How much concrete do I need for a slab?

People type this into search every day: how much concrete do I need.

The usual answer is “length times width times thickness.” That is the pour. It is not the order. A slab that is a centimetre low in the middle still drinks an extra batch. Bags are sold by a yield printed on the sack, not by the rectangle on your sketch.

This page is a labeled example. It is not a structural specification, and it is not a mix design.

Volume first, then waste, then bags

The concrete volume calculator uses:

length × width × depth × (1 + waste)

The pour is a rectangular volume. Sub-base and reinforcement are excluded. Confirm structural specifications with a qualified professional. The tool says all of that.

Depth is entered in centimetres. The formula divides by 100 so the result is cubic metres.

QUIKRETE’s Concrete Mix 1101 data sheet is the bag-yield source I am using. An 80 lb (36.2 kg) bag yields approximately 0.60 ft³ (17 L). A 60 lb bag yields approximately 0.45 ft³ (12.7 L). A 40 lb bag yields approximately 0.30 ft³ (8.5 L). A 50 lb bag is listed at 0.375 ft³ (10.6 L). A 90 lb bag, where sold, is 0.675 ft³ (19.1 L). The product is for pours 2 in (50 mm) thick or more, including floor slabs and patios.

Their mixing and placing training works a 10 ft by 6 ft by 4 in slab. Volume is 20 cu ft. They divide by 0.60 and buy 33 bags of 80 lb mix, or by 0.45 and buy 44 bags of 60 lb mix. The same pages say slabs are generally poured 4 in (0.33 ft) or 6 in (0.5 ft) thick. If that 4 in slab averages 4.5 in, they raise the volume to 22.5 cu ft and the 80 lb count to 38 bags.

Their public bag calculator rounds up to the next whole bag and does not add extra for an uneven substrate or waste. The 8% default in Calculum is a planning allowance you type. It is not a building code.

The published 5 m by 3 m by 12 cm slab

Here are the calculator defaults:

• length: 5 m
• width: 3 m
• depth: 12 cm
• waste allowance: 8%
• yield per bag: 0.012 m³

Before waste: 5 × 3 × 12 ÷ 100 = 1.80 m³

With waste: 1.80 × 1.08 = 1.944 m³

On a US display those volumes are 2.35 yd³ and 2.54 yd³.

80 L batches: 1.944 ÷ 0.08 = 24.3, rounded up to 25

Bags at 0.012 m³: 1.944 ÷ 0.012 = 162, already a whole number, so 162

5 m displays as 16.40 ft. 3 m displays as 9.84 ft. 12 cm displays as 4.72 in. Floor area is 15 m², which the square footage tool prints as 161.5 ft² when you put 5 m by 3 m in room 1 and zero the other rooms.

Buy from the bare 1.80 m³ and you order 1.80 ÷ 0.012 = 150 bags, and 1.80 ÷ 0.08 = 22.5, rounded up to 23 batches. The 8% default adds 12 bags and 2 mixer counts.

5% and 10% can buy the same mixer count

Hold 5 m, 3 m, 12 cm, and 0.012 m³ bags. Change only waste.

• 0% waste: 1.80 m³, 2.35 yd³, 23 batches, 150 bags
• 5% waste: 1.89 m³, 2.47 yd³, 24 batches, 158 bags
• 8% waste (default): 1.944 m³, 2.54 yd³, 25 batches, 162 bags
• 10% waste: 1.98 m³, 2.59 yd³, 25 batches, 165 bags

8% and 10% both need 25 of the 80 L batches because 1.944 ÷ 0.08 and 1.98 ÷ 0.08 sit on either side of 25 after the ceiling. Bags still move, 162 to 165, because 0.012 m³ is a smaller slice.

QUIKRETE’s 20 cu ft walkthrough did not add waste. 20 ÷ 0.60 is 33.33, and they bought 33. Our card would ceiling that leftover third of a bag. Type their yield and their dimensions if you want their count.

Depth is the lever that surprises people

Hold 5 m, 3 m, 8% waste, and 0.012 m³ bags. Change only depth.

• 10 cm: before waste 1.50 m³ (1.96 yd³). With waste 1.62 m³ (2.12 yd³). 21 batches. 135 bags.
• 12 cm (default): 1.80 m³ / 1.944 m³. 25 batches. 162 bags.
• 15 cm: before waste 2.25 m³ (2.94 yd³). With waste 2.43 m³ (3.18 yd³). 31 batches. 203 bags.

