Where bags stop making sense
The single most useful thing this calculator tells you is when to stop buying bags. A 10 × 10 ft slab at 4 in thick is about 33 cubic feet — roughly 56 eighty-pound bags, or over two tonnes of material to carry, open and mix by hand.
The problem is not the money. A slab has to be poured as one continuous piece: if the first section starts to set before the last is placed, you get a cold joint running through the middle, which is a built-in crack line. One person mixing bags cannot keep ahead of that on anything much over 50 square feet.
The thresholds worth remembering: under half a cubic yard, bags and a mixing tub are fine. Half to one cubic yard, hire a mixer or get help. Over a cubic yard, order ready-mix — even with a short-load fee it will be cheaper than your weekend.
| Volume | Approach | Rough bag count (80 lb) |
|---|---|---|
| Under 0.5 yd³ | Bags, tub and shovel | Up to about 22 |
| 0.5 – 1 yd³ | Hired mixer, or several people | 22 – 45 |
| 1 – 3 yd³ | Ready-mix, expect a short-load fee | Not practical |
| Over 3 yd³ | Ready-mix, standard load | Not practical |
Thickness and the base underneath
Four inches is the standard for a shed base, patio or footpath. Five to six inches suits a driveway or anything a car sits on. Six inches reinforced is the figure for a garage or a hot tub, and anything structural should be designed rather than estimated.
What matters as much as the slab is what is under it. A compacted gravel sub-base does two jobs: it lets water drain rather than sitting under the slab and freezing, and it provides uniform bearing so the slab is supported evenly instead of bridging soft spots. A slab poured on bare soil is being asked to span whatever settles beneath it, and that is the usual reason one cracks within a couple of years.
Why the over-dig allowance exists
The arithmetic assumes a neat rectangular box. Real excavations are not neat. Edges slump, the base is rarely perfectly level, and formwork bows outward slightly under the weight of wet concrete.
Ten per cent is a sensible default and what the calculator starts at. Go to 15% for a hand-dug hole in loose or sandy ground. You can drop to 5% for a well-formed slab with rigid shuttering on a compacted base.
Running out mid-pour is much worse than having a little spare, because the joint between the old and new concrete will always be the weak line.
Common questions
How many 80 lb bags in a cubic yard?
About 45. An 80 lb bag yields roughly 0.60 cubic feet and a cubic yard is 27 cubic feet.
How thick should a shed base be?
Four inches (100 mm) over a compacted gravel sub-base of similar depth is the normal specification for a garden shed.
Can I pour the bags in dry and add water?
It is done for fence posts and works acceptably in damp ground, but water distribution is uneven and the result is weaker. For a slab, mix it properly.