Calculator 01 / Concrete
Concrete Calculator
Concrete is ordered by volume — length × width × thickness — so bag count depends on each bag's yield, not just the area you are covering. Figure cubic yards and bag count for slabs, footings, round columns, and stairs. Enter your measurements, pick a shape, and add a waste buffer before you order.
How this calculator works
Concrete volume is length × width × thickness, converted to cubic feet and then to cubic yards (concrete is sold by the cubic yard for ready-mix, or by the bag for small pours). This tool applies the same formulas ready-mix suppliers and bagged-mix manufacturers use:
The stairs formula treats each step as a stacked rectangular block, which is the standard approximation used for a solid stair pour (no separate stringer or void beneath).
Worked example: a 20 × 10 ft slab
A 20 ft × 10 ft patio slab at 4 in thick, 10% waste, priced in 60 lb bags:
- Volume = 20 × 10 × (4 ÷ 12) = 66.7 cu ft
- With 10% waste = 73.3 cu ft
- Cubic yards = 73.3 ÷ 27 = 2.72 yd³
- Bags = 73.3 ÷ 0.45 = 163 bags
That bag figure is the useful part of this example. A hundred and sixty-three 60 lb bags is nearly five tons of material to carry, open and mix, and every one of them has to be mixed to the same consistency or the slab cures unevenly. At 2.7 cubic yards this is firmly a ready-mix job. Bagged concrete stops making sense somewhere around a cubic yard — roughly 45 to 90 bags depending on size — and that threshold arrives far sooner than most people expect.
Bags, yields and when to stop bagging
Bag yield is the finished volume one bag produces, not the weight of the powder. It is printed on the sack, and these are the standard figures.
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Ready-mix has its own arithmetic. Suppliers generally have a minimum load and charge a short-load fee below a few cubic yards, so a small pour can cost more per yard than a large one — worth a phone call before deciding, because the crossover point is a price question, not just a volume one.
Typical slab thickness
Thickness is a structural decision that belongs to your plans and your local code, not to a table on a website. These are the common starting points, and every one of them assumes a properly prepared and compacted base beneath.
| Pour | Common thickness | Notes |
|---|---|---|
| Walkway or patio | 4" | Foot traffic only |
| Residential driveway | 4–6" | Thicker where vehicles turn |
| Garage floor | 4–6" | 6" if anything heavy is stored |
| Shed or workshop base | 4" | Thicken the edge as a perimeter beam |
| Heavy vehicle or equipment | 6–8"+ | Engineered, with reinforcement designed for it |
For footings, use the slab tab — a strip footing is simply a long, narrow slab, and a pad footing is a short one. Footing size and depth are structural and frost-related, and they must come from your plans, an engineer, or the local frost-depth requirement rather than from a general table.
Where concrete orders go wrong
- Ordering exactly what the maths says. Concrete has no second chances — a pour that runs short mid-slab leaves a cold joint you cannot undo. The waste factor exists for uneven subgrade, over-dig and spillage, and subgrade is almost never as flat as it looks.
- Forgetting the base settles. A slab poured on soft or uncompacted fill uses more concrete than its nominal thickness, because the wet weight pushes down into it.
- Bagging a job that wants ready-mix. See above. Beyond about a cubic yard, bags cost more in labour and time than the delivery saves, and consistency suffers.
- Ignoring the pour window. Volume says nothing about whether you can place and finish it before it goes off. Hot, dry or windy conditions shorten that window considerably.
- Leaving out reinforcement and control joints. Neither shows up in a volume calculation, and both determine whether the slab cracks where you chose or where it likes.
Where these numbers come from
Yield, slump and curing all move the number of bags or yards you actually need. These are the bodies that publish the underlying guidance.
- American Concrete Institute — Mix design, placement, curing and the ACI documents most specifications reference.
- Portland Cement Association — Cement and concrete fundamentals, including how water content changes strength.
- National Ready Mixed Concrete Association — Ordering ready-mix: minimum loads, short-load charges, slump and delivery windows.
How much waste factor should I add?
5–10% covers most slab pours. Add more (10–15%) for uneven or unexcavated subgrade, hand-dug footings, or irregular forms where over-digging is likely.
When should I order ready-mix instead of bags?
As a rule of thumb, pours over roughly 1 cubic yard (about 60–70 bags of 60 lb mix) are usually cheaper and faster with a ready-mix delivery than hand-mixing bags.
Does this account for rebar or wire mesh volume?
No — reinforcement takes up a negligible fraction of pour volume and is not subtracted. This calculator estimates gross concrete volume only.
Can I save, share, or print a calculation?
Yes — use the Save, Copy link, and Print buttons under your results. Saved calculations can be grouped into a named project on the My Projects page for a combined job estimate with cost totals. Everything is stored only in your browser, never uploaded — see the Privacy Policy for details.
Estimates only. Confirm final quantities with your ready-mix supplier or bagged-mix manufacturer, and always order slightly more than the calculated minimum.