How much concrete do I need for a 3 × 3 m slab 10 cm thick?

0.90 m³, which is 1.18 cubic yards: 77 bags of 25 kg mix or 97 bags of 20 kg mix, weighing about 2,160 kg once poured. With a 10% allowance it is 0.99 m³, 85 bags of 25 kg or 106 of 20 kg. Switch to Metric to enter meters and centimeters, or use the 3×3 m, 10 cm preset.

Why is the weight more than the bags add up to?

The weight line is the poured volume at a typical 2,400 kg/m³, while a bag label gives the dry mix. For the 10 × 10 ft, 4 in slab the tool shows about 4,994 lb, but 56 bags of 80 lb weigh 4,480 lb. Use bags × bag weight for what you lift and haul, and the weight line for a rough idea of the load on what the slab sits on.

How much does a yard of concrete weigh?

About 4,045 lb (1,835 kg) at the 2,400 kg/m³ this calculator uses, a typical density for normal-weight concrete. The real figure depends on the mix. QUIKRETE lists about 140 lb/ft³ for its bagged concrete mix, which is lighter.

How much concrete do I need for a round slab, a post hole or a round footing?

For a round patio or pad, choose Round slab: 10 ft across and 4 in thick is 78.54 sq ft and 26.18 ft³, or 0.97 cubic yards and 44 bags of 80 lb mix. For a post hole, choose Column and enter the diameter, the height and how many. One column 12 in across and 48 in tall is 3.14 ft³, or 6 bags of 80 lb mix. Four of them are 12.57 ft³ and 21 bags. A post standing in the hole is not subtracted, so a hole with a post in it needs less.

How does the waste allowance change the result?

It is added to everything you order. With 10% on the 10 × 10 ft, 4 in slab, the volume goes from 1.23 to 1.36 cubic yards and the 80 lb bags from 56 to 62. The net volume without waste stays visible under the result.

Should I buy bags or order a ready-mix truck?

The calculator shows both so you can price them. A 20 × 20 ft slab 4 in thick is 4.94 cubic yards: 223 bags of 80 lb mix, or part of one truckload of about 10 cubic yards. From 450 ft³ (16.7 cubic yards, about 150 square yards at 4 in) the page adds a large-pour note suggesting ready-mix: a 30 × 40 ft slab 5 in thick is 18.52 cubic yards, or 834 bags of 80 lb mix. That threshold is a judgement about mixing by hand, not a supplier rule. Truck sizes, minimum orders and delivery fees vary by supplier, so ask yours.

Concrete Calculator

Concrete calculator for square and round slabs, footings and columns: cubic yards or m³, bags of 40 to 80 lb or 20 and 25 kg (exact and rounded up), weight. How it works

Length (ft)

Width (ft)

Thickness (in)

Waste allowance (%)

Common slab sizes

Volume to order

1.23 yd³

Required material

Copy the volume to order

Project area: 100.00 sq ft
Net volume (no waste): 1.23 yd³
Cubic feet to order: 33.33 ft³
Cubic metres to order: 0.94 m³
Waste allowance: 0%

Pre-mixed bags (waste included)

40 lb bags

0.3 ft³ (8.5 L) per bag

Exact count before rounding up: 111.11

112

50 lb bags

0.375 ft³ (10.6 L) per bag

Exact count before rounding up: 88.89

60 lb bags

0.45 ft³ (12.7 L) per bag

Exact count before rounding up: 74.07

80 lb bags

0.6 ft³ (17 L) per bag

Exact count before rounding up: 55.56

Yields are from the QUIKRETE Concrete Mix No. 1101 data sheet (about 0.0075 ft³ per pound of mix). Other brands and mixes differ: check the bag.

Total weight

Weight of the volume to order4,994 lb

Uses 2,400 kg/m³ (about 150 lb/ft³), a typical figure for normal-weight concrete (140 to 150 lb/ft³ is the usual range).

1.23 yd³ is within one truckload of about 10 yd³ (7.65 m³). Truck sizes vary by supplier.

Shape

About the Concrete Calculator

A 10 × 10 ft slab 4 in thick takes 1.23 cubic yards of concrete: 56 bags of 80 lb mix or 75 bags of 60 lb mix. The concrete calculator multiplies length × width × thickness (π × radius² × thickness for a round slab, π × radius² × height for a round column), adds the waste allowance you set and divides by the yield of each bag size. Under each count it prints the exact figure before rounding up, 55.56 and 74.07 bags here, so you can see how close the pour is to needing one more.

Concrete and bags for common slab sizes

Slab	Cubic yards	Cubic feet	60 lb bags	80 lb bags
8 × 8 ft, 4 in thick	0.79	21.33	48	36
10 × 10 ft, 4 in thick	1.23	33.33	75	56
12 × 12 ft, 4 in thick	1.78	48.00	107	80
10 × 20 ft, 4 in thick	2.47	66.67	149	112
20 × 20 ft, 4 in thick	4.94	133.33	297	223
8 × 8 ft, 6 in thick	1.19	32.00	72	54
10 × 10 ft, 6 in thick	1.85	50.00	112	84
12 × 12 ft, 6 in thick	2.67	72.00	160	120
10 × 20 ft, 6 in thick	3.70	100.00	223	167
20 × 20 ft, 6 in thick	7.41	200.00	445	334

No waste allowance. Bags are rounded up to whole bags at 0.45 ft³ per 60 lb bag and 0.6 ft³ per 80 lb bag. Set a waste percentage in the tool to cover uneven ground and spillage.

