Concrete Calculator (slab)

How the concrete estimate is computed

The math is volume, then a conversion to how concrete is sold:

• Volume = length × width × thickness in feet. A 10 × 10 ft slab at 4″ is 10 × 10 × (4 ÷ 12) = 33.33 ft³ = 1.23 yd³.
• Bags = volume ÷ the bag's yield, rounded up. An 80 lb bag yields 0.6 ft³, so 33.33 ÷ 0.6 = 56 bags before waste.
• Ready-mix is ordered by the cubic yard in 0.25-yard steps, with your overage added.

Bag yields — and what a yard of bagged concrete really costs

Yields come from the QUIKRETE #1101 and Sakrete High-Strength data sheets, which match at the common 80 lb size. Fast-setting and specialty mixes yield differently. The last column is the one worth staring at:

Bag	Yield	Bags per yd³	Est. price	Est. cost per yd³
40 lb	0.3 ft³	90	≈ $4.00	≈ $360
50 lb	0.38 ft³	72	≈ $5.00	≈ $360
60 lb	0.45 ft³	60	≈ $5.50	≈ $330
80 lb	0.6 ft³	45	≈ $6.50	≈ $293
90 lb	0.68 ft³	40	≈ $7.50	≈ $300

The 80 lb bag is the cheapest way to buy bagged concrete, at about $293/yd³ — still roughly 77% more than delivered ready-mix (≈ $165/yd³). Small bags are convenient, not cheap: 40 lb costs about $68 more per yard for identical concrete.

Worked example: 10 × 10 ft slab at 4″

• Volume: 33.33 ft³ = 1.23 cubic yards
• Bags (80 lb): 56 with no waste, 62 with 10% overage (≈ $403)
• Ready-mix: 1.5 cubic yards after the ¼-yard round-up, ≈ $248 before fees

Tip: running short means a cold joint or an expensive second trip — that's why a 10–15% overage is standard. Order a little extra, not a little less.

Every common slab size, worked out

All 20 sizes are computed at the standard 4″ residential thickness with a 10% waste allowance. Follow any row for the joint layout, rebar takeoff, pour plan and cost for that exact footprint.

Slab	Area	Ready-mix order	80 lb bags	Recommended
8×8 slab	64 sq ft	1 yd³	40	Buy bags
8×10 slab	80 sq ft	1.25 yd³	49	Buy bags
8×12 slab	96 sq ft	1.5 yd³	59	Order ready-mix
10×10 slab	100 sq ft	1.5 yd³	62	Order ready-mix
10×12 slab	120 sq ft	1.75 yd³	74	Order ready-mix
10×16 slab	160 sq ft	2.25 yd³	98	Order ready-mix
10×20 slab	200 sq ft	2.75 yd³	123	Order ready-mix
12×12 slab	144 sq ft	2 yd³	88	Order ready-mix
12×14 slab	168 sq ft	2.5 yd³	103	Order ready-mix
12×16 slab	192 sq ft	2.75 yd³	118	Order ready-mix
12×20 slab	240 sq ft	3.5 yd³	147	Order ready-mix
12×24 slab	288 sq ft	4 yd³	176	Order ready-mix
16×16 slab	256 sq ft	3.5 yd³	157	Order ready-mix
16×20 slab	320 sq ft	4.5 yd³	196	Order ready-mix
20×20 slab	400 sq ft	5.5 yd³	245	Order ready-mix
20×24 slab	480 sq ft	6.75 yd³	294	Order ready-mix
24×24 slab	576 sq ft	8 yd³	352	Order ready-mix
24×30 slab	720 sq ft	10 yd³	440	Order ready-mix
30×30 slab	900 sq ft	12.25 yd³	550	Order ready-mix
30×40 slab	1,200 sq ft	16.5 yd³	734	Order ready-mix

The order column includes waste and the ¼-yard round-up, so it runs ahead of true volume: only 2 of the 20 sizes hold less than 1 cubic yard of actual concrete, which is where bags become the default.

