How many bags of concrete do I need?
It comes down to one thing: the volume of your slab. Measure the length and width, choose a thickness, multiply them together, then convert that volume into cubic yards or into a number of bags. A cubic yard is 27 cubic feet, so a bag's yield tells you how many bags fill a yard.
To get bags, divide the volume in cubic feet by the bag's yield — about 0.60 ft³ for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb, 0.30 for 40 lb. Pick a bag size in the calculator, tap Custom for any other size, or switch to Metric for kg bags — it does the rest the instant you type.
Rectangle or circle
For a rectangular slab — a patio, driveway, or walkway — the area is simply length × width. For a round pad or a tube footing, it's π × radius². Pick the shape at the top of the calculator and the right measurement boxes appear.
Worked example
A 4 ft × 8 ft slab poured 4 inches thick:
32 sq ft × 0.33 ft = 10.7 cu ft = 0.4 cubic yards — about 18 × 80 lb bags (or 24 × 60 lb, 36 × 40 lb). Add ~10% for waste and round up.
How thick should a concrete slab be?
- 4 inches — patios, walkways, and shed or garden slabs
- 5–6 inches — driveways and garage floors
- 6–8 inches — heavy loads, footings, and RV or truck parking
Tap a thickness chip in the calculator to drop in one of these common depths without typing.
Bags per cubic yard
One cubic yard is 27 cubic feet. Divide 27 by a bag's yield to get how many bags make a yard. These are the standard yields the calculator uses (Quikrete / Sakrete figures):
| Bag size | Yield (ft³) | Bags / yd³ |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
Yields are for standard mixes; fast-setting, high-strength, and fibre-reinforced blends can run 10–15% lower, so check your bag's printed yield. Buying in kg? Switch to Metric for 20, 25, 30, and 40 kg bags. Mixing 45+ bags by hand is a big job — for anything near a full yard, ready-mix delivery is usually cheaper; switch Sold by to Cubic yard to plan that.
Common questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. An 80 lb bag of concrete mix yields about 0.60 cubic feet, so you need 45 bags to make a yard. A 60 lb bag yields about 0.45 cubic feet, which is 60 bags per yard, and a 40 lb bag yields about 0.30 cubic feet, which is 90 bags per yard. For anything approaching a full yard, ready-mix delivery is usually cheaper and faster than mixing bags by hand.
How many bags of concrete do I need for a 4×8 slab?
For a 4 ft by 8 ft slab poured 4 inches thick you need about 0.4 cubic yards of concrete, which is roughly 18 bags of 80 lb mix, 24 bags of 60 lb, or 36 bags of 40 lb. Add about 10 percent extra for waste and uneven ground. Enter your own dimensions above to get exact bag counts.
How much does one cubic yard of concrete cover at 4 inches?
One cubic yard placed 4 inches thick covers about 81 square feet, roughly a 9 ft by 9 ft slab. At 6 inches thick a yard covers about 54 square feet. Divide your slab's square footage by these figures for a quick yardage estimate, or use the calculator for an exact number.
How do I calculate concrete for a slab?
Multiply length by width by thickness to get the volume, keeping thickness in feet, so 4 inches is 0.33 ft. That gives cubic feet; divide by 27 to get cubic yards. To get bags, divide the cubic feet by the bag's yield: 0.60 for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb. The calculator above does all of this instantly as you type.
Should I order extra concrete for waste?
Yes. Spillage, uneven subgrade, and over-excavation mean a little extra is wise, so ordering about 10% more is a common rule of thumb. Concrete also sets fast and can't be topped up mid-pour, so it's better to have slightly too much. Tap “+ Add waste calc” in the project total to add a percentage buffer.
Does this concrete calculator work in metric?
Yes. Switch to "Metric" to enter your slab in metres and thickness in centimetres. The result is shown in cubic metres, with bag counts and cost, so you can plan in whichever units you prefer.