Concrete Slab Calculator

Enter the length, width and thickness of your slab to get the concrete in cubic yards and bags, plus the rebar and crushed-stone base for the same footprint. Add several slabs to price a patio, walkway and pad together.

Dimensions

Length

Width

Thickness

Quantity

Extra for waste

Bag size

Price per bag

Ready-mix per yd³

Rebar grid

Rebar spacing

Gravel base depth

Concrete needed

2.61cubic yards

70.4 cu ft · 1.99 m³ · includes 10% extra

At 2.61 cubic yards, order ready-mix: about 2.75 yd from the plant, or a truck plus a pump if access is tight.

Premixed bags

40 lb bags (0.3 cu ft each): 235
50 lb bags (0.375 cu ft each): 188
60 lb bags (0.45 cu ft each): 157
80 lb bags (0.6 cu ft each): 118

Cost estimate

118 × 80 lb bags: $767.00
Ready-mix, 2.75 yd³ ordered: $481.25
Wet weight: 10,560 lb (5.28 tons)

Slab extras

Rebar at 18" on center: 251 linear ft
20 ft bars (incl. 10% laps): 14
Crushed stone base, 4": 2.73 yd³ · 3.68 t

How the slab estimate works

The calculator multiplies length × width × thickness to get cubic feet, adds your waste allowance, and divides by 27 for cubic yards. Three more numbers come from the same footprint:

Bags — cubic feet divided by the bag yield (0.45 cu ft for 60 lb, 0.60 cu ft for 80 lb), rounded up.
Rebar — bars both ways at the chosen spacing, held 3 inches in from each edge, then converted to 20 ft sticks with 10% added for lap splices and offcuts.
Gravel base — footprint × base depth, plus 15% because loose crushed stone compacts by roughly that much.

The ÷ 324 shortcut

For a slab, square feet × thickness in inches ÷ 324 gives cubic yards directly. The 324 is 12 inches per foot × 27 cubic feet per yard. A 20 × 20 ft slab at 4 inches: 400 × 4 ÷ 324 = 4.94 yd³. At 5 inches: 6.17 yd³. Each extra inch of thickness on a 400 sq ft slab costs about 1.2 more yards.

Thickened edges and uneven ground

Slabs that sit directly on the ground often have a thickened edge — a perimeter strip 8–12 inches wide poured 8–12 inches deep. Add it as a second pour using the Footing / wall shape on the concrete calculator, with the slab perimeter as its length. If your subgrade varies, measure the depth at several points and use the average; a half-inch dip across a 400 sq ft pad adds 0.6 yards.

Worked examples

Example 1 — 12 × 16 ft patio, 4 inches thick, rebar at 18 inches

192 sq ft × 4 ÷ 324 = 2.37 cubic yards, 2.61 yd with 10% extra — order 2.75 yards of ready-mix, or 118 bags of 80 lb mix. Rebar at 18 inches on center: 8 bars along the length and 11 across, 250 linear feet, or 14 twenty-foot bars with laps. A 4-inch crushed-stone base needs about 2.7 loose yards, 3.7 tons.

Example 2 — 24 × 24 ft two-car garage floor

At 4 inches: 576 × 4 ÷ 324 = 7.11 yd³. At 6 inches: 10.67 yd³, and 11.73 yd³ with 10% extra — about 12 yards, more than a single truck on many routes, so ask the plant whether it will send two. Rebar at 18 inches: 32 bars, 752 linear feet, 42 twenty-foot sticks. Four inches of base: 8.2 yards loose, about 11 tons.

Concrete for common slab sizes

4-inch slabs (patios, walks, shed floors)

Slab	Cubic yards	40 lb bags	60 lb bags	80 lb bags
8 × 10 ft	0.99	89	60	45
10 × 10 ft	1.23	112	75	56
10 × 12 ft	1.48	134	89	67
12 × 12 ft	1.78	160	107	80
12 × 16 ft	2.37	214	143	107
12 × 20 ft	2.96	267	178	134
14 × 20 ft	3.46	312	208	156
20 × 20 ft	4.94	445	297	223

6-inch slabs (driveways, garages)

Slab	Cubic yards	40 lb bags	60 lb bags	80 lb bags
10 × 20 ft	3.7	334	223	167
12 × 20 ft	4.44	400	267	200
12 × 24 ft	5.33	480	320	240
20 × 20 ft	7.41	667	445	334
24 × 24 ft	10.67	960	640	480
24 × 30 ft	13.33	1200	800	600

