A 10 ft × 10 ft slab at 4 inches thick needs 56 bags of 80 lb concrete, 74 bags at 60 lb, or 112 bags at 40 lb. That’s 1.23 cubic yards of concrete, and roughly two and a quarter tons of bags to buy, carry, open and mix.
The number is easy. The more useful question is whether bags are the right choice at this size, and for most people the honest answer is no.
Bag counts by thickness
| Thickness | Volume | 40 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|
| 3 in | 0.93 yd³ | 84 | 56 | 42 |
| 4 in | 1.23 yd³ | 112 | 74 | 56 |
| 5 in | 1.54 yd³ | 139 | 93 | 70 |
| 6 in | 1.85 yd³ | 167 | 111 | 84 |
Those figures are before any wastage. Add 5 to 10% for spillage, uneven sub-base and the concrete that stays stuck in the mixer, and a 4-inch slab realistically needs around 60 bags of 80 lb rather than 56.
How the volume is worked out
Four inches expressed in feet is 0.333, so:
10 × 10 × 0.333 = 33.3 ft³
A cubic yard is 27 cubic feet, so 33.3 ÷ 27 = 1.23 cubic yards.
To get bag counts, divide the volume by the yield printed on the bag. An 80 lb bag of standard premix yields 0.60 ft³, so 33.3 ÷ 0.60 = 55.5, rounded up to 56. The bag calculator runs this for every common bag size at once, including UK 20 kg and 25 kg sacks.
What 56 bags actually means
This is the part the arithmetic hides. Fifty-six 80 lb bags is 4,480 lb of material — well over two tons. Each one has to be lifted off a pallet, carried to the mixer, cut open and tipped in.
A 3.5 cubic foot mixer takes two 80 lb bags per batch comfortably. That’s 28 batches. At roughly five minutes per batch including loading, mixing, tipping and moving the concrete, you’re looking at two and a half hours of continuous work — and the concrete you mixed in batch one is already setting while you’re on batch twenty.
That last point matters more than the labour. Every batch is a separate mix with its own water content, and a slab poured from 28 slightly different batches over three hours has cold joints running through it and inconsistent strength across its area.
Cost compared with ready-mix
| Option | Quantity | Cost | Effective per yd³ |
|---|---|---|---|
| 80 lb bags at $6.00 | 60 bags | $360 | $270 |
| 60 lb bags at $4.75 | 78 bags | $371 | $278 |
| Ready-mix, 1.5 yd³ | 1.5 yd³ | $218 + $110 short load | $219 |
| Ready-mix, combined with another pour | 1.5 yd³ | $218 | $145 |
Even with a short-load fee, ready-mix comes out cheaper at this volume — and someone else does the mixing. The gap widens considerably if you can combine the slab with another pour on the same day.
When bags still make sense
Bags win in a handful of specific situations, and they’re worth recognising:
- No truck access. If the pour is behind a house with a narrow side gate, bags carried through beat a pump you can’t afford.
- You want to work in stages. A slab poured in formed sections over several weekends is possible with bags and impossible with a delivery.
- Very small top-ups. A few bags to finish a section, or to set posts alongside the main pour.
- Remote sites where no supplier will deliver.
Outside those, the crossover point is around 30 bags. Above that, price a delivery before committing.
If you are going with bags
A few things make the difference between a decent slab and a bad one:
Hire a mixer, don’t mix by hand. A petrol or electric mixer is £30 to £50 a day and transforms both the consistency and your back. Hand-mixing 56 bags is not realistic.
Measure the water. Use a marked bucket and use the same amount every batch. Eyeballing it is the single biggest cause of weak bagged concrete — see our guide on how much water goes in a bag for the figures.
Get help. One person mixing and one placing and screeding roughly halves the elapsed time, which directly reduces the cold-joint problem.
Buy by the pallet. 42 bags of 80 lb come on a standard pallet and the per-bag price drops noticeably. You’ll need a second part-pallet for a 4-inch slab.
Why the sub-base matters more than the slab
People agonise over bag counts and then lay the slab on whatever happens to be there. That is backwards. A 4-inch slab on 4 inches of properly compacted stone will outperform a 6-inch slab on soft ground every time, because concrete is strong in compression and weak in bending — and unsupported ground makes a slab bend.
Dig out to 8 inches below finished level for a 4-inch slab. Put down 4 inches of crushed stone in two layers, compacting each with a plate. The stone should feel solid underfoot with no give at all before any concrete arrives.
Pouring in sections to beat the clock
If bags are your only option at this size, don’t try to pour the whole slab as one continuous piece. You physically can’t mix fast enough, and the result is a slab full of unplanned cold joints in random places.
Instead, divide the 10 × 10 into two or four bays with proper formwork between them, and pour one bay at a time. Each bay becomes a complete pour with a deliberate, formed joint at its edge rather than an accidental weakness in the middle.
A 10 × 10 split into two 10 × 5 bays means 28 bags per bay — about 90 minutes of mixing each, which is achievable. You can do them on consecutive days if you need to. Leave the formwork between bays in place until the first side has set, then strip it and pour the second against the finished face.
Where the bays meet, either run reinforcement bars through the joint or accept it as a control joint and seal it afterwards. For a shed base or patio, a sealed joint is perfectly acceptable and arguably better — it gives the slab a defined place to move.
Everything else you need for the job
The bags are the obvious cost. These are the ones people forget until the day:
| Item | Quantity for 10×10 | Cost |
|---|---|---|
| Sub-base stone, 4 in compacted | ~1.5 tons | $60–$110 |
| Formwork timber, 2×4 | 40 linear ft | $45–$70 |
| Stakes and screws | 16 stakes | $25–$40 |
| Vapour barrier | 110 ft² | $25–$40 |
| Mixer hire | 1 day | $40–$60 |
| Plate compactor hire | 1 day | $70–$100 |
| Screed board, float, edger | — | $60–$120 |
That’s another $325 to $540 on top of the bags, which is why the realistic all-in DIY cost for a 10 × 10 slab lands around $700 rather than the $360 the bag count suggests.
Common questions
How many 80 lb bags of concrete in a cubic yard?
45 bags. One 80 lb bag yields 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45.
How much does a 10×10 concrete slab cost?
Around $360 in bags at 4 inches, or $218 to $330 in ready-mix depending on whether a short-load fee applies. Add sub-base stone, formwork timber and mixer hire and the realistic total is $500 to $700 DIY.
Can one person pour a 10×10 slab?
Technically yes with bags and a mixer, over about four hours. The result will be worse than a two-person pour because of the time gap between the first and last batch. If you’re working alone, form the slab in two halves with a proper joint between them.
How thick should a 10×10 slab be?
Four inches for a shed base, patio or garden building. Five to six inches if a vehicle will sit on it. All of them want at least 4 inches of compacted sub-base underneath.
How long does a 10×10 slab take to cure?
Foot traffic after 24 to 48 hours, vehicles after seven days, full design strength at 28 days. Keep it damp for the first week — concrete cures by chemical reaction with water, not by drying out.