Concrete Calculator
Calculate how many cubic yards of concrete, bags, and pounds of weight you need for a rectangular slab, footing, wall, or circular column.
How we calculate it
Volume = Length × Width × Depth for a rectangular slab, footing, or wall, or Volume = π × (Diameter / 2)² × Height for a circular column or slab (plus your waste factor), converted to cubic yards by dividing by 27. Bag counts assume a 60lb bag yields about 0.45 ft³ and an 80lb bag about 0.6 ft³. Metric bag yields (25kg/40kg) are estimated from the same material density. Weight is Volume × Density, using the poured-concrete density in the field above (independent from the bag density).
Understanding your estimate
Order too little concrete and the truck comes up short, or you're mixing bags by hand at six in the evening trying to finish a pour that has to go in as one piece. Order too much and you're paying for concrete that ends up in the truck's washout pit. The formula behind this calculator is straightforward. The two rounding decisions built into it are what actually save you that second trip to the yard.
A patio slab, worked out
Say you're pouring a 10 ft by 12 ft patio, 4 in thick, with the default 10% waste factor.
- Depth in feet: 4 in ÷ 12 = 0.33 ft.
- Raw volume: 10 × 12 × 0.33 = 40 ft³.
- Add the waste factor: 40 × 1.10 = 44 ft³. That's what you order, not the slab's theoretical volume.
- Divide by 27 for cubic yards, the unit ready-mix trucks price by: 1.63 yd³.
- Bagged pre-mix: 44 ÷ 0.45 = 97.8, rounded up to 98 bags of 60 lb. Or 44 ÷ 0.6 = 73.3, rounded up to 74 bags of 80 lb.
- At the default 150 lb/ft³ density, that same 44 ft³ weighs 6,600 lb once it's poured.
Why the bag count always rounds up
97.8 becomes 98, never 97. This calculator never rounds a purchase quantity down, anywhere. Coming up one bag short mid-pour means a trip back to the store while a half-finished slab cures unevenly in the meantime. One leftover bag costs you nothing you weren't already planning to spend. The waste factor already covers spillage and slab variance; rounding up on top of it protects the one number in this whole job you can't undo once the bags are mixed.
Why the waste factor is a field, not a fixed markup
The waste factor defaults to 10%, but it's a field you can change, not a number baked into the formula. Five to ten percent on top of the calculated volume is standard practice, for two reasons that have nothing to do with each other: spillage during the pour, and the fact that a subgrade is rarely perfectly level, so what actually gets poured rarely matches the exact volume you measured on paper. A small job with tight formwork might get away with 5%. A DIY pour on ground you haven't graded yourself is closer to 10%, sometimes more. We don't guess which one applies to your job. You do.
Why the weight and the bag count don't match
Multiply it out and 6,600 lb won't match either bag count: 98 bags at 60 lb comes to 5,880 lb, 74 bags at 80 lb comes to 5,920 lb. That's not a rounding artifact, and it's not sloppy math. The bag counts assume dry, bagged pre-mix. The weight uses the density of concrete after it's mixed and poured, which packs more mass into the same cubic foot than the dry material does. Two different physical states, two different densities, both correct for what they measure. If you're figuring out what a wheelbarrow can carry, use the poured weight, not the bag math.
No density presets, on purpose
Our Gravel Calculator ships with named density presets, Light, Standard, Heavy, because gravel's density genuinely swings by material. Concrete doesn't get that treatment here. Ready-mix concrete for a residential job stays dense and mixed at a fairly consistent ratio no matter which supplier delivers it, so one editable number with a sensible default does the job without pretending there are meaningful categories to pick from. If you know your mix's exact density, from a batch ticket or a supplier spec sheet, type it in. Otherwise the default is a reasonable one to trust.
Rectangular covers more than slabs
Footings and walls use the same length times width times depth math as a slab, so pick Rectangular for those too. Circular exists mostly for sonotubes, the cardboard tube forms used for deck post footings; round slabs are rare in a backyard job. That's also why depth and diameter are entered in inches, not feet, while a sonotube's height still gets its own foot-based field. Nobody measures a slab's thickness in fractions of a foot. A post footing's height is a different story.
What this calculator won't tell you
It won't tell you what any of this costs. Ready-mix pricing swings by region, by supplier, and by the day you call, and bag prices at the hardware store change often enough that a number we hardcoded today would be wrong by the time you read this. Volume, bag count, and weight are physical quantities that don't change. Price is a phone call.