Bags or ready-mix? The short-load fee decides

Comparing per-yard prices is the mistake nearly everyone makes, because ready-mix isn't sold at its per-yard price on a small pour. Suppliers add a short-load fee below roughly 4 cubic yards — typically $50–$150 — because dispatching a truck and driver costs the same whether the drum is full or a quarter full. Here is what that does to the real delivered rate:

Slab	Order	Bags cost	Delivered, fees in	Effective $/yd³
8×8 slab	1 yd³ / 40 bags	≈ $260	≈ $215–$315	$215–$315
10×10 slab	1.5 yd³ / 62 bags	≈ $403	≈ $298–$398	$198–$265
12×16 slab	2.75 yd³ / 118 bags	≈ $767	≈ $504–$604	$183–$220
16×20 slab	4.5 yd³ / 196 bags	≈ $1,274	≈ $743	$165
20×20 slab	5.5 yd³ / 245 bags	≈ $1,593	≈ $908	$165
24×30 slab	10 yd³ / 440 bags	≈ $2,860	≈ $1,650	$165

The 8×8 slab pours only 1 yd³, and the fee alone pushes its effective rate to $215–$315 per cubic yard against a headline price of $165. Past the threshold the fee disappears — the 24×30 slab takes 10× the concrete at the list rate. Always ask for the delivered total including fees, not the per-yard price.

Across all 20 sizes at these estimates, bags never come out cheaper than the truck at the low end of the fee. They edge ahead only on the smallest slabs (8×8 slab, 8×10 slab, 8×12 slab), and only if the fee lands at the top of its range. So don't choose bags to save money — choose them for what money can't buy on a small pour: no minimum order, no delivery window, no truck access, and the freedom to stop when it rains. Ready-mix buys the opposite: one continuous placement, no cold joints, one consistent mix. Full reasoning in bags vs ready-mix concrete.

How thick should the slab be?

Thickness is set by what will sit on the slab, not by budget. 4″ is the residential standard for patios, sheds, walkways and foot traffic; 5–6″ is the range for vehicles and heavy loads. What the extra inches cost:

Slab	4″	5″	6″	4″ → 6″
10×12 (120 sq ft)	1.75 yd³ · ≈ $289	2.25 yd³ · ≈ $371	2.5 yd³ · ≈ $413	+ $124
12×16 (192 sq ft)	2.75 yd³ · ≈ $454	3.5 yd³ · ≈ $578	4 yd³ · ≈ $660	+ $206
20×20 (400 sq ft)	5.5 yd³ · ≈ $908	7 yd³ · ≈ $1,155	8.25 yd³ · ≈ $1,361	+ $454

Concrete scales exactly with depth, but nothing else in the job does — excavation, forms, base, reinforcement and finishing are identical at 4″ and 6″. So the step-up looks alarming against the concrete and trivial against the project: across those three slabs it is roughly 17–19% of what a contractor charges to install the slab. Note too that a nominal 4″ slab poured over a high subgrade finishes at 3.5″ in those spots, so if your base isn't dead flat, order to the thicker figure. Full comparison in concrete slab thickness: 4″ vs 6″.

Control joints: deciding where it cracks

Concrete shrinks as it cures and it will crack; the only question is whether it cracks along a line you chose. The rule: maximum spacing in FEET is 2–3 times the slab thickness in INCHES — so 4″ → 8–12 ft, 5″ → 10–15 ft, 6″ → 12–18 ft. Cut to ¼ of the slab depth (1″ on a 4″ slab) and keep every panel near square, inside the 1.5:1 limit — a long thin panel cracks across its middle however close the joints are.