12 cm to 15 cm is 3 cm. That 25% thicker pour adds 2.43 − 1.944 = 0.486 m³, 6 batches, and 41 bags.

10 cm is 3.94 in. 12 cm is 4.72 in. 15 cm is 5.91 in. QUIKRETE’s 4 in and 6 in notes sit next to those two thicker inputs. I am not going to pick a thickness for a patio or a driveway. That is the structural check the tool asks you to make.

A low spot that turns 12 cm into an average of 13 cm on this slab, still 8% waste:

13 ÷ 100 × 5 × 3 × 1.08 = 2.106 m³

Bags: 2.106 ÷ 0.012 = 175.5, rounded up to 176

That extra centimetre is 14 bags above the 162-bag default. The training PDF’s 4 in to 4.5 in warning is the same idea on their 10 ft by 6 ft rectangle.

Bags depend on the yield printed on the sack

The default yield is 0.012 m³, which is 12 L. A 60 lb QUIKRETE bag is listed at approximately 12.7 L. Close, not identical. Type the number on your bag.

An 80 lb bag at 17 L is 0.017 m³. Keep the default 1.944 m³ slab and change only yield:

Bags: 1.944 ÷ 0.017 = 114.35, rounded up to 115

The 80 L batch count stays 25, because that line does not use bag yield.

0.012 m³ displays as about 0.42 ft³. 0.017 m³ is about 0.60 ft³, the 80 lb line on the data sheet.

80 L is 0.08 m³. That batch size is a mixer-sized bucket the calculator uses, not a QUIKRETE sack. The data sheet asks for at least 1 cu ft (28 L) of mixer capacity per 80 lb bag. Do not treat 25 batches as 25 bags.

Ready-mix is ordered in cubic metres, or cubic yards on a US ticket. The default with waste is 1.944 m³ / 2.54 yd³. Plants have their own minimum load. I am not going to invent that minimum.

A small patio and a thicker strip

Same 8% waste and 0.012 m³ bags.

A 3 m by 2 m by 10 cm pad:

Before waste: 3 × 2 × 0.10 = 0.60 m³ (0.78 yd³)

With waste: 0.648 m³ (0.85 yd³)

9 batches. 54 bags.

An 8 m by 3 m by 15 cm strip:

Before waste: 8 × 3 × 0.15 = 3.60 m³ (4.71 yd³)

With waste: 3.888 m³ (5.09 yd³)

49 batches. 324 bags.

324 bags of 60-lb-class mix is a different job from 54 bags. At some point the ready-mix truck is the thing you call. The card will still do the volume either way.

A 10 ft by 10 ft by 4 in square, converted so the calculator can take it: 3.048 m by 3.048 m by 10.16 cm, 8% waste.

With waste the card prints 1.33 yd³, 13 batches, 85 bags at 0.012 m³, or 60 bags at 0.017 m³. Before waste it prints 1.23 yd³. I used those metre inputs so the tool can run. A tape that already reads feet can stay in feet if you convert first, or switch the site profile.

Mulch and pavers are the same rectangle with different leftovers

The mulch, soil, or gravel calculator is area times depth times a settling allowance, then bags.

Defaults: 8 m by 4 m, 7 cm, 40 L bags, 10% settling.

Volume: 8 × 4 × 0.07 × 1.10 = 2.464 m³ (3.22 yd³)

Litres: 2,464, shown as 651 gal

Area: 32 m² (344.4 ft²)

Bags: 2464 ÷ 40 = 61.6, rounded up to 62

That is bedding or a path, not a structural slab. Do not pour 2.464 m³ of concrete because a mulch card said 62 bags.

The paving stone quantity calculator is the surface you lay instead of a poured slab:

area × (1 + waste) ÷ paver area, then full packs

Defaults: 6 m by 4 m, 20 cm by 10 cm pavers, 8% waste, 40 pavers per pack.

Base area: 24 m² (258.3 ft²)

Order area: 25.92 m² (279 ft²)

Calculated pavers: 1,296

Full packs: 33, so you order 1,320 pavers

The flooring and paint posts are the same idea on a floor and a wall: area, waste, then the unit the store sells. Concrete adds depth, so the unit is a volume.

A pour that fails still costs a second truck. The moving cost contingency is the same habit on a different job.

Open the concrete volume calculator with the three dimensions you measured and the yield on the bag in the shed. If the job is soil or gravel, use the mulch, soil, or gravel calculator. If the surface is pavers, use the paving stone calculator. For anything that holds a building or a car, have the thickness checked before you order.