Concrete Calculator
Cubic yards, cubic feet and bags of concrete for a slab, footing or post hole pad.
Quick answer
- A 10 × 10 ft slab at 4 in thick is 1.235 cu yd (33.3 cu ft) of concrete before waste.
- With the standard 10% waste allowance, order 1.358 cu yd — or 62 bags of 80-lb pre-mix if you mix it yourself.
- One cubic yard takes about 45 bags of 80-lb mix (60 bags of 60-lb, 90 bags of 40-lb), so at roughly 1 cu yd and above a ready-mix truck usually beats bags on effort.
- Bag counts are always rounded UP to whole bags — you cannot buy part of a bag.
Concrete Calculator
| Volume (exact) | 33.3 cu ft |
| Volume (exact) | 1.235 cu yd |
| Order volume (with waste) | 1.358 cu yd |
| Pre-mix bags needed (with waste) | 62 |
A 10 × 10 ft slab, 4 inches thick, is 33.3 cu ft = 1.235 cu yd before waste; with 10% waste you order 1.358 cu yd, or 62 bags of 80-lb pre-mix.
Enter the length, width and thickness of your slab to get the exact concrete volume in cubic yards and cubic feet — plus how many pre-mix bags you need if you are mixing it yourself instead of ordering a truck.
Bag counts use the standard published yields for pre-mix concrete: one 80-lb bag makes about 0.6 cubic feet and one 60-lb bag about 0.45 cubic feet. Always add waste (10% is the usual allowance) for spillage, uneven ground and a slightly thicker-than-planned slab, and note that bag counts are always rounded UP to whole bags — you cannot buy part of a bag.
The formula
Worked example
A 10 × 10 ft slab, 4 inches thick, is 33.3 cu ft = 1.235 cu yd before waste; with 10% waste you order 1.358 cu yd, or 62 bags of 80-lb pre-mix.
The example is pre-loaded in the result panel — press Calculate with your own numbers to replace it.
Concrete formulas by shape
| Shape | Volume formula |
|---|---|
| Slab / pad | Length × Width × (Thickness in inches ÷ 12) |
| Footing / strip | Length × Width × Depth (all in feet) |
| Round hole / column | π × Radius² × Depth (all in feet) |
| Steps | Sum of each step: Length × Width × Height (all in feet) |
This calculator works the rectangular slab case; for every shape, divide cubic feet by 27 to get cubic yards.
Pre-mix bag yields
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Yields are the manufacturers' published figures. Bags per cubic yard are this calculator's counts for 27 cu ft with no waste allowance.
Common slab sizes at 4 in thick
| Slab size | Volume (cu yd) | 80-lb bags (exact) | 80-lb bags (with 10% waste) |
|---|---|---|---|
| 8 × 8 ft | 0.79 | 36 | 40 |
| 10 × 10 ft | 1.235 | 56 | 62 |
| 10 × 12 ft | 1.481 | 67 | 74 |
| 12 × 12 ft | 1.778 | 80 | 89 |
| 16 × 20 ft | 3.951 | 178 | 196 |
| 20 × 20 ft | 4.938 | 223 | 245 |
Volumes and bag counts are this calculator's outputs for each size at 4 in thick; bag counts are rounded up to whole bags.
Common mistakes
- Forgetting the waste allowance — subgrade dips, bowed forms and spillage mean the exact volume is rarely enough; 10% is the standard cushion.
- Entering thickness in feet instead of inches (or the reverse) — a 4-inch slab entered as 4 ft multiplies the order by 12.
- Comparing bag counts against truck prices without the yield: one cubic yard is about 45 bags of 80-lb mix, not a handful.
- Mixing bags for a pour above roughly 1 cu yd — the hauling and mixing workload, not the price, is usually what breaks the job.
When this estimate may be wrong
- Bag yields (0.60 cu ft per 80-lb bag, 0.45 per 60-lb bag) are the manufacturers' published figures; actual yield varies slightly with water content and consolidation.
- The estimate assumes a level subgrade at a uniform thickness — an uneven base can add 10% or more to the real volume.
- Ready-mix suppliers often enforce a minimum load and charge short-load fees below it, so the order volume here may not match the billable volume.
Frequently asked questions
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick that slab is 33.3 cu ft. Dividing by the 0.6 cu ft yield of an 80-lb bag gives 56 bags before waste, or 62 bags with a 10% waste allowance. At that quantity a ready-mix truck (1.36 cu yd) is usually cheaper and far less work.
Why add 10% waste?
Real slabs are never perfect: the subgrade dips, forms bow slightly and some concrete spills. Ordering 5–10% extra is far cheaper than a second delivery or a cold joint partway through the pour.
How thick should a slab be?
4 inches is standard for patios, walkways and sheds. Driveways and garage floors typically use 4–6 inches, often with rebar or wire mesh. Heavy vehicles or poor soil call for 6 inches or more — check local code.
Bags or ready-mix — which is better?
It comes down to volume. One cubic yard of concrete is about 45 bags of 80-lb pre-mix (60 bags of 60-lb), and every one has to be hauled, lifted and mixed — the 10 × 10 ft example slab above works out to 62 bags with a 10% waste allowance. At roughly 1 cubic yard and above, a ready-mix truck usually wins on effort and finish quality; below that, bags avoid delivery minimums. Ready-mix short-load fees for small orders vary by supplier and can narrow the gap, so compare the delivered price both ways.
Category: Construction & Home calculators
Sources & methodology
Volume is length × width × thickness for a rectangular slab, converted at 27 cu ft per cubic yard. Bag counts divide the waste-adjusted volume by the manufacturer's published yield per bag and round up to whole bags.
Content last reviewed by CalcHub Editorial.
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