Concrete Bag Calculator: How Many Bags?

How many bags of premixed concrete you need: 40, 50, 60 and 80 lb or 20, 25 and 30 kg bags side by side, for any slab, hole, column or step.

Premixed concrete bags are sold by weight but what matters is how much concrete each one makes. Typical US bags make 0.30 ft³ (40 lb), 0.375 ft³ (50 lb), 0.45 ft³ (60 lb) and 0.60 ft³ (80 lb). Metric bags make about 1 m³ per 108 × 20 kg bags, which works out at roughly 86 × 25 kg or 72 × 30 kg bags. Enter what you're filling, whether a slab, post holes, a tube form or steps, and the calculator lists the bags needed in every size side by side, so you can compare carrying fewer heavy bags against more light ones. Totals are rounded up to whole bags, and the extra for waste is included.

Units
Bag sizes
Pour
Area
ft
ft
in
Feet and inches work too: type 12'6 or 12.5.
%
Bags
62

80 lb bag (0.60 ft³)

Concrete
1.36 yd³

36.67 ft³ · 1.038 m³

Weight when mixed
5,494 lb

2.75 tons

Premixed bags needed by size
80 lb bag (0.60 ft³)62
60 lb bag (0.45 ft³)82
50 lb bag (0.375 ft³)98
40 lb bag (0.30 ft³)123
20 kg bag113
25 kg bag89
30 kg bag74
That's 62 × 80 lb bags. At this size many people order ready-mix instead: ask suppliers for 1.36 cubic yards, and check their minimum load and how they round.
How it was worked out
  1. Area: 10 ft × 10 ft = 100 ft²; × 4 in (0.333 ft) deep = 33.33 ft³
  2. 33.33 ft³ ÷ 27 = 1.23 yd³
  3. Plus 10% extra = 36.67 ft³ (1.36 yd³)
  4. 80 lb bag (0.60 ft³): 36.67 ft³ ÷ 0.6 ft³ made per bag = 61.11, rounded up to 62 bags

Bag yields are typical figures: 0.30, 0.375, 0.45 and 0.60 ft³ for 40, 50, 60 and 80 lb bags, and about 108 × 20 kg bags per m³. Check the yield printed on the bag you buy. Mixed concrete is taken as 2,400 kg/m³ (about 4,050 lb per cubic yard).

Common use cases

  • Buy the right number of bags for fence posts, a small slab or steps
  • Compare 60 lb and 80 lb bags, or 20 kg and 25 kg bags, for the same job
  • Decide whether a job is small enough for bags or needs ready-mix
  • Work out the total weight before loading a car or trailer

How to use

  1. Choose your units and bag sizes, then what you're pouring.
  2. Enter the sizes, and keep or change the 10% extra.
  3. Read the bag count for your chosen size, and the table of every size.
  4. Add a price per bag to compare the cost of each size.

FAQ

How many bags for a 10 × 10 ft slab 4 inches thick?

The slab is 33.3 ft³. Without extra that's 56 × 80 lb, 75 × 60 lb, 89 × 50 lb or 112 × 40 lb bags. With 10% extra, 62 × 80 lb bags.

How many bags are in a cubic yard or a cubic metre?

A cubic yard takes about 45 × 80 lb, 60 × 60 lb, 72 × 50 lb or 90 × 40 lb bags. A cubic metre takes about 108 × 20 kg bags. Both before extra.

When is ready-mix better than bags?

Somewhere around a cubic yard (about 0.75 m³) many people switch: that's 45 × 80 lb bags to lift and mix by hand. Ready-mix arrives in one go, which also avoids joints between batches. Suppliers have minimum loads and charges for small loads, so get a price for both.

Are all bags the same?

No. Fast-setting, high-strength and fibre mixes can yield a little differently, so check the yield printed on the bag you're buying and adjust the extra if needed.

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