Quikrete Calculator for Post Holes

The Quikrete calculator for post holes works out how many 50-pound bags of concrete mix you need to set a fence, deck, or mailbox post. You enter the post dimensions, hole depth and diameter, and the calculator returns the number of bags required, accounting for the post volume that displaces concrete. This page explains the math behind that calculation, gives a step-by-step example, and covers the practical details that affect your final bag count.

Formula

The volume of concrete needed for a post hole is the volume of the hole minus the volume of the portion of the post that sits inside it. For a cylindrical hole and a cylindrical post, the formula is:

Concrete volume (ft³) = π × (hole radius)² × hole depth − π × (post radius)² × hole depth

Where:

• hole radius = hole width / 2, converted to feet (divide inches by 12)
• post radius = post diameter / 2, converted to feet
• hole depth = depth of the hole in feet (use the actual depth of the concrete, not the total hole depth if you place gravel at the bottom)

The result is the volume of concrete in cubic feet. To convert to bags, divide by the volume one bag produces. A standard 50-pound bag of Quikrete concrete mix yields about 0.375 cubic feet (or 0.0139 cubic yards). The calculator above uses a factor of 40 lb per cubic foot (which is equivalent to 50 lb / 0.375 ft³) to convert weight to volume.

Worked Example

Using the default values from the calculator:

• Post diameter = 4 in (radius = 2 in = 0.167 ft)
• Hole depth = 2 ft
• Hole width = 12 in (radius = 6 in = 0.5 ft)
• Bag size = 50 lb

Step 1: Hole volume = π × (0.5)² × 2 = π × 0.25 × 2 = 1.5708 ft³

Step 2: Post volume (inside hole) = π × (0.167)² × 2 = π × 0.02778 × 2 = 0.1745 ft³

Step 3: Concrete volume = 1.5708 − 0.1745 = 1.3963 ft³

Step 4: Bags needed = 1.3963 ft³ × (40 lb/ft³) / 50 lb = 1.117 bags → round up to 2 bags

The calculator shows 2 bags and a concrete volume of 1.40 ft³ (rounded to two decimals).

Why the Post Volume Matters

Many DIYers mistakenly calculate the hole volume only, ignoring the post. That can overestimate the concrete needed by 20% or more, depending on post thickness relative to hole width. For a 4×4 post (actual 3.5 in) in a 12-inch hole, the post occupies about 8.5% of the hole volume. For a 6×6 post (actual 5.5 in) in the same hole, it’s about 21%. The calculator subtracts the post volume so you don’t buy extra bags.

How to Choose Hole Dimensions

Concrete post footings need to be wide enough to resist frost heave and lateral loads. A common rule of thumb is to make the hole width three times the post width, and the depth at least one-third to one-half the above-ground post height. For a 4×4 post, that means a 12-inch diameter hole; for a 6×6 post, an 18-inch hole. Depth depends on local frost depth – check with your building department for the required depth in your area. The calculator lets you adjust these values to match your project.

Common Mistakes and How to Avoid Them

• Mixing units: Always convert inches to feet before calculating. The calculator does this for you, but if you use a manual formula, forgetting to divide by 12 is the most common error.
• Using total hole depth when gravel is present: If you place 6 inches of gravel at the bottom for drainage, the concrete depth is only the remaining depth. Enter the concrete depth, not the dug depth.
• Rounding too early: Round the final bag count up, not the intermediate volume. Rounding the volume to 1.4 ft³ and then converting to bags might give 1.49 bags, which still rounds up to 2, but for larger holes early rounding can cause a one-bag error.
• Ignoring post shape: If you use a square post, the volume is width × width × depth. The calculator assumes a round post; for a square post, adjust the volume manually or use the equivalent diameter (a 4×4 post has a diagonal of 5.66 in, but the volume is 3.5×3.5×depth).

Bag Sizes and Yields

Quikrete concrete mix comes in 40, 50, 60, and 80-pound bags. The yield per bag varies by product – standard concrete mix yields about 0.3 ft³ for a 40-lb bag, 0.375 ft³ for a 50-lb bag, 0.45 ft³ for a 60-lb bag, and 0.6 ft³ for an 80-lb bag. The calculator uses a generic conversion of 40 lb per cubic foot, which is accurate for standard mix. For fast-setting concrete, the yield is similar. Always check the bag label for the exact yield, as some blends may differ slightly.

Alternatives to Quikrete Calculator

Quikrete’s own website has a project calculator, but you can also use the formula above with any spreadsheet. For irregular holes (e.g., dug with a clamshell digger), measure the average width and depth. If you’re setting multiple posts, multiply the per-post bag count by the number of posts, then round up to the nearest whole bag. For large jobs, consider ordering ready-mix concrete by the cubic yard – one cubic yard equals 27 cubic feet, so divide the total concrete volume by 27.

Safety and Practical Tips

• Wear gloves and eye protection when handling dry concrete mix – it’s caustic.
• Follow the mixing instructions on the bag; usually, you add water to the mix in a wheelbarrow or mixing box.
• For fence posts, pour the concrete slightly above ground level and slope it away from the post to shed water.
• Allow concrete to cure for at least 24-48 hours before attaching fence panels or applying weight to the post.
• If the hole is deeper than 3 feet, consider using a tube form to keep the concrete column straight and reduce the amount of mix needed.

Frequently Asked Questions

How many 50lb bags of concrete do I need for a 4×4 post?
For a typical 4×4 post (3.5 in actual) set in a 12-inch diameter hole that is 2 feet deep, you need about 1.4 cubic feet of concrete, which is roughly 2 bags of 50-lb mix. If the hole is deeper or wider, adjust accordingly.

Can I use the calculator for square posts?
Yes, but the formula assumes a round post. For a square post, calculate the post volume as width × width × depth and subtract that from the hole volume. For a 4×4 post, that’s 3.5 in × 3.5 in × 24 in = 294 in³, which is 0.17 ft³ – similar to the round post assumption.

What if I use gravel at the bottom of the hole?
Subtract the gravel depth from the hole depth before entering it into the calculator. The concrete only fills the space above the gravel. For example, if the hole is 30 inches deep and you add 6 inches of gravel, enter 2 feet (24 inches) as the depth.

Do I need to add extra concrete for a bell-shaped hole?
If the hole is wider at the bottom than the top (common for frost protection), the volume is larger than a straight cylinder. Measure the width at the widest point and use that as the hole width, or calculate the volume as a cone frustum for more accuracy.

Is fast-setting concrete better for posts?
Fast-setting concrete sets in about 20-40 minutes, which is convenient for posts that need to be held upright without bracing. The yield per bag is similar to standard mix, so you can use the same calculator. Just follow the mixing instructions on the bag.

How do I convert bags to cubic yards?
One 50-lb bag yields about 0.375 ft³, so 27 ft³ / 0.375 ft³ = 72 bags per cubic yard. For other bag sizes, divide 27 by the yield in cubic feet per bag.

Related

• How to Calculate Precipitation Rate
• How to Calculate Gallons from Inches
• How to Calculate Gallons to Pounds

in d | Post diameter | in | 4 | 0 | 24
in h | Hole depth | ft | 2 | 0 | 10
in w | Hole width | in | 12 | 0 | 36
in bagsize | Bag size | lb | 50 | 10 | 100
out bags | Bags needed | bags | ceil(round((pi * ((w/12)/2)^2 * h - pi * ((d/12)/2)^2 * h) * 0.5 / (bagsize/40), 6)) | 0
out vol | Concrete volume | ft³ | (pi * ((w/12)/2)^2 * h - pi * ((d/12)/2)^2 * h) * 0.5 | 2