Concrete for a 8×8 slab

What a 8×8 slab is usually for

• Small garden shed
• Generator or HVAC condenser pad
• Trash-bin enclosure
• Rain-barrel or pump pad

At 64 square feet this is the smallest footprint most people ever pour, and it is squarely in bag territory — a single afternoon, one rented mixer, no truck. It is also the size where the excavation and the base gravel take longer than the concrete itself, because the forms are quick but a level, compacted subgrade is the same amount of work per square foot no matter how small the pad is.

Mixing a 8×8 pad before it sets

That is a realistic one-day job for one determined person with a rented mixer, or a comfortable morning for two. Mix continuously and keep placing into the edge of what you have already poured so each batch knits into the last before it starts to stiffen. The order of operations is form, base, mix, finish, cure.

Budget for the tools as well as the concrete: a mixer runs about $70/day, a plate compactor for the base about $80/day, and a concrete saw for the joints about $60/day.

Why bags win on a 8×8 pad

At 0.79 cubic yards, this slab sits just below the point where a truck usually makes sense. Here is the comparison with the fees included:

Ready-mix is the better call at this size, even carrying the short-load fee. Beyond the price, a truck delivers the whole slab as one continuous placement — no cold joints between batches, no risk of the first section setting while you mix the last, and a consistent mix throughout. Mixing 40 bags to match it is a full day of heavy work with a worse result.

The short-load fee is what people miss. Suppliers charge it below roughly 4 cubic yards, typically $50–$150, because sending a truck out costs them the same whether it is full or not. Always ask for the delivered total including fees rather than the per-yard rate. The full reasoning is in bags vs ready-mix concrete.

Thickness on a 8×8 pad

Thickness is the one input that changes the order most, and it is chosen by what will sit on the slab rather than by budget. Here is the same 64 sq ft footprint at each common depth:

Going from 4″ to 6″ on this slab adds roughly $0 in materials — about 0% more. That is the entire cost of the decision, and it is why the advice for anything carrying a vehicle is to just pour the extra inches. Removing and replacing a cracked slab costs many times the difference.

The 3.5″ row is not a recommendation, it is a reality check: a “4 inch” slab poured on a subgrade that is a little high in places genuinely finishes at 3.5″ in those spots. If your base is not dead flat, order to the thicker figure. More on this in concrete slab thickness: 4″ vs 6″.

Control joints on a 8×8 pad

Concrete shrinks as it cures and it will crack — the only question is whether it cracks where you decided or where it chose. Control joints are the decision. The rule of thumb is a maximum spacing in feet of about two to three times the slab thickness in inches, so at 4″ that is 8–12 ft.

At 8×8 this slab fits inside a single panel at the maximum allowed spacing, so the rule does not force any joints on you. That said, a hard-freeze climate or a subgrade you are not confident about is a good reason to cut one anyway — a tooled joint disappears within a season, whereas a random shrinkage crack is permanent. The sequence is in forming and pouring a small slab.

Does a 8×8 pad need mesh?

Reinforcement does not stop concrete cracking; it holds the crack tight so the slab keeps acting as one piece. For a residential slab-on-grade of 64 sq ft, either of these is conventional:

Whichever you use, it has to sit in the middle of the slab to do anything. Mesh laid on the ground and walked over during the pour is the most common wasted material on a DIY slab — it ends up at the bottom, where it carries no tension at all. Use chairs or bar supports, and lift as you go if you have no other option.

Watch out on a pad this size

Equipment pads get poured too thin because they look small. A condenser or standby generator concentrates real weight onto four feet, so keep the full 4 inches and compact the base properly — a pad that settles unevenly puts a permanent twist into the equipment mounted on it.

The arithmetic

Volume = 8 × 8 × (4 ÷ 12) = 21.33 ft³, then ÷ 27 = 0.79 cubic yards. An 80 lb bag yields 0.60 ft³, so 21.33 ÷ 0.60 = 36 bags before waste; add 10% and round up to 40 bags. Ready-mix rounds up to the quarter yard, giving 1 yd³. Every constant behind those steps is documented in how we calculate, and the reasoning is in how much concrete do I need?

Other slab sizes

• Concrete for a 8×10 slab
• Concrete for a 8×12 slab
• Concrete for a 10×10 slab
• Concrete for a 10×12 slab
• Concrete for a 10×16 slab

See all slab sizes →