How far will my concrete go?
Enter how many bags or yards you have and how thick you want the slab.
How the concrete calculator works
The calculator finds the area of each shape, multiplies by how many you have, subtracts cut-outs, then multiplies by the slab thickness. It adds an allowance for waste, converts to cubic yards, and divides by the yield of the bag you pick to give a bag count. Weight and truck loads come from the same volume. Use a circle for round pads and columns, and a 1 ft wide rectangle for a footing run.
With extra = volume × (1 + extra %)
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ yield per bag, rounded up
Area formulas used for each shape
| Shape | Formula |
|---|---|
| Rectangle | length × width |
| Circle | π × (diameter ÷ 2)² |
| Triangle | ½ × base × height |
| Known area | entered directly in sq ft (or sq m if you chose meters) |
Worked examples
Example 1: a 10 × 10 ft patio slab, 4 inches thick
10 × 10 = 100 sq ft. At 4 in (⅓ ft) that is 33.3 cu ft, or 36.7 cu ft with 10% extra. That is 1.36 cubic yards. In 80 lb bags (0.6 cu ft) it is 62 bags, or 82 bags of 60 lb. At this size ready-mix is worth pricing.
Example 2: a 12 × 20 ft driveway pad, 6 inches thick
12 × 20 = 240 sq ft. At 6 in (½ ft) that is 120 cu ft, or 132 cu ft with 10% extra. That is 4.9 cubic yards, so order 5 yards from a ready-mix truck. In bags it would be 220 bags of 80 lb, which is not practical.
Example 3: a 40 ft footing, 12 inches wide and 8 inches deep
Enter a 40 × 1 ft rectangle at 8 in deep. That is 26.7 cu ft, or 29.3 cu ft with 10% extra. That is 1.09 cubic yards, or 49 bags of 80 lb. Check local code for footing sizes.
Example 4: a round pad 4 ft across, 4 inches thick
Area = π × 2² = 12.6 sq ft. At 4 in that is 4.2 cu ft, or 4.6 cu ft with 10% extra. That is 0.17 cubic yards, or 8 bags of 80 lb.
How much do bags of concrete yield?
| Bag size | Typical yield | Bags in 1 cubic yard | Area covered at 4 in |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 0.9 sq ft |
| 50 lb | 0.375 cu ft | 72 | 1.1 sq ft |
| 60 lb | 0.45 cu ft | 60 | 1.35 sq ft |
| 80 lb | 0.60 cu ft | 45 | 1.8 sq ft |
Yields differ a little by brand, so use the number printed on your bag and enter it by picking the closest size.
Typical slab thickness by use
| Use | Common thickness | Notes |
|---|---|---|
| Walkway, patio, shed pad | 4 in | On a compacted gravel base |
| Garage floor, light vehicles | 4 to 5 in | Often with mesh or rebar |
| Driveway | 5 to 6 in | Thicker for heavy trucks |
| Steps, heavy equipment | 6 in and up | Ask a contractor or check code |
These are common rules of thumb. Local building codes, soil and loads decide the real figure.
Mistakes to avoid
- Ordering exactly what the maths says. Forms and ground are never perfect. Order 5 to 10% extra, because a short pour leaves a cold joint.
- Mixing up thickness units. 4 inches is ⅓ foot, not 4. Pick the thickness unit in the tool and check the result looks sensible.
- Counting only the slab. Footings, thickened edges and steps need their own concrete. Add each as another area.
- Planning to mix too many bags by hand. Past about a cubic yard (45 bags of 80 lb), compare ready-mix delivery, even with a short-load fee.
- Skipping the base and the reinforcement. A compacted gravel base and the right mesh or rebar matter as much as the volume. Check your local code.
How far does a bag or a yard of concrete go?
The area a pour covers depends on its thickness. This table uses 80 lb bags, which commonly yield about 0.6 cubic feet each. Check the yield printed on your bag, and use the calculator above for your exact slab.
| You have | Covers at 4 in deep | Covers at 5 in deep | Covers at 6 in deep |
|---|---|---|---|
| 10 bags of 80 lb (0.6 cu ft each) | 18 sq ft | 14 sq ft | 12 sq ft |
| 45 bags of 80 lb (about 1 cubic yard) | 81 sq ft | 65 sq ft | 54 sq ft |
| 2 cubic yards | 162 sq ft | 130 sq ft | 108 sq ft |
| 5 cubic yards | 405 sq ft | 324 sq ft | 270 sq ft |
| 10 cubic yards | 810 sq ft | 648 sq ft | 540 sq ft |
No extra allowance is included here. Add 5 to 10% for waste and uneven ground.
