Calculate concrete volume for slabs, footings and round columns in cubic yards and feet, and convert the volume into pre-mix bags by their stated yields. Runs entirely in your browser — your numbers never leave this device.
Bag counts and yardage include the waste allowance above. Bag counts are whole-bag ceilings — you cannot buy a fraction of a bag — based on standard yields of 0.6 ft³ per 80 lb bag and 0.45 ft³ per 60 lb bag.
| Bag size | Yield |
|---|---|
| 40 lb | 0.30 ft³ |
| 60 lb | 0.45 ft³ |
| 80 lb | 0.60 ft³ |
Concrete is a volume problem in three shapes. A slab is length times width times thickness; a footing is the same product with depth in place of thickness; a round column or post hole is the circle area, π times the radius squared, times its height. Every dimension must be in the same unit before multiplying — the classic error is a 4-inch slab thickness left in inches against feet for length and width, which inflates the volume twelvefold — so this calculator converts thickness and depth to feet internally.
The volume is reported in cubic feet and cubic yards (1 yd³ = 27 ft³), because ready-mix is ordered by the yard while bagged pre-mix is counted by yield. Bag yields are printed on the bags and in the table here: an 80 lb bag yields 0.6 ft³ of placed concrete, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.30 ft³. Dividing the volume by the yield gives the bag count, and the count is always rounded up — a fraction of a bag still has to be bought.
Real pours lose concrete to uneven subgrade, spillage and forms that are not perfectly true, so a waste allowance — 10% in the first worked example — is added before the bag count is finalised. The result is a planning estimate: confirm bag yields against the product in hand, since yields are the manufacturer's figures and differ between brands.
Waste. The raw volume of 33.3 ft³ divides to about 56 × 80 lb bags, but uneven ground, spillage and form tolerances consume concrete on every real pour. The example applies a 10% allowance first — 36.7 ft³ — which divides to 62 bags. Ordering the raw figure risks running short mid-pour.
27 — a yard is 3 feet, and 3 × 3 × 3 = 27. Ready-mix is ordered in cubic yards and bags are counted in cubic feet, so this conversion sits in the middle of almost every concrete estimate.
No — yields are stated by the manufacturer and the common figures are printed in the table (80 lb = 0.6 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³). Check the bag in hand; if its stated yield differs, the bag count changes in proportion.
This page estimates volume for planning and buying. It is not structural or code advice: footing sizes, slab thickness, reinforcement and mix specification for load-bearing work are set by local code and, where required, an engineer.
No. The calculation runs entirely in the browser and nothing entered is uploaded, stored or tracked.
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