Fast Setting Concrete Calculator

1. Enter the number of posts or holes.
2. Enter the hole diameter and depth in inches (rules of thumb: hole three times the post, depth half the above-ground height).
3. Pick the post shape and size so its volume is subtracted, or choose No post to fill the whole hole.
4. Adjust the bag yield and waste if needed, then read the bags.

Here is what each result means:

Fast-setting concrete is a pre-mixed concrete with an accelerator, sold in 50 pound bags, that sets hard in 20 to 40 minutes. It is made for setting posts: fence, mailbox, deck, sign and basketball posts.

Its selling point is the no-mix method. You stand the post in the hole, pour the dry mix in around it, then add water to the dry mix in the hole, with no mixing in a barrow or mixer. Because you fill the hole, you size it by the hole volume, minus the space the post takes. A 50 pound bag yields about 0.375 cubic feet of concrete.

The hole is a cylinder, so its volume is pi times the radius squared times the depth. The post displaces some of that volume, so it is subtracted. Dividing by the 0.375 cubic foot bag yield gives the bags, rounded up.

1. Find the hole volume: pi times the hole radius squared times the depth, in feet.
2. Subtract the post volume, for a round or square post of the same depth.
3. Add a waste allowance, divide by 0.375 cubic feet per bag, and round up for one hole.
4. Multiply by the number of holes for the total.

Suppose you are setting a 6 inch round post in an 18 inch diameter hole, 24 inches deep.

Calculation: hole volume = pi x (9/12)^2 x (24/12) = 3.53 cu ft. The post takes pi x (3/12)^2 x 2 = 0.39 cu ft, leaving 3.14 cu ft. Dividing by 0.375 is 8.4, so 9 bags per hole, which matches the manufacturer guidance.

Pour those bags in dry around the post, then add water: the no-mix method. For several posts, multiply the per-hole bags by the number of holes.

Both set posts, but they work differently. Fast-setting is made for the no-mix method and speed; standard bagged concrete is mixed and cheaper per bag.

For a slab or a larger pour, the concrete bag calculator and standard mix are usually the better value.

Hole Size

The diameter matters most, because volume rises with the square of the radius. A slightly wider hole takes noticeably more concrete.

Post Size and Shape

A bigger post displaces more concrete, lowering the count. A square post of the same width displaces more than a round one.

Bag Yield

Yields differ by product. The 0.375 cubic feet here is for 50 lb fast-setting concrete; change it if your bag states another figure.

Waste and Oversized Holes

Augered holes are rarely exact, and some mix spills. A small allowance covers it.

Setting Posts

Fence, mailbox, deck, sign and basketball posts, where a quick set lets you move on the same day.

The No-mix Method

Where mixing is awkward, pour the dry mix into the hole around the post and add water, with no barrow or mixer.

Repairs That Cannot Wait

Small, time-critical fixes where standard concrete would set too slowly.

1. Forgetting to Subtract the Post

The post fills part of the hole. Ignoring it over-orders, though a small surplus is no bad thing.

2. Mixing It First

Fast-setting concrete is made for the no-mix method and sets fast. Mixing a batch can leave it setting before you place it.

3. Too Little Water

Follow the directions for water added to the hole. Too little leaves dry pockets that never gain strength.

4. Hole Too Small

A hole about three times the post diameter gives enough concrete around the post. A tight hole is weak and hard to fill evenly.

5. Using It for a Slab

Fast-setting concrete is for posts and repairs. For a slab, use standard concrete and the concrete calculator.

The arithmetic is exact for the hole and post sizes you enter; the bag count depends on the real yield and how the hole is dug.

It depends on the hole. A 4 inch post in a 10 inch hole about 2 feet deep takes 2 to 3 bags, while a 6 inch post in an 18 inch hole 2 feet deep takes about 9. Enter your hole and post size for the exact count.

About 0.375 cubic feet of mixed concrete, the same as a standard 50 pound bag. A cubic foot takes between two and three bags, and a cubic yard about 72.

Stand the post in the hole, pour the dry mix in around it to about 3 to 4 inches below grade, then add water to the dry mix in the hole. Do not mix it first. It sets hard in 20 to 40 minutes.

A common rule is a hole about three times the post diameter, and a depth of about one-half the above-ground post height. A 4 inch post then takes roughly a 10 to 12 inch hole.

Yes, for accuracy, because the post fills part of the hole. This calculator subtracts a round or square post. A small surplus of concrete is fine, so the exact subtraction is not critical.

It reaches a working strength quickly and a comparable final strength, often around 4,000 psi at 28 days. The difference is speed, not a much higher final strength.

It is not ideal for slabs because it sets too fast to place and finish a large area. Use standard concrete and the concrete calculator for slabs.

Follow the bag directions, typically a set amount poured over the dry mix in the hole. Too little water leaves weak, dry pockets; too much weakens the mix.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• QUIKRETE Concrete Mix product data (QUIKRETE; bag sizes and yields).
• QUIKRETE concrete and bag calculator (QUIKRETE; bag-count and post-setting method).
• National Ready Mixed Concrete Association (NRMCA; concrete materials).
• American Cement Association (Portland Cement Association) (American Cement Association; concrete basics).