Frequently asked questions
How do you calculate cubic yards of concrete?
Multiply length × width × thickness, all in feet, to get cubic feet, then divide by 27. A 10 × 10 ft slab that is 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. Add 5 to 10% on top for spillage and uneven ground, which takes it to about 1.36 cubic yards.
How many 80 lb bags of concrete are in a cubic yard?
Most 80 lb bags yield about 0.6 cubic feet, so a cubic yard (27 cubic feet) takes about 45 bags. A 60 lb bag yields about 0.45 cubic feet, so it takes about 60, and a 40 lb bag yields about 0.3, so it takes about 90. Check the yield printed on your bag, because brands differ.
How much extra concrete should I order?
Many contractors order 5 to 10% extra, because forms are rarely perfectly level, the ground is never perfectly flat, and some concrete is lost in the truck and tools. Running short is much worse than having a little left over. The calculator adds 10%, and you can change it.
How thick should a concrete slab be?
A common rule of thumb is 4 inches for patios, walkways and shed pads, 5 to 6 inches for driveways and spots that carry vehicles, and thicker for heavy loads. Soil, climate, reinforcement and local building codes all matter, so check them before you pour.
Should I use bagged concrete or ready-mix?
Bags suit small jobs such as a post footing, a step or a small pad. Up to about a cubic yard means around 45 bags of 80 lb, which is a lot of mixing. For bigger slabs ready-mix is faster and gives a more even result, but many suppliers charge a short-load fee for small orders. Enter both prices in the calculator to compare.
How much does a cubic yard of concrete weigh?
Concrete is commonly quoted at about 150 pounds per cubic foot, which is around 4,000 pounds, or 2 tons, per cubic yard. The calculator uses 150 and you can change it. This matters for your truck, wheelbarrow and the base underneath.
Do I need rebar or wire mesh?
Many slabs have wire mesh or rebar to hold cracks tight, and some building codes ask for it for driveways, garage floors and footings. This calculator only works out the volume of concrete, not the steel. Check your local code or ask a contractor what your job needs.
How much concrete do I need for a 10x10 slab?
A 10 × 10 ft slab is 100 square feet. At 4 inches thick it needs 33.3 cubic feet, or 1.23 cubic yards. At 5 inches it is 1.54 cubic yards and at 6 inches 1.85. Add 5 to 10% extra. A 4 inch slab with 10% extra is about 62 bags of 80 lb.
How many bags of concrete do I need for a 12x12 slab?
A 12 × 12 ft slab is 144 square feet. At 4 inches thick it needs 48 cubic feet, which is about 80 bags of 80 lb at 0.6 cubic feet each, or about 88 with 10% extra. That is 1.8 cubic yards, so ready-mix delivery is worth pricing.
How much concrete do I need for a 4x8 slab?
A 4 × 8 ft slab is 32 square feet. At 4 inches thick that is 10.7 cubic feet, or 0.4 cubic yards. With 10% extra it is 11.7 cubic feet, which is about 20 bags of 80 lb. Small slabs like this are usually done with bags.
How many bags of concrete do I need for a fence post?
A typical hole 10 inches wide and 24 inches deep holds about 1.1 cubic feet, and a 4 × 4 post takes up around 0.2 of that. That leaves roughly 0.9 cubic feet, which is about 1.5 to 2 bags of 80 lb. Deeper holes for taller fences take more. Check your local code for depth and width.
How much does a 60 lb bag of concrete cover?
A 60 lb bag commonly yields about 0.45 cubic feet. That covers about 1.35 square feet at 4 inches thick, 1.8 square feet at 3 inches and 2.7 square feet at 2 inches. Check the yield printed on your bag.
How much does ready-mix concrete cost per yard?
Prices depend on your region, the strength of the mix, the distance and any short-load or fuel fees, so there is no single figure. Get two or three quotes from local suppliers, then enter the price per cubic yard in the calculator to compare it with bagged concrete.
How long does concrete take to cure?
Concrete is often firm enough to walk on after a day or two and ready for light use in about a week, but it keeps gaining strength for around 28 days. Heat, cold and the mix change this, so follow your supplier’s advice and keep new concrete damp and protected.
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