Concrete Slab & Pad Calculator
For patios, shed pads, walkways and driveways. Enter the size in feet and inches (or meters) and get cubic yards, bag counts, weight, rebar and a scaled drawing of your slab.
A 10×10 ft slab, 4 in thick needs 1.23 yd³ of concrete: 62 bags of 80 lb mix with 10% extra.
ft³ · m³ · exact yd³ + %
Slab weight ≈ lb · mixing water ≈ gal
Concrete for common slab sizes
| Slab size | Thickness | Exact volume | With 10% extra | 80 lb bags | 60 lb bags | Weight |
|---|---|---|---|---|---|---|
| 4 × 4 ft | 4 in | 0.20 yd³ | 0.22 yd³ | 10 | 14 | 773 lb |
| 8 × 8 ft | 4 in | 0.79 yd³ | 0.87 yd³ | 40 | 53 | 3,093 lb |
| 10 × 10 ft | 4 in | 1.23 yd³ | 1.36 yd³ | 62 | 82 | 4,833 lb |
| 10 × 12 ft | 4 in | 1.48 yd³ | 1.63 yd³ | 74 | 98 | 5,800 lb |
| 12 × 12 ft | 4 in | 1.78 yd³ | 1.96 yd³ | 88 | 118 | 6,960 lb |
| 10 × 20 ft | 4 in | 2.47 yd³ | 2.72 yd³ | 123 | 163 | 9,667 lb |
| 16 × 16 ft | 4 in | 3.16 yd³ | 3.48 yd³ | 157 | 209 | 12,373 lb |
| 20 × 20 ft | 4 in | 4.94 yd³ | 5.43 yd³ | 245 | 326 | 19,333 lb |
| 12 × 24 ft | 4 in | 3.56 yd³ | 3.91 yd³ | 176 | 235 | 13,920 lb |
| 24 × 24 ft | 4 in | 7.11 yd³ | 7.82 yd³ | 352 | 470 | 27,840 lb |
| 10 × 20 ft | 6 in | 3.70 yd³ | 4.07 yd³ | 184 | 245 | 14,500 lb |
| 20 × 20 ft | 6 in | 7.41 yd³ | 8.15 yd³ | 367 | 489 | 29,000 lb |
Bag counts include 10% extra and are rounded up. Weight uses 145 lb/ft³ for the exact volume.
Concrete pads for sheds, hot tubs and AC units
A pad is just a small slab, so the same formula applies. What changes is thickness: a shed pad is usually 4 in, while a filled hot tub can weigh several thousand pounds and is commonly set on 5 to 6 in. Check the tub or shed maker's installation sheet, which often states the minimum pad size and thickness.
| Pad | Size | Thickness | Volume (+10%) | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| Shed pad | 8 × 10 ft | 4 in | 1.09 yd³ | 49 | 66 |
| Shed pad | 10 × 12 ft | 4 in | 1.63 yd³ | 74 | 98 |
| Shed pad | 12 × 16 ft | 4 in | 2.61 yd³ | 118 | 157 |
| Hot tub pad | 8 × 8 ft | 6 in | 1.30 yd³ | 59 | 79 |
| Hot tub pad | 10 × 10 ft | 6 in | 2.04 yd³ | 92 | 123 |
| AC / generator pad | 3 × 3 ft | 4 in | 0.12 yd³ | 6 | 8 |
| Grill pad | 6 × 6 ft | 4 in | 0.49 yd³ | 22 | 30 |
Gravel base under the slab
Most slabs sit on a compacted gravel base, commonly about 4 in deep, which drains water and gives the concrete an even bed. It is a separate order from the concrete: the base for a 10×10 ft slab at 4 in is another 1.23 yd³ of gravel. Excavate for both layers, so for a 4 in slab on a 4 in base you dig about 8 in below the finished height.
Concrete for a driveway
Driveways carry vehicles, so they are usually poured thicker than a patio: 5 in is common for cars and 6 in where heavier trucks park or soil is weak. A typical single-car driveway is 10 to 12 ft wide and a double is 20 to 24 ft. The table shows each 20–24 ft run; multiply by the number of runs for a longer drive.
| Section | Thickness | Exact volume | Order (10% extra, ¼ yd³ steps) | 80 lb bags |
|---|---|---|---|---|
| 10 × 20 ft | 5 in | 3.09 yd³ | 3.50 yd³ | 153 |
| 10 × 20 ft | 6 in | 3.70 yd³ | 4.25 yd³ | 184 |
| 12 × 24 ft | 5 in | 4.44 yd³ | 5.00 yd³ | 220 |
| 12 × 24 ft | 6 in | 5.33 yd³ | 6.00 yd³ | 264 |
| 20 × 24 ft | 5 in | 7.41 yd³ | 8.25 yd³ | 367 |
| 20 × 24 ft | 6 in | 8.89 yd³ | 10.00 yd³ | 440 |
| 24 × 24 ft | 6 in | 10.67 yd³ | 11.75 yd³ | 528 |
Above 2–3 yd³, ready-mix is usually the practical choice: 264 bags for a single 12×24 ft section at 6 in is a full day of mixing.
