Fast Setting Concrete Calculator

1. Choose the shape: a round post hole (diameter by depth) or a slab or pad (length by width by thickness).
2. Enter the dimensions. Keep the hole depth and slab sizes in feet, and diameters and thickness in inches, as labelled.
3. For a post hole, add the post width so the post is subtracted. Set the quantity of posts and a 5 to 10 percent waste allowance.
4. Read the result: the number of 50 pound bags, rounded up. Add a price per bag for the cost.

Here is what each result line means:

Fast-setting concrete is a pre-blended mix of cement, sand and gravel with accelerators that make it harden in minutes. It is the standard product for setting fence, mailbox and sign posts, because you pour the dry mix straight into the hole and add water, with no mixing.

QUIKRETE Fast-Setting Concrete Mix is designed to set hard in about 20 to 40 minutes, so a post is plumb and firm quickly. It is sold in 50 pound bags that yield roughly 0.375 cubic feet each, and this calculator divides the hole or pad volume by that yield. It is made for posts and small pads; for a larger pour where you need working time, a standard mix sized with the concrete mix calculator or a ready-mix order is usually the better choice.

For a post hole the volume is pi times the radius squared times the depth, with the radius taken as half the hole diameter. The post sitting in the hole displaces concrete, so its volume is subtracted.

1. Find the hole volume in cubic feet from the diameter and depth.
2. Subtract the post volume, worked out the same way from the post width, so you only buy the concrete that fills the gap.
3. Multiply by the number of posts and add the waste allowance.
4. Divide by 0.375 cubic feet per 50 pound bag and round up to whole bags.

For a slab or pad the volume is simply length times width times thickness, with thickness converted from inches to feet.

Suppose you are setting a fence post in a 10 inch diameter hole 2 feet deep, with a 4 inch post and a 10 percent waste allowance.

Calculation: hole volume = pi x (5/12)^2 x 2 = 1.09 cu ft. The 4 inch post displaces pi x (2/12)^2 x 2 = 0.17 cu ft, leaving 0.92 cu ft. Adding 10 percent waste gives 1.01 cu ft, and at 0.375 cubic feet a bag that is 1.01 / 0.375 = 2.7, rounded up to 3 bags.

The lesson: three 50 pound bags per post means you pour the dry mix in, soak it, and the post is set in well under an hour, far faster than mixing and waiting for standard concrete to cure.

Hole Size

A common rule is to dig the hole three times the post width and one third to one half the post height deep. A wider or deeper hole holds more concrete, so the bag count climbs quickly with diameter.

Post Size

The post displaces concrete. A 4 by 4 post in a 10 inch hole leaves more gap to fill than a 6 by 6 in the same hole, so subtracting the post keeps the estimate honest.

Number of Posts

Bags scale with the number of holes. Set the quantity to the whole fence line and the tool totals the bags in one step.

Waste and Over-digging

An auger or clamshell rarely cuts a clean cylinder. A 5 to 10 percent allowance, plus rounding up to whole bags, covers the extra.

Fast-setting mix and standard concrete mix both set posts, but they trade speed for working time. Fast-setting is poured dry and sets in minutes; standard mix is batched wet and gives you time to plumb and brace.

If you would rather batch standard concrete for the posts, size the cement with the cement calculator and price the job with the concrete cost calculator.

Setting Fence Posts

The signature use. Dig the hole, stand the post, pour the dry bags around it and soak with water; the post is firm in under an hour.

Mailbox, Sign and Deck Posts

Any single post that needs to be plumb and set quickly suits fast-setting mix, with no mixer to clean afterwards.

Small, Quick Pads

A small equipment pad or steppingstone that you want to use the same day can use fast-setting mix, though a standard mix is cheaper once the pad gets large.

1. Not Subtracting the Post

Filling the whole hole volume over-buys, because the post itself takes up space. Enter the post width so only the gap is counted, though a little extra is cheap insurance.

2. Mixing Up the Yield with Standard Bags

A 50 pound fast-setting bag yields about 0.375 cubic feet, less than an 80 pound standard bag. Do not reuse the 0.60 figure or you will under-buy.

3. Mixing It When You Should Pour It Dry

For posts the mix goes into the hole dry and is soaked in place. Pre-mixing wastes the fast set and leaves you rushing.

4. Using It for a Big Pour

The short set time fights you on anything large. Past a pad or two, switch to a standard mix or ready-mix for cost and working time.

5. Ignoring Frost Depth

The bag count does not know your frost line. Set the hole depth from local code so the footing sits below frost, then let the tool size the bags.

The arithmetic is exact and the bag yield is the manufacturer’s published figure; the usefulness depends on the hole size, the post you subtract and the waste allowance you enter.

It depends on the hole and the post. For a 10 inch hole 2 feet deep with a 4 inch post, you need about 3 fifty-pound bags after subtracting the post and adding waste. A smaller 8 inch hole takes about 2 bags. Enter your sizes above for the exact count.

About 0.375 cubic feet of set concrete. Because a cubic yard is 27 cubic feet, it takes roughly 72 fifty-pound bags to equal a full cubic yard, which is why fast-setting mix suits posts and small jobs rather than large pours.

No. For posts you pour the dry mix into the hole around the post and then add water, with no mixing. It sets hard in about 20 to 40 minutes. Pads can be mixed, but the short set time leaves little working time.

A common rule is to bury one third to one half of the post height and dig the hole about three times the post width. Local code and frost depth set the minimum. This tool sizes the bags once you enter the hole depth and diameter.

QUIKRETE Fast-Setting Concrete Mix is designed to set hard in about 20 to 40 minutes, so a post is firm quickly. Full strength develops over the following days, as with ordinary concrete.

It reaches a usable strength much faster, which is its advantage, and cures to a comparable strength over time. The trade-off is a short working time and a higher cost per cubic foot, so it is chosen for speed, not for large volumes.

About 72. A cubic yard is 27 cubic feet and each 50 pound bag yields roughly 0.375 cubic feet, so 27 divided by 0.375 is 72 bags. That quantity is why a large pour is cheaper with standard or ready-mix concrete.

For a small pad you can, and it will be usable quickly. For anything larger the short set time and higher cost make a standard concrete mix or a ready-mix delivery the better option.

Yes, because the post occupies part of the hole. Entering the post width subtracts its volume so you only buy the concrete that fills the gap, though rounding up and the waste allowance keep a small margin.

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

• QUIKRETE Fast-Setting Concrete Mix (No. 1004) (QUIKRETE; sets in 20 to 40 minutes, pour dry into the hole and soak, 50 lb bag).
• National Ready Mixed Concrete Association (NRMCA; standard and ready-mixed concrete for larger pours).
• American Cement Association (American Cement Association; what concrete and cement are).