Concrete Volume & Bags Calculator
Calculate the concrete volume of a slab or footing from length, width and depth, and convert it into the number of premix bags to buy — for six standard US and metric bag sizes.
Input
Unit for length and width. Depth below is always in inches, since slab thickness is conventionally specified that way.
Pick the bag you plan to buy. Yield is the approximate finished concrete per bag.
Output
| Metric | Value |
|---|---|
| No data yet | |
Guides
Pouring a slab, footing, or small pad and want to know how many bags of premix concrete to buy? This calculator turns three measurements — length, width, and depth — into a volume, then converts that volume into a bag count for the exact bag size you plan to use.
The formula
For a rectangular slab or footing, volume is simply:
Volume = Length × Width × Depth
Enter length and width in feet, meters, or inches, and enter depth in inches — slab and footing thickness is almost always specified in inches (a "4-inch slab", a "6-inch footing") regardless of what unit the plan-view dimensions use. Everything is converted internally to cubic feet, then also shown in cubic yards and cubic meters, so you can cross-check against a ready-mix truck's ticket (ordered by the cubic yard) or a metric spec sheet.
Bag yield and bag count
Bagged premix concrete lists a "yield" on the label — the volume of finished, mixed concrete one bag produces once combined with water. Yield isn't the same as the bag's dry weight, and it varies by size:
| Bag | Approx. yield |
|---|---|
| 40 lb | ~0.30 ft³ |
| 60 lb | ~0.45 ft³ |
| 80 lb | ~0.60 ft³ |
| 20 kg | ~0.33 ft³ |
| 25 kg | ~0.41 ft³ |
| 30 kg | ~0.50 ft³ |
These are standard figures published by major premix brands (Quikrete, Sakrete, and similar), averaged. Once total volume is known:
Bags Needed = ceil(Total Volume ÷ Bag Yield)
The result always rounds up to the next whole bag — a fractional remainder still needs a full extra bag, since bags aren't sold in partial units.
How to use it
- Enter the length and width of the area you're pouring, and pick their unit (feet, meters, or inches).
- Enter the depth (thickness) in inches.
- Pick the bag size you intend to buy from the dropdown.
- The results table updates automatically with the total volume in cubic feet, cubic yards, and cubic meters, plus the number of bags needed.
FAQ
Does this account for waste or spillage? No — the figures above are the exact volume of the space you described. Always buy a small overage: uneven subgrade, spillage during mixing, and over-excavation typically consume an extra 5–10% of material. Round your bag count up further, or buy one or two extra bags, before you start.
Why is depth always in inches even if I pick meters for length/width? Because that's how concrete work is specified in practice — a "4-inch slab" is a fixed convention independent of the plan-view unit. This avoids the error-prone step of converting a thickness spec to decimal feet or meters before typing it in.
Can I use this for a footing, pad, or other rectangular pour, not just a slab? Yes — the math is identical for any rectangular volume: a footing, a pad, a sidewalk section, or a shed base. Just treat "depth" as the pour's thickness.
Does it work for non-rectangular shapes like columns or circular slabs? No — this tool covers rectangular volumes only. For cylindrical columns or circular slabs, calculate volume separately (π × radius² × height) before converting to bags.
Privacy
All calculations run entirely in your browser — nothing you enter is sent to a server or stored.