Concrete waits for no one once water hits it, and the cost of one extra bag is trivial compared to the cost of a cold joint or a second trip to the store. Measure carefully, pour confidently, and your posts will stand straight for decades.