Concrete Bags Calculator
Find out exactly how many bags of pre-mixed concrete your project needs, in 40, 60, or 80 lb sizes. Enter the dimensions and the calculator converts volume to bag count using each bag's real yield, so you avoid the classic mistake of buying too few. Ideal for small pours where ordering ready-mix by the yard isn't worth it.
Concrete Bags Diagram
How the Formula Works
Calculate concrete volume
Volume = Length × Width × Depth Convert all dimensions to feet first: inches ÷ 12. Example: a 10 ft × 10 ft slab at 4″ thick = 10 × 10 × (4/12) = 33.33 ft³ = 1.23 yd³.
Apply waste factor
Adjusted Volume = Volume × (1 + Waste %) A 10% waste factor is standard for straightforward pours. Use 15% for complex forms, irregular shapes, footings with uneven bases, or when working with large crews where spills are likely.
Determine bag yield
Yield: 40 lb = 0.30 ft³ | 50 lb = 0.375 ft³ | 60 lb = 0.45 ft³ | 80 lb = 0.60 ft³ Bag yield is the volume of mixed concrete produced by one bag. It is printed on every bag label. Quikrete and Sakrete use these standard yields; specialty mixes may differ — use the Custom yield option in that case.
Calculate bags required
Bags = ⌈Adjusted Volume ÷ Yield per Bag⌉ Always round up — you cannot use a partial bag, and running short mid-pour causes cold joints that weaken the slab. The net bag count (no waste) is also shown so you can see exactly how many bags the pure volume requires.
Convert units
1 yd³ = 27 ft³ = 0.7646 m³ Ready-mix concrete is ordered by the cubic yard. If your project is under roughly 1 yd³ (27 ft³), bags are typically more economical and convenient. Above 1 yd³, a ready-mix delivery is usually faster and cheaper per unit.
Frequently Asked Questions
A 10 ft × 10 ft slab at 4″ thick = 33.33 ft³ = 1.23 yd³. At 0.45 ft³ per 60 lb bag you need about 74 bags (with 10% waste) or 56 bags net. At 0.60 ft³ per 80 lb bag you need about 56 bags with waste or 42 bags net. Always add at least 10% for waste.
An 80 lb bag of standard concrete mix (Quikrete, Sakrete) yields 0.60 ft³ of mixed concrete. That means you need approximately 45 bags to fill one cubic yard (27 ft³ ÷ 0.60 = 45).
80 lb bags are more economical per cubic foot and reduce the total number of bags to mix. 60 lb bags are easier to lift and carry — a good choice for solo DIY projects or when working overhead. Both produce the same mix strength; the only difference is weight and yield per bag.
Bagged concrete is practical for projects under about 1 cubic yard. Above 1 yd³, a ready-mix truck is typically faster, cheaper per unit, and produces a more consistent mix. The break-even point depends on local ready-mix delivery minimums — many suppliers have a 1–2 yd³ minimum.
In the US, a 60 lb bag typically costs $5–$7 and an 80 lb bag $7–$10 at home improvement stores. That puts bagged concrete at roughly $130–$200 per cubic yard — comparable to or more expensive than ready-mix ($150–$200/yd³ delivered), which is why ready-mix is preferred for larger pours.
For residential patios and walkways, 4″ (100 mm) is standard. Driveways that see passenger vehicles should be 4–6″. Slabs carrying heavy trucks or equipment should be 6″ or more. Interior floor slabs are typically 3.5–4″. Footings are sized by structural requirements — commonly 8–12″ for residential foundations.
Most standard concrete mixes reach initial set in 1–2 hours and are walkable after 24 hours. Full structural strength (3,000–4,000 psi) is reached at 28 days. Fast-setting mixes (e.g. Quikrete Fast-Setting) can bear weight in 3–4 hours. Temperature affects cure time: cold weather slows curing, hot weather accelerates it.
Use 10% waste for typical rectangular slabs and straightforward pours. Use 15% for complex forms, curved shapes, irregular footings, or thick pours where estimating exact volume is difficult. Never round down on bags — running out of concrete mid-pour causes cold joints that permanently weaken the structure.