Field Guide
How Much Concrete Do I Need? Slabs, Footings, Bags & Ready-Mix
Whether you're pouring a patio from bags or ordering a truck for a driveway, the math is the same: find the volume in cubic feet, add a waste allowance, then convert to cubic yards, bags, or a ready-mix order.
1. How to Calculate Concrete (The Math Explained)
Concrete is sold by the cubic yard (ready-mix) or by the bag. Your measurements come in feet and inches, so convert everything to feet, find the volume, then convert. Here's the exact process this calculator uses.
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1
Convert inches to feet
Slab thickness, tube diameters and footing sizes are usually given in inches. Divide by 12.
Feet = Inches ÷ 12 Example: 4 in ÷ 12 = 0.333 ft -
2
Find the volume in cubic feet
Use the formula for your pour type. Round footings use the circle area (π × radius²), where the radius is half the diameter.
Slab: Length (ft) × Width (ft) × Thickness (in ÷ 12) Round: π × (Diameter (in) ÷ 24)² × Depth (ft) × Quantity Footing: Length (ft) × Width (in ÷ 12) × Depth (in ÷ 12) Example (slab): 10 × 10 × 0.333 = 33.3 ft³ Example (round): 3.1416 × 0.5² × 4 × 6 = 18.85 ft³ Example (footing): 30 × 1.333 × 0.667 = 26.7 ft³ -
3
Add waste, then convert to cubic yards
Subgrade is never perfectly flat and forms bow a little, so add about 10%. Then divide by 27, the number of cubic feet in a cubic yard.
With Waste = Cubic Feet × 1.10 Cubic Yards = With Waste ÷ 27 Example: 33.3 × 1.10 = 36.7 ft³ → 36.7 ÷ 27 = 1.36 yd³ -
4
Convert to bags or a ready-mix order
Divide cubic feet by the bag yield and round up. For ready-mix, round the cubic yards up to the next quarter yard, since that's how plants batch and sell it.
80 lb bags = Cubic Feet ÷ 0.60 (round up) 60 lb bags = Cubic Feet ÷ 0.45 (round up) Example: 36.7 ÷ 0.60 = 61.1 → 62 bags of 80 lb 36.7 ÷ 0.45 = 81.5 → 82 bags of 60 lb Ready-mix: 1.36 yd³ → order 1.50 yd³
Quick concrete conversions
- 1 cubic yard = 27 cubic feet
- 1 cubic yard ≈ 45 bags of 80 lb, or 60 bags of 60 lb
- At 4 in thick, 1 cubic yard covers about 81 sq ft of slab
- At 6 in thick, 1 cubic yard covers about 54 sq ft
- A full ready-mix truck carries about 8–10 cubic yards
2. Bagged Concrete vs. Ready-Mix: Which Should You Buy?
The break-even point is mostly about labor and minimum-order fees, not just material price. Hand-mixing 45 bags of 80 lb concrete means lifting 3,600 lbs, plus the water, mixing and finishing before it sets. A ready-mix truck delivers the same cubic yard in minutes.
| Pour size | 80 lb bags | 60 lb bags | Usually best |
|---|---|---|---|
| 0.5 yd³ (13.5 ft³) | 23 | 30 | Bags: post holes, small pads, steps |
| 1 yd³ (27 ft³) | 45 | 60 | Either: compare bag cost vs. a short-load fee |
| 2 yd³ (54 ft³) | 90 | 120 | Ready-mix, even with a short-load fee |
| 5 yd³ (135 ft³) | 225 | 300 | Ready-mix |
Ready-mix ordering tips
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Short-load fees. Many plants charge an extra fee on orders under about 1–3 cubic yards, because a nearly empty truck costs them almost as much to send. Ask for the fee when you get a quote.
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Order in quarter yards, rounded up. Plants batch in ¼ yd³ steps. Running short mid-pour can leave a weak cold joint, and a second delivery costs far more than a little extra concrete.
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Plan truck access. A loaded mixer is heavy and can crack driveways and sidewalks. If the chute can't reach the forms, ask about a pump or line up wheelbarrows and helpers before the truck arrives.
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Be ready when it arrives. Forms set, subgrade compacted and damp, tools and finishers on site. Most plants expect the truck to be emptied within about 90 minutes of batching.
3. Recommended Concrete Slab Thickness
How thick should a concrete slab be? These are common starting points for residential work on a well-compacted gravel base. Go thicker for heavy loads, poor soil, or freeze-thaw climates.
| Application | Thickness | Notes |
|---|---|---|
| Sidewalks & patios | 4 in | Foot traffic, patio furniture, grills and hot tubs on a separate pad |
| Residential driveways | 4–5 in | 5 in for pickups, SUVs and occasional delivery trucks |
| Garage floors | 4–6 in | Thicker for heavy vehicles, lifts, or workshop equipment |
| Heavy-use & RV pads | 6 in | RVs, boats on trailers, dumpsters and equipment |
| Footings & piers | Per local code | Width, depth and frost depth are set by your building code and soil |
Structural work: check your code first
Footings, foundation walls, structural slabs and deck piers carry real loads. Local building codes, your building department and a licensed engineer govern their size, depth below frost line, reinforcement and concrete strength. Use this calculator for quantities once those dimensions are set.
4. Frequently Asked Questions
How many 80 lb bags of concrete make a cubic yard?
An 80 lb bag of pre-mixed concrete yields about 0.60 cubic feet, so one cubic yard (27 ft³) takes about 45 bags. With 60 lb bags (about 0.45 ft³ each) you need about 60 bags per cubic yard.
Always round up to whole bags, and check the yield printed on the bag; it varies slightly by brand.
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab at 4 in thick is 100 sq ft × 0.333 ft = 33.3 cubic feet, or 1.23 cubic yards. With a 10% waste allowance that becomes 36.7 ft³ (1.36 yd³): about 62 bags of 80 lb, or a 1.50 yd³ ready-mix order rounded up to the nearest quarter yard.
How do I calculate concrete for a sonotube or round footing?
Use the cylinder formula: Volume = π × radius² × depth, where the radius is half the diameter, in feet. A 12 in tube has a 0.5 ft radius, so a tube 4 ft deep holds 3.1416 × 0.25 × 4 = 3.14 cubic feet. Six of them hold 18.85 ft³ (0.70 yd³), about 32 bags of 80 lb before waste.
In the calculator above, choose Round Footing and enter the diameter in inches, the depth in feet, and the number of tubes.
How thick should a concrete slab be?
For typical residential work on a compacted gravel base:
- Sidewalks and patios: 4 in
- Residential driveways: 4–5 in
- Garage floors: 4–6 in
- Heavy-use and RV pads: 6 in
Footings, foundations and structural slabs are sized by your local building code or a licensed engineer.
Should I use bagged concrete or order ready-mix?
Bags make sense for small pours under about 1 cubic yard (45 bags of 80 lb). Above that, ready-mix is usually cheaper and far less work. Orders under about 1–3 cubic yards often carry a short-load fee, so for pours in that range compare the fee against the cost of bags and the labor of mixing them.
How much extra concrete should I order?
Plan on about 10% extra to cover uneven subgrade, form bulge and spillage. Running short mid-pour can leave a weak cold joint, so a little left over is much cheaper than a second delivery. This calculator adds 10% by default (the slider goes from 0 to 20%) and also shows the raw volume.
How many cubic yards are in a concrete truck?
A full ready-mix truck typically carries 8 to 10 cubic yards, depending on the truck and local weight limits. This calculator assumes about 10 yd³ per truck. Ready-mix is sold in quarter-yard steps, so the order quantity is rounded up to the nearest 0.25 yd³.