Concrete Slab & Bag Calculator
Enter a slab's length, width, and thickness to get cubic feet, cubic yards, and how many 40lb, 60lb, or 80lb premix concrete bags you'll need.
Concrete needed (with waste)
1.36 cu yd
36.7 cu ft · 1.04 m³
Premix bags needed
| Bag size | Bags |
|---|---|
| 40 lb | 123 |
| 60 lb | 82 |
| 80 lb | 62 |
Bag yields used: 40lb ≈ 0.30 cu ft, 60lb ≈ 0.45 cu ft, 80lb ≈ 0.60 cu ft — the yields most premix concrete brands publish, but check your specific bag's label since yields vary slightly by brand and mix. Ready-mix (delivered by truck) is usually cheaper than bags above roughly 1-1.5 cubic yards; this is here for the bag-by-bag DIY case.
About this tool
How to use it
- Enter the slab's length and width in feet, and its thickness in inches.
- Enter an extra percentage for waste — 10% is a common allowance for spillage, uneven ground, and over-excavation.
- Press Calculate Concrete, or just keep typing — the results update live.
- Check the bag table for how many 40lb, 60lb, or 80lb premix bags cover that volume, and pick whichever size is easiest for you to lift and mix.
Why it matters
Concrete is sold by volume, but slab dimensions are usually thought of in length, width, and a thickness in inches — converting between the two, then converting cubic feet to the cubic yards a ready-mix supplier quotes in, or to a number of premix bags for a DIY pour, is easy to get wrong by a factor of 12 (inches vs feet) or 27 (cubic feet vs cubic yards) if done by hand.
When to use it
Useful when planning a patio, walkway, footing, or slab pour, when deciding whether a job is small enough for bagged premix or big enough to order ready-mix by the truck, or when double-checking a contractor's quoted volume against your own measurements.
Good to know
Bag yields (40lb ≈ 0.30 cu ft, 60lb ≈ 0.45 cu ft, 80lb ≈ 0.60 cu ft) are the typical figures most premix concrete brands publish, not a universal standard — check your specific bag's label, since yield varies slightly by brand and mix design. The waste percentage accounts for spillage, an uneven sub-base, and slight over-excavation; 10% is a commonly used starting allowance, but a rougher site may need more. Above roughly 1 to 1.5 cubic yards, ready-mix delivered by truck is usually more cost-effective and far less labor than mixing dozens of bags by hand.
Common questions
Why does the calculator ask for thickness in inches but length and width in feet?
That matches how slabs are normally measured and specified — a patio is usually described as, say, '10 by 12 feet, 4 inches thick' rather than all in the same unit. The calculator handles the inches-to-feet conversion internally before computing volume.
How much waste percentage should I use?
10% is a commonly used starting point that covers typical spillage and an uneven sub-base. Add more (15-20%) if the ground isn't well leveled or compacted, since low spots will need extra concrete to fill.
Should I use bags or order ready-mix?
For small jobs (roughly under 1 cubic yard) bags are usually more practical. Above about 1 to 1.5 cubic yards, ordering ready-mix by the truck is typically cheaper per unit of concrete and dramatically less physical labor than mixing dozens of bags by hand.
Why do the three bag sizes give different bag counts for the same job?
Each bag size yields a different volume of cured concrete once mixed (roughly 0.30, 0.45, and 0.60 cubic feet for 40lb, 60lb, and 80lb bags respectively) — a bigger bag naturally means fewer bags for the same total volume, but also more weight to lift and mix per batch.
Does this account for rebar, wire mesh, or a gravel sub-base?
No — this calculates the concrete volume for the slab itself only. Reinforcement and sub-base material are separate parts of the job with their own quantities to work out.