Yield per bag of concrete mix

Bag	Yield per bag	Bags needed	Where the yield comes from
40 lb	0.3 ft³ (8.5 L)	90 per cubic yard	QUIKRETE data sheet
50 lb	0.375 ft³ (10.6 L)	72 per cubic yard	QUIKRETE data sheet
60 lb	0.45 ft³ (12.7 L)	60 per cubic yard	QUIKRETE data sheet
80 lb	0.6 ft³ (17 L)	45 per cubic yard	QUIKRETE data sheet
20 kg	0.331 ft³ (9.4 L)	107 per cubic meter	Derived from 0.0075 ft³ per lb
25 kg	0.413 ft³ (11.7 L)	86 per cubic meter	Derived from 0.0075 ft³ per lb

Yields are the approximate figures on the QUIKRETE Concrete Mix No. 1101 data sheet. It lists no metric bags, so the 20 kg and 25 kg rows are worked out from the same yield per pound. Other brands and mixes differ: check the bag.

How to use this tool

Pick Slab, Round slab, Footing or Column, and Imperial (feet and inches) or Metric (meters and centimeters). A round slab takes its diameter in feet or meters; footings and columns take a count.
Enter the sizes and a waste allowance: 0, 5, 10 or 15%, or your own figure up to 100%.
Read the volume to order, the slab area, the bags of each size with the exact count before rounding up, the weight and the truckloads. Open Cost Estimation to price each bag size.

Frequently asked questions

How much concrete do I need for a 3 × 3 m slab 10 cm thick?: 0.90 m³, which is 1.18 cubic yards: 77 bags of 25 kg mix or 97 bags of 20 kg mix, weighing about 2,160 kg once poured. With a 10% allowance it is 0.99 m³, 85 bags of 25 kg or 106 of 20 kg. Switch to Metric to enter meters and centimeters, or use the 3×3 m, 10 cm preset.
Why is the weight more than the bags add up to?: The weight line is the poured volume at a typical 2,400 kg/m³, while a bag label gives the dry mix. For the 10 × 10 ft, 4 in slab the tool shows about 4,994 lb, but 56 bags of 80 lb weigh 4,480 lb. Use bags × bag weight for what you lift and haul, and the weight line for a rough idea of the load on what the slab sits on.
How much does a yard of concrete weigh?: About 4,045 lb (1,835 kg) at the 2,400 kg/m³ this calculator uses, a typical density for normal-weight concrete. The real figure depends on the mix. QUIKRETE lists about 140 lb/ft³ for its bagged concrete mix, which is lighter.
How much concrete do I need for a round slab, a post hole or a round footing?: For a round patio or pad, choose Round slab: 10 ft across and 4 in thick is 78.54 sq ft and 26.18 ft³, or 0.97 cubic yards and 44 bags of 80 lb mix. For a post hole, choose Column and enter the diameter, the height and how many. One column 12 in across and 48 in tall is 3.14 ft³, or 6 bags of 80 lb mix. Four of them are 12.57 ft³ and 21 bags. A post standing in the hole is not subtracted, so a hole with a post in it needs less.
How does the waste allowance change the result?: It is added to everything you order. With 10% on the 10 × 10 ft, 4 in slab, the volume goes from 1.23 to 1.36 cubic yards and the 80 lb bags from 56 to 62. The net volume without waste stays visible under the result.
Should I buy bags or order a ready-mix truck?: The calculator shows both so you can price them. A 20 × 20 ft slab 4 in thick is 4.94 cubic yards: 223 bags of 80 lb mix, or part of one truckload of about 10 cubic yards. From 450 ft³ (16.7 cubic yards, about 150 square yards at 4 in) the page adds a large-pour note suggesting ready-mix: a 30 × 40 ft slab 5 in thick is 18.52 cubic yards, or 834 bags of 80 lb mix. That threshold is a judgement about mixing by hand, not a supplier rule. Truck sizes, minimum orders and delivery fees vary by supplier, so ask yours.

Slabs, footings, columns and metric pours

Pick Slab, Round slab, Footing or Column; footings and columns take a count. The result gives the volume to order, the slab area, bag counts with the exact figure before rounding up, the weight and the truckloads. The waste allowance is added to all of them; the net volume stays visible. From 450 ft³ (16.7 cubic yards) the page suggests ready-mix instead of bags. Metric takes meters and centimeters with 20 kg and 25 kg bags, and Cost Estimation prices each bag size.

How the volume and bags are calculated

slab or footing = length × width × thickness

round slab = π × (diameter ÷ 2)² × thickness

column = π × (diameter ÷ 2)² × height

A round slab 10 ft across and 4 in thick is 78.54 sq ft and 26.18 ft³ (0.97 cubic yards): 44 bags of 80 lb mix.

bags = volume to order ÷ yield per bag, rounded up

27 ft³ make one cubic yard. Bag yields come from the QUIKRETE Concrete Mix No. 1101 data sheet:

40 lb bag: 0.3 ft³
50 lb bag: 0.375 ft³
60 lb bag: 0.45 ft³
80 lb bag: 0.6 ft³

That is 0.0075 ft³ per pound of mix, so one cubic yard takes 45 bags of 80 lb mix or 60 bags of 60 lb mix. The 20 kg (0.331 ft³) and 25 kg (0.413 ft³) yields are derived from the same per-pound figure.

Weight uses 2,400 kg/m³ (about 150 lb/ft³), a typical figure for normal-weight concrete, so a cubic yard weighs about 4,045 lb. That is the weight of the poured concrete, not of the bags: the bag yields imply about 133 lb of dry mix per cubic foot. The truck line assumes about 10 cubic yards per load, a common mixer size.

Fun Fact: QUIKRETE's data sheet for Concrete Mix No. 1101 lists a typical compressive strength of 2,500 psi after 7 days and 4,000 psi after 28 days.

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