How to calculate concrete for a slab
- Measure length and width. Measure the inside of your forms in feet. Convert inches to feet by dividing by 12, so 10 ft 6 in is 10.5 ft.
- Convert the thickness to feet. Divide the slab thickness in inches by 12. A 4 in slab is 0.333 ft; 6 in is 0.5 ft.
- Multiply for cubic feet. Length × width × thickness in feet gives the volume in cubic feet.
- Divide by 27 for cubic yards. One cubic yard is 3 ft × 3 ft × 3 ft = 27 ft³. Ready-mix is sold by the cubic yard.
- Add extra for waste. Multiply by 1.05 to 1.10 to cover uneven ground, form flex and spillage.
- Convert to bags if mixing yourself. Divide cubic feet by the bag yield (0.60 ft³ for an 80 lb bag) and round up.
12 ft × 12 ft × (4 ÷ 12) ft = 48 ft³
48 ft³ ÷ 27 = 1.78 yd³
48 ft³ × 1.10 = 52.8 ft³ ÷ 0.60 = 88.0 → 88 bags of 80 lb mix
Mixing bagged concrete
For each 80 lb bag of standard concrete mix, the manufacturer's data sheet calls for about 6 pints (3 quarts) of clean water to start, and no more than about 9 pints in total. Add water slowly: extra water makes the mix easier to place but weaker once it cures.
- Water for the job: 62 bags for a 10×10 ft slab need roughly 47–70 gallons.
- Batches: divide the bag count by what your mixer takes per batch; a wheelbarrow or tub handles one 80 lb bag at a time.
- Crew: 40 or more bags is hard work for one person. Line up help and keep each batch placed before the previous one starts to set.
Slab thickness guide
| Project | Typical thickness | Concrete per 100 ft² | 80 lb bags per 100 ft² (+10%) |
|---|---|---|---|
| Walkway, patio, shed pad | 4 in | 1.23 yd³ | 62 |
| Hot tub pad, car driveway | 5 in | 1.54 yd³ | 77 |
| Heavy driveway, garage floor | 6 in | 1.85 yd³ | 92 |
Typical values only. Soil, frost depth, reinforcement and local code decide the final thickness.
How much does a concrete slab weigh?
Normal-weight concrete weighs 140 to 150 lb per cubic foot, or roughly 3,800 to 4,050 lb per cubic yard. Multiply the slab volume in cubic feet by 145 for a working estimate. A 10×10 ft slab at 4 in is 33.33 ft³, so it weighs about 4,833 lb, close to 2.4 tons. That matters when you plan a pour on a deck or roof, or need to haul old slab away.
Common mistakes
- Mixing inches and feet. Multiplying 10 × 10 × 4 gives 400, not 33.3. Thickness must be in feet (4 ÷ 12) before you multiply.
- No extra. Ordering the exact volume almost always comes up short. Subgrade is never perfectly flat.
- Forgetting thickened edges. If the slab edges are turned down for a footing, calculate that strip separately and add it.
- Ordering ready-mix to the decimal. Suppliers deliver in steps, usually ¼ yd³. Round up, not down.
Questions
How many bags of concrete do I need for a 10x10 slab?
A 10×10 ft slab 4 in thick is 1.23 cubic yards (33.33 ft³). With 10% extra that is 62 bags of 80 lb mix, 82 bags of 60 lb or 123 bags of 40 lb.
How many yards of concrete for a 20x20 slab?
At 4 in thick, a 20×20 ft slab needs 4.94 cubic yards, or 5.43 yd³ with 10% extra. At this size most people order ready-mix rather than 245 bags of 80 lb mix.
How thick should a concrete slab be?
Common practice is 4 in for patios, walkways and shed pads, and 5 to 6 in where vehicles or heavier loads sit, such as driveways and garage floors. Soil, frost depth and local code can require more, so check before you pour.
How much extra concrete should I order?
Plan for 5 to 10% extra. Soft or uneven subgrade, form flex and spillage all use concrete, and running short mid-pour leaves a cold joint. This calculator adds 10% by default; set it to 0 to see the exact volume.
How many 80 lb bags are in a yard of concrete?
45. One cubic yard is 27 ft³ and an 80 lb bag yields about 0.60 ft³ (27 ÷ 0.60 = 45). With 60 lb bags it is 60, with 40 lb bags 90.
How much does a 10x10 concrete slab weigh?
At 4 in thick it holds 33.33 ft³ of concrete. Normal-weight concrete is 140 to 150 lb per cubic foot, so the slab weighs about 4,667 to 5,000 lb.
How much rebar do I need for a 10x10 slab?
With #4 bars 12 in apart each way and 3 in from the edges, you need 10 bars each way, 190 linear feet in total, or 11 20 ft bars with 10% extra for laps and cuts. At 18 in spacing it drops to 8 bars.
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Sources: bag yields and mixing water from the QUIKRETE® Concrete Mix No. 1101 data sheet (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³; 6–9 pt water per 80 lb bag). Concrete density 140–150 lb/ft³: Iowa DOT Materials IM 340. Rebar weights: ASTM A615 nominal values (#3 0.376, #4 0.668, #5 1.043 lb/ft). Calculations last checked 2026-11.