Work out the concrete a slab, footing, pad or round column needs in cubic feet and cubic yards, how many 40, 60 or 80 lb bags that is, and what the finished pour weighs.
Measure the finished pour, not the form. Add 5–10% for over-excavation, spillage and an uneven sub-base, bags never quite fill the hole you dug.
Every concrete question is a volume question. A slab is length × width × thickness. A round column is π × radius² × height. Once that is in cubic feet the rest is arithmetic: divide by 27 for cubic yards, or by the yield of one bag for the bag count.
One cubic yard is 27 cubic feet, a cube three feet on every side. It is the unit ready-mix is sold in, and it weighs roughly 4,000 lb once cured. That weight is the real reason most people stop carrying bags somewhere around half a yard.
For a round form, the diameter you type is the nominal inside diameter printed on the tube. Sonotube-style forms are waxed cardboard and the printed size already refers to the inside, so the concrete diameter is the nominal figure, not the outside of the roll.
| Bag size | Mixed yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
| 90 lb | 0.675 cu ft | 40 |
Yields are for standard 4,000 psi bagged mix. A 90 lb bag is not sold in every region, and the bigger the bag the cheaper the concrete per cubic foot, an 80 lb bag costs 20–25% less per cubic foot than a 40 lb bag of the same mix.
| Slab thickness | Coverage per 80 lb bag | Bags per 100 sq ft |
|---|---|---|
| 2 in | 3.60 sq ft | 28 |
| 3 in | 2.40 sq ft | 42 |
| 4 in | 1.80 sq ft | 56 |
| 5 in | 1.44 sq ft | 70 |
| 6 in | 1.20 sq ft | 84 |
| 8 in | 0.90 sq ft | 112 |
A 10 × 10 ft patio at 4 in thick is 33.3 cu ft of concrete, 56 bags of 80 lb, or 1.23 cubic yards. That is the point where hand mixing stops being sensible.
| Application | Typical thickness | Reinforcement |
|---|---|---|
| Garden path, footpath | 3 in | Usually none |
| Patio, sidewalk, shed floor | 4 in | Mesh or #3 bar on 18 in centres |
| Driveway, garage floor | 4–6 in | #4 bar on 12–16 in centres |
| Footing depth (IRC minimum) | 6 in | See the footing table below |
Bagged mix is designed to be poured 2 in thick or more. Below 2 in it is a repair patch, not a slab. Below 4 in on a driveway it will crack under a loaded vehicle regardless of how well it is mixed.
The International Residential Code sets minimum footing width by how many storeys the footing carries and by the bearing capacity of the soil. On the 1,500 psf soil the code presumes when no soil test exists:
| Condition (conventional light-frame) | 1,500 psf soil | 2,000 psf soil |
|---|---|---|
| 1 storey | 12 in wide | 12 in |
| 2 storeys | 15 in wide | 12 in |
| 3 storeys | 23 in wide | 17 in |
Footings must be at least 6 in thick, and the projection beyond the wall on each side must be at least 2 in and no more than the footing thickness (IRC R403.1.1). Footings also have to bear below the local frost line, 12 in below grade is the floor in every climate, and 36–48 in in the northern states.
This table is the prescriptive minimum for ordinary light-frame houses on presumed soil. A footing carrying a structure should be designed, it is sized by load and soil, not by a rule of thumb.
The practical crossover is around half a cubic yard to a cubic yard. Below it, bags let you work at your own pace. Above it, a truck delivers consistent concrete in minutes and usually costs less than the bags plus your time: 45 bags of 80 lb is 3,600 lb of material to carry, mix and place, and the whole pour has to be finished before it starts to stiffen.
If you mix by hand, mix only what you can place in about 20 minutes, keep a second person on the mixer while you screed, and never add extra water to make it flow, extra water is what turns 4,000 psi concrete into 2,500 psi concrete and a dusty, cracking surface.
Bag yields (0.30 / 0.375 / 0.45 / 0.60 / 0.675 cu ft for 40 / 50 / 60 / 80 / 90 lb) are the published figures on the Quikrete Concrete Mix data sheet for product 1101, the same mix sold under most store brands; the same sheet gives the unit weight of approximately 140 lb per cubic foot that this calculator uses for its weight line, and the 2 in minimum pour depth. Minimum footing widths come from International Residential Code Table R403.1, with the 6 in minimum thickness and 2 in minimum projection from R403.1.1. Footing depth rules come from R403.1.4 (12 in below grade) and R403.1.4.1 (below the local frost line). Slab thicknesses follow ordinary residential practice. Confirm everything against your local building department before you pour.
Work out the volume in cubic feet, add your waste allowance, then divide by the yield of one bag. A 10 × 10 ft slab at 4 in thick is 33.3 cu ft, about 56 bags of 80 lb (0.60 cu ft each), or 75 bags of 60 lb, or 1.23 cubic yards.
45. A cubic yard is 27 cubic feet and an 80 lb bag yields 0.60 cu ft, so 27 ÷ 0.60 = 45 bags. That is 3,600 lb of bagged material for one cubic yard of finished concrete.
π × radius² × height, with the radius in feet. A 12 in diameter tube has a 6 in radius, which is 0.5 ft, so the area is π × 0.25 = 0.785 sq ft. At 8 ft tall that is 6.28 cu ft, or about 11 bags of 80 lb. Remember the printed diameter is the inside of the tube.
Length × width × depth, converted to feet. Footings are a minimum of 6 in deep and, on presumed 1,500 psf soil, 12 in wide for a one-storey light-frame house, 15 in for two storeys and 23 in for three. Footings must also bear below the local frost line.
About 4,000 lb, or roughly 2 tons, at a typical 140–150 lb per cubic foot. That is why a cubic yard is the practical limit for wheelbarrow work and why most suppliers have a minimum delivery quantity.
Yes, 5% for a formed slab on a firm, level sub-base, and 10% for anything poured into an excavated hole, where the hole is almost never exactly the size you dug. Round up to whole bags: a leftover bag stores, a short pour does not.