Estimate concrete volume, ready-mix yards, or bag count with unit conversion and wastage controls.
How this calculator works
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.
Common questions
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.
The math behind a concrete order isn’t hard. The expensive part is being wrong: order short and you pay a short-load surcharge plus the cost of a second truck; order long and you pay for a yard you have to dispose of. This calculator gives you both numbers — the one that physically fits your form, and the one you actually order — so the conversation with the supplier starts from a defensible figure.
Inputs and outputs at a glance
Enter slab length, width, and thickness in any unit system, then tune two planning factors: a wastage percentage (extra to order against form irregularities and finish losses) and an optional compaction-or-expansion percentage. The calculator returns base volume in cubic yards, the inflated order volume, and — if you supply a price per cubic yard — an estimated material cost.
A sample pour
Say you’re pouring a 12 ft × 16 ft patio at 4 inches. The base volume is length × width × thickness, normalised to cubic yards:
Add 10% wastage and the order volume — the number you take to the supplier — is:
At $230 per yard the supplier’s quote should land around $600 plus delivery. If your supplier short-loads anything under three yards, that 2.61 number tells you exactly where you sit on the pricing band — and whether dropping the wastage to 7% (and accepting the risk) saves enough to matter.
Where this stops being useful
This is a rectangular-volume calculator. It handles patios, shed pads, garage floors, walkways, and simple strip footings or pier pours well. It is not the right tool for multi-pitch slabs, curved or radial pours, or foundation systems with grade beams and pile caps. For those, use it as a first-pass sanity check and confirm against a structural drawing or a supplier walkthrough.
On wastage: 10% by default, with a floor of 7%
10% is the figure most ready-mix supply houses recommend as a starting point for ground-level slab work, and it’s the calculator’s default. Bump to 15% if your formwork is irregular, your subgrade is sloping, or you’re pouring on a windy day with the finishing crew under pressure. Drop to 7% only if you’ve poured this exact form before and know your losses; going below that on a single-truck pour rarely saves enough to outweigh the short-load risk.
The two formulas to take away:
One cubic yard equals 27 cubic feet (≈ 0.76 cubic metres). Delivery, short-load surcharges, and tax are not included — confirm those with your supplier. If you’re scoping something larger than a weekend pad — a multi-pour foundation, a structural slab, a job where sequencing matters more than the math — a sit-down with a contractor who can validate the numbers on-site is usually cheaper than getting them wrong.
Other calculators on this site
If you’re scoping a whole project, you’ll usually need more than one of these. They share the same approach — plain math, honest about edge cases, with the practical notes someone who has done the work would actually need.
- Asphalt Calculator. Tonnage for a driveway, parking patch, or walkway pour, including the depth-to-tonnage curve a supplier will quote against.
- Drywall Calculator. Sheets of drywall and bundles of mud for a renovation room, plus the cut and waste allowance.
- Excavation Calculator. Cubic yards of dirt to remove for a basement, pool, or footing — including the bulking factor that surprises first-timers.
- Fence Calculator. Lineal feet of fence, post count, rails, and gates for a property line that is rarely perfectly straight.
- Fill Calculator. Cubic yards of fill for a regrade, a backyard level, or a low spot, plus a sensible compaction factor.
- Lumber Calculator. Board feet for a deck, frame, or addition, in the rough framing pattern a supplier will price against.
- Paint Calculator. Gallons of paint for a room, plus the coverage difference between primed and unprimed surfaces.
- Tile Calculator. Square footage of tile for a floor or wall, plus the breakage and cut waste a real installation needs.
