Estimate concrete volume, ready-mix yards, or bag count with unit conversion and wastage controls.
How this is calculated
Slab volume in cubic feet, then converted to yards for ready-mix or to bags for small jobs.
cubic_feet = length × width × thickness × (1 + wastage%) × (1 + compaction%)cubic_yards = cubic_feet ÷ 27bags = ceil( cubic_feet ÷ cu_ft_per_bag )Ready-mix is sold by the cubic yard, often with a half-yard minimum. Bags suit small jobs, post holes, and patches.
60 lb bags yield ≈ 0.45 cu ft; 80 lb bags ≈ 0.6 cu ft. A cubic yard ≈ 60× 60-lb bags or 45× 80-lb bags; past about 0.5 cu yd, ready-mix wins on price and labour.
Bags or ready-mix?
Switch to ready-mix once you cross about 0.5 cubic yards. At that point, the bags-vs-truck math favours the truck on price, time, and consistency, and small ready-mix suppliers will deliver as little as a half-yard.
What thickness do I need?
4″ for a typical residential slab, walk, or patio. 5–6″ if vehicles will sit on it. 6″+ with rebar for driveways used by trucks or RVs. Always over a properly compacted granular base.
Should I add wastage?
I default to 10%. Drop to 7% if your forms are tight and the subgrade is dead level, hold at 10% for the typical residential pour, push to 15% on irregular forms or sloping subgrade. Wastage covers over-excavation, sub-base unevenness, and the small thickness variation between forms. Order a touch heavy; running short mid-pour is far worse than throwing a wheelbarrow extra in the back.
Bags or a ready-mix truck: sizing a concrete pour
This one answers two questions at once for a homeowner pouring a shed pad or a contractor confirming a slab: how much concrete the pour needs, and whether you should be mixing bags or calling for a truck. Once you know the yardage, the bag-versus-truck decision usually makes itself.
A worked example
Take a 10 ft by 12 ft shed pad at a standard 4-inch residential thickness. Four inches is 0.333 of a foot, so 120 square feet times 0.333 gives you 40 cubic feet. Divide by 27 and that is 1.48 cubic yards, so you would order 1.5. To see why the truck wins here, count the bags: an 80-pound bag yields about 0.6 cubic feet, so 40 cubic feet is 67 bags. Mixing 67 bags by hand for a pad that a truck delivers in one chute is a long, hard Saturday.
Common mistakes
- Putting thickness in as inches instead of feet. A 4-inch slab is 0.333 feet, not 4. Enter 4 and the answer comes out twelve times too big.
- Ignoring the half-yard minimum and short-load fee. Ready-mix plants charge for a minimum load, so a job that calls for 0.4 yards still bills at the minimum.
- Not accounting for an uneven subgrade. A slab that dips to 5 inches in a few spots eats yardage faster than you would guess, so allow a little extra rather than screeding air.
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