Bagged concrete vs ready mix: where the DIY line is

Bagged concrete vs ready mix comes down to how much concrete you need, and the math takes two minutes. Length times width times thickness, all in feet, gives you cubic feet. 27 cubic feet is a cubic yard. Under about a third of a yard, bags are the right call. Past a full yard you’re mixing 45-plus bags against a clock, and that’s how cold joints get into a slab.

How much concrete does your slab actually need?

It’s three numbers and one division. Multiply length by width by thickness, all in feet. So a 4-inch slab is 0.333 feet thick, and a 5-inch slab is 0.417. That gives you cubic feet. Divide by 27 for cubic yards, or divide by the yield printed on the bag for a bag count.

Makers print about 0.60 cubic feet of finished concrete per 80-pound bag, and about 0.45 per 60-pound bag. That number is on the bag in your hand. Use it for the division.

Two pallets of bags sitting on the pad looks like plenty, right up until you divide. A 12x16 pad at 4 inches takes 107 bags, and at 5 inches it takes 133. Nobody hand mixes that on purpose. People do it because they bought the bags before they did the math.

Where does the line actually fall?

• Under about 1/3 of a yard (roughly 15 bags): bags. One person and a wheelbarrow or a mixing tub can handle it. Think post holes, mower pads, an air conditioner base, or a stoop.
• 1/3 to 1 yard (roughly 15 to 45 bags): it’s a judgment call. A rented mixer and a helper make it easy. One person with a hoe won’t keep up.
• Over 1 yard: call a plant. Bags can reach that volume. The clock can’t.
• Anything outdoors where it freezes: order from a plant. Air-entrained concrete is non-negotiable on outdoor flatwork, and that air comes from the plant, not from a bag you shook out yourself.

The freeze rule catches people on acreage more than any other. An outdoor slab in Oklahoma goes through 60 to 70 freeze-thaw cycles a winter. Air entrainment puts tiny air bubbles in the mix, and those give freezing water somewhere to grow into. It’s a batch-plant add-in, so ask for it on the phone when you order.

So is ready mix better than mixing bags for a shed foundation? For a 10x10 shed pad at 4 inches, yes. That’s 1.2 yards and about 56 bags, just past the line. And since it sits outside, it needs air-entrained plant concrete too.

When we pour a pad like that, it’s our Slab Boss Pour. Ready-mix concrete comes in on a truck, and we set the forms, level it, float it, and give it a broom finish, with basic ground prep included. Welded wire mesh goes inside, and a spray-on curing coat goes on after we finish. A 10x10 pad (100 square feet) runs $700 to $3,400 before the drive cost, our trip out to your place. The low end is our standard pad, and the top end adds every option, up to a line pump. The quote tool prices your size, and its thickness slider covers a 5-inch pad.

What is a cold joint, and why does it matter on a hand-mixed slab?

A cold joint is the seam that forms when one batch of concrete starts to set before the next batch lands against it. On a truck pour, the whole slab shows up within a few minutes. It stays soft while the crew screeds and floats it, so there’s no seam. Hand mixing 100-plus bags takes hours. The first wheelbarrow you dumped is getting stiff while you’re still tearing open bag 40, and the concrete you place against it never knits into one piece. What you get is a weak plane running through the slab where the two loads met. You also get a color line on top, because the older concrete set at a different water content. Sealer won’t hide it, and neither will a finish. That’s why the yard line matters more than the dollar gap between bags and a truck.

Heat makes that window even shorter. A pour that gives you plenty of working time in April gives you a lot less on a 95 degree July afternoon, out on open acreage with wind across it. The top starts losing water the moment it lands. Our seasonal pour guide for Oklahoma covers which months make this easier.

How do you read a bag of concrete mix?

Bagged concrete mix comes with four things printed on the front of the bag, and those four decide if it belongs in your slab. The PSI rating is the design strength. 3,000 PSI is common on general-purpose mix, and 4,000 PSI is what a slab you drive on wants. The product name tells you if it’s for slabs at all. Sand mix and sand topping mix have no gravel in them, so they’re for patching and thin toppings, not a slab you park on. Fast-setting mix is a fence-post product. It gives you almost no working time, and that’s the opposite of what you need to screed and float a surface. And the water amount in the mixing steps is the most you should add, not the amount to aim for. Every extra gallon you add to make the mud easier to move comes back out of the finished strength.

Water you add to make the mix easier to work costs you strength and density. It hurts the top first, because the extra water rises with the bleed water and leaves a soft, chalky skin. A slab mixed wet across 100 bags is a slab that dusts. Mix to the low end of the water range on the bag, and let a mixer do the work instead of the water.

What does a ready mix truck actually buy you?

One pour, start to finish, with no seams. The rest of the ticket is the cost of getting it to you.

ASTM C94 is the rule book for ready-mixed concrete. By default, it says a load has to be out of the truck within 90 minutes of the water hitting the cement, or 300 turns of the drum, whichever comes first. That’s the industry drawing a line around the exact problem hand mixing has. With ready mix, the concrete shows up as one batch, at one water content, and it goes down before any of it starts to set.

Ask a plant three things when you call:

• The smallest load they’ll deliver. Most will bring less than a full truck. The question is how much less.
• The short-load charge. Below a plant’s full-load amount, they add a charge per yard. On a 2 or 3 yard pour, it’s the biggest surprise on the ticket, so get it up front.
• How much unloading time is included. Trucks bill for waiting past a set time. Have your forms built, your base packed, and your help standing there before it rolls up.

Thickness, reinforcement, and timing live elsewhere

Thickness and steel are separate choices from where you buy the concrete, and getting them wrong costs more than either mix choice. A slab you drive on wants 4 inches at least, over a packed base, and more for trucks and trailers. Our guide on how thick a concrete driveway should be works through the loads. Steel on clay that swells and shrinks (expansive clay) is its own topic, covered in whether you need rebar.

Timing is the third one. You’ll hear “28 days” a lot. That’s a strength milestone, and it’s not the same clock a sealer or coating cares about. If a finish is going on this slab later, cure time versus dry time explains why those are two separate waits.

The math comes before the purchase

Cubic feet, then cubic yards, then bags. Under a third of a yard, buy bags and enjoy the afternoon. Over a yard, call a plant and ask the three questions above before you decide anything. The short-load fee is the number that really moves the comparison, and nobody quotes it until you ask.

If the division comes back at 107 bags, that pad wanted a truck. No version of that weekend ends with a slab you’re happy with. Better to find that out with a tape measure than with a wheelbarrow.