Cubic yards by slab area and thickness

Area	4" deep	5" deep	6" deep
100 sq ft	1.23 yd³	1.54 yd³	1.85 yd³
150 sq ft	1.85 yd³	2.31 yd³	2.78 yd³
200 sq ft	2.47 yd³	3.09 yd³	3.7 yd³
250 sq ft	3.09 yd³	3.86 yd³	4.63 yd³
300 sq ft	3.7 yd³	4.63 yd³	5.56 yd³
400 sq ft	4.94 yd³	6.17 yd³	7.41 yd³
500 sq ft	6.17 yd³	7.72 yd³	9.26 yd³
600 sq ft	7.41 yd³	9.26 yd³	11.11 yd³
750 sq ft	9.26 yd³	11.57 yd³	13.89 yd³
1,000 sq ft	12.35 yd³	15.43 yd³	18.52 yd³

Recommended slab thickness

Project	Depth	Notes
Walkway	4 in	2–3 in sub-base is enough on firm soil.
Patio	4 in	Over 4 in of compacted base; slope away from the house.
Shed floor	4 in	6 in or a thickened edge for a workshop with heavy tools.
Hot tub pad	6 in	A filled tub with people can exceed 5,000 lb.
Driveway	5–6 in	4 in only for light cars on very stable soil.
Garage floor	5–6 in	Rebar or mesh, vapor barrier under heated spaces.
RV or equipment pad	6–8 in	#4 rebar at 12–16 in on center.
Gravel sub-base	4–6 in	Crushed stone or road base, compacted in 2-in lifts.

Material weights for slab work

Wet concrete is about 150 lb per cubic foot. The base materials below are sold by the ton, which is why the calculator converts the base volume to tons; for any other area, the gravel calculator does the same conversion.

Material	lb / cu ft	lb / cu yd	tons / cu yd	Typical use
Gravel (general, 3/4")	104	2,800	1.4	Driveways, drainage, sub-base
Pea gravel	100	2,700	1.35	Paths, play areas, between pavers
Crushed stone (#57)	100	2,700	1.35	Drainage, concrete sub-base
Crusher run / road base	111	3,000	1.5	Compactable base for driveways and pavers
River rock	100	2,700	1.35	Decorative beds, dry creek beds
Decomposed granite	100	2,700	1.35	Paths and patios, compacts firm
Sand (dry)	100	2,700	1.35	Paver bedding, leveling, play sand
Topsoil	81	2,200	1.1	Lawns, raised beds, grading
Fill dirt	74	2,000	1	Filling holes and low spots
Compost	37	1,000	0.5	Soil amendment, top dressing
Mulch (shredded hardwood)	22	600	0.3	Flower beds, around trees
Wood chips / playground chips	19	500	0.25	Paths, playgrounds
Concrete (wet, normal weight)	150	4,050	2.03	150 lb per cubic foot
Asphalt (hot mix, compacted)	145	3,915	1.96	145 lb per cubic foot

Frequently asked questions

How much concrete do I need for a 10 × 10 slab?

A 10 × 10 ft slab 4 inches thick is 100 × 4 ÷ 324 = 1.23 cubic yards, or 33.3 cubic feet. That is 56 bags of 80 lb mix or 75 bags of 60 lb mix before waste. With 10% extra, plan on 62 × 80 lb bags. At 6 inches thick the same pad needs 1.85 cubic yards.

How many bags of concrete for a 12 × 12 slab?

At 4 inches thick a 12 × 12 slab is 48 cubic feet (1.78 cubic yards): 80 bags of 80 lb mix or 107 bags of 60 lb mix, plus waste. At that size, a short-load ready-mix delivery is usually less work and costs about the same.

Does a 4 inch slab need rebar?

A 4-inch patio or walkway on well-compacted ground does not strictly need rebar; control joints and a good base matter more. Rebar (usually #3 or #4 at 18–24 inches on center) or welded wire mesh is worth adding for driveways, garage floors, slabs on clay or fill, and anything carrying a hot tub or vehicle. It holds cracks tight rather than preventing them.

How much gravel goes under a concrete slab?

Four inches of compacted crushed stone or road base is the usual sub-base for patios and garage floors; use 6 inches on clay or poorly draining soil. Order about 15% more loose stone than the finished volume, because it compacts. For a 12 × 16 ft patio, 4 inches of base is 2.37 cubic yards finished, about 2.7 yards (3.7 tons) loose.

How far apart should control joints be in a slab?

A common rule is 2 to 3 times the slab thickness in feet: 8–12 ft apart for a 4-inch slab, 10–15 ft for 6 inches. Keep the panels close to square, cut joints to a quarter of the slab depth, and cut them within a day of the pour.

How long before I can walk or drive on a new slab?

You can usually walk on it after 24–48 hours and drive a car on it after about 7 days. Concrete reaches most of its design strength at 28 days, so wait that long before parking heavy trucks or equipment on it. Keep it moist for the first week for a stronger surface.