33.33
cu ft
1.23
cu yd
1.36
cu yd
82
bags
62
bags
5,500
lb
33.33
cu ft
1.23
cu yd
1.36
cu yd
82
bags
62
bags
5,500
lb
The Concrete Calculator determines the exact volume of concrete needed for any slab, footing, wall, or pour project, and translates that volume into the number of pre-mixed concrete bags required. Whether you are pouring a patio, a sidewalk, a driveway, fence post footings, or a small foundation, accurate material estimation prevents costly overruns and mid-pour shortages.
Concrete volume is expressed in cubic yards (the unit used by ready-mix suppliers for truck deliveries) or cubic feet (the unit most useful when calculating bag counts). One cubic yard equals 27 cubic feet. Ready-mix concrete is typically ordered by the yard for projects over 1 cubic yard (approximately 45 bags at 80 lb); for smaller projects, pre-mixed bags from a home improvement store are more practical.
Standard concrete slab thicknesses: 4 inches for sidewalks, patios, and driveways on stable ground; 6 inches for driveways subject to heavy vehicle traffic; 8-12 inches for structural foundations. The most common residential applications use 4-6 inch slabs. This calculator works for all thicknesses — simply enter the desired depth.
A 60 lb bag of ready-mix concrete yields approximately 0.45 cubic feet when mixed; an 80 lb bag yields approximately 0.60 cubic feet. Always add a 10% buffer to your bag count for spillage, overfill, and uneven subgrades — this calculator already provides the exact minimum; consider rounding up by at least one extra bag.
Volume in Cubic Feet: $$V_{\text{ft}^3} = \text{Length} \times \text{Width} \times \frac{\text{Depth (in)}}{12}$$
Volume in Cubic Yards: $$V_{\text{yd}^3} = \frac{V_{\text{ft}^3}}{27}$$
60 lb Bags Needed (0.45 cu ft yield each): $$\text{Bags}_{60} = \left\lceil \frac{V_{\text{ft}^3}}{0.45} \right\rceil$$
80 lb Bags Needed (0.60 cu ft yield each): $$\text{Bags}_{80} = \left\lceil \frac{V_{\text{ft}^3}}{0.60} \right\rceil$$
For a 10×10 ft patio at 4" depth: Volume = 10 × 10 × (4/12) = 33.33 cu ft = 1.23 cu yd; 60 lb bags = ⌈33.33/0.45⌉ = 75; 80 lb bags = ⌈33.33/0.60⌉ = 56.
If your project requires more than 1 cubic yard (27 cubic feet), consider ordering ready-mix concrete from a supplier — it will be cheaper and less labor-intensive than mixing dozens of bags. For projects over 3 cubic yards, ready-mix delivery is almost always the practical choice. For small projects under 0.5 cubic yards, pre-mixed bags are ideal. Always add at least one extra bag beyond the calculated minimum to account for uneven subgrades and waste.
Inputs
Results
A 12×12 ft patio at 4" depth needs 1.78 cubic yards — at this scale, either 80 bags at 80 lb each or ordering a small ready-mix delivery are both viable options.
Inputs
Results
A small post footing at 3×2 ft and 8" deep requires just 4 cubic feet — 7 bags at 80 lb is a quick, manageable pour for a single person.
Order ready-mix concrete (delivered by truck) when your project exceeds 1 cubic yard (about 45 bags at 80 lb). Mixing bags by hand for large projects is exhausting, time-consuming, and risks inconsistent mix ratios. Ready-mix is typically $120-$180 per cubic yard delivered, versus $5-$7 per 80 lb bag — bags become cost-prohibitive for large pours.
Standard residential applications use 3,000-4,000 PSI concrete. Pre-mixed bags are typically 4,000-5,000 PSI when properly mixed. For structural applications (footings, foundations, retaining walls), consult a structural engineer. For driveways subject to heavy trucks or freeze-thaw cycles, use 4,000+ PSI concrete with air entrainment.
For patios and sidewalks on stable, undisturbed soil: wire mesh (6×6 W1.4×W1.4) is often sufficient. For driveways, garage floors, and areas with clay or expansive soil: rebar (#3 or #4, 18" grid) is recommended. Footings and foundations almost always require rebar. Place reinforcement at mid-depth of the slab for optimal tension resistance.
Concrete gains strength progressively: it reaches approximately 70% strength at 7 days and 99% strength at 28 days. You can walk on a slab after 24-48 hours, drive on it after 7 days, and it reaches full design strength at 28 days. Keep new concrete moist (wet burlap, curing blanket, or curing compound) for at least 7 days to maximize strength.
The water-to-cement (w/c) ratio is the weight of water divided by the weight of cement in the mix. Lower ratios (0.40-0.45) produce stronger, more durable concrete but are harder to work with. Higher ratios (0.55+) are easier to place but produce weaker concrete prone to cracking. Adding excess water to bag mix 'to make it easier' is the most common DIY mistake — it significantly reduces final strength.
Key strategies: use proper water ratio (do not over-water), install control joints (saw cuts 1/4 of slab depth, spaced at 2-3 times the slab thickness in feet), use adequate subgrade compaction, cure properly for 7+ days, and avoid rapid drying in hot or windy conditions. Some cracking in concrete is normal and does not indicate structural failure.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
How helpful was this calculator?
Be the first to rate!