Slab (4″)	Panels	Panel size	Cuts	Linear ft of joint
10×10 slab	1 × 1 = 1	10 × 10 ft	0	0 ft
12×12 slab	1 × 1 = 1	12 × 12 ft	0	0 ft
10×20 slab	1 × 2 = 2	10 × 10 ft	1	10 ft
12×24 slab	1 × 2 = 2	12 × 12 ft	1	12 ft
20×20 slab	2 × 2 = 4	10 × 10 ft	2	40 ft
24×30 slab	2 × 3 = 6	12 × 10 ft	3	78 ft

The 10×10 and 12×12 rows need no cuts: at 4″ the maximum spacing is 12 ft, so a 12 ft square is the largest slab the rule lets you pour as one panel. Permitted isn't wise, though — in a hard-freeze climate, or over a subgrade you don't trust, most finishers still cut it. A tooled joint disappears within a season; a shrinkage crack never does.

Timing matters as much as layout. Cut the joints the same day, roughly 6 to 12 hours after finishing — once the surface carries a saw without ravelling, but before shrinkage has done its work. Wait a week and you're cutting joints into cracks that already formed. A walk-behind saw rents for about $60/day; on a small pad a hand groover run along a straightedge does the same job free.

The base matters more than the extra inch

With a fixed budget, spend it under the slab rather than in it. Four to six inches of compacted gravel — crushed stone with fines, not smooth pea gravel — does more for a slab's life than an extra inch of concrete, at a fraction of the cost. It supports the slab uniformly, drains water away from the underside, and stops frost heave lifting one corner:

Slab	Concrete (4″)	Gravel base at 4″	Gravel base at 6″
10×12 slab	1.48 yd³	1.48 yd³	2.22 yd³
16×20 slab	3.95 yd³	3.95 yd³	5.93 yd³
20×24 slab	5.93 yd³	5.93 yd³	8.89 yd³

You're moving as much gravel as concrete, or half as much again — price it with the gravel calculator first. Compact it in lifts of 2–3 inches rather than dumping the full depth and rolling over it once: a plate compactor rents for about $80/day and can't densify more than a few inches at a time. Loose fill is the most common cause of a pad that cracks in year one, and nothing on the surface reveals it until it does.

Why the waste factor is 10%, and why it goes on before the round-up

Ten percent isn't padding for its own sake. It covers spillage from the barrow and chute, concrete left in the drum, forms bowing under wet concrete, and — the big one — a subgrade that isn't level. Half an inch of extra depth on a nominal 4″ slab is 13% more concrete on its own, and half an inch of dip is invisible across a 12 ft form. On the 10 × 10 example the allowance is 6 extra bags, about $39 — cheap against a cold joint, the permanent weak line you get where fresh concrete meets concrete that has begun to set.

Order of operations matters too. This calculator applies the waste percentage to the volume, then rounds up to whole bags once. The shortcut — round to whole bags first, then add 10% — rounds twice and charges waste on a partial bag you were never going to place. Across these 20 sizes it over-orders on 8 of them. Every constant is listed in how we calculate.

Curing is not drying

Concrete doesn't dry to gain strength — it hydrates, a reaction between cement and water that runs for weeks as long as the water is there. Concrete that dries too fast stops gaining strength permanently, so the goal for the first week is the opposite of what most people assume: keep it damp. Mist it, cover it with plastic or wet burlap, or use a curing compound — starting within hours of finishing, not the next morning.

Age	Typical strength	What it will take
1 day	≈ 15–20%	Keep it damp. Strip forms only if you must.
3 days	≈ 40%	Light foot traffic. Still curing — keep it damp.
7 days	≈ 65–70%	Normal foot traffic; a passenger car at a pinch.
28 days	100% (design strength)	Full load — vehicles, machinery, structures.

The rule: foot traffic after 24–48 hours, vehicles after 7 days, full design load at 28 days. Parking on a four-day-old pad leaves no evidence until the cracks appear months later. Hot, dry or windy weather makes it harder, and below about 40 °F hydration slows to a crawl — a cold-weather pour needs blankets and more time.

Still deciding whether to pour it yourself? See DIY vs hiring a pro for a concrete slab, slab cost by size, how to form and pour a small slab, or browse the guides.