Concrete Calculator
Cubic yards, cubic metres, bags and weight for any pour
Decimals are fine, so a 30-inch path is 2.5 ft. Thickness, stair rise and run and column diameters are entered in inches because that is how they are specified and sold.
Full breakdown
| Item | Amount | Detail |
|---|
Bags needed by size
| Bag | Bags needed | Yield & total weight |
|---|
How to Use
- 1Pick what you are pouring and only the fields that shape needs appear.
- 2Enter the measurements. Large dimensions in feet, thickness in inches.
- 3Leave the waste allowance at 10% unless the ground is unusually flat or rough.
- 4Read the cubic yards to order, and open the bag table if it is a small pour.
The Formulas
- Slab · footing · walllength × width × thickness
- Round columnπ × radius² × height
- Round slab or ring(π ÷ 4) × (outer² − inner²) × thickness
- Stairswidth × run × rise × n(n+1) ÷ 2
- Cubic yardscubic feet ÷ 27
- Cubic metrescubic feet ÷ 35.3147
Quick Examples
Tap an example to load it above.
Everything about the Concrete Calculator
How much to order, in the units your supplier actually uses.
This concrete calculator works out how much concrete a pour needs and tells you the answer in cubic yards and cubic metres at the same time, because that is how ready-mix is ordered on either side of the Atlantic. It also adds a waste allowance, counts the bags for a small job, and estimates the weight.
Five shapes cover almost every domestic pour: a flat slab, a footing or wall, a round column or post hole, a round slab or ring, and a flight of stairs. Choose one and only the measurements that shape needs appear.
The mistake that ruins pours: mixing units. A 4-inch slab is 0.333 ft thick, not 4 ft: get that wrong and the order is twelve times too big. This calculator fixes the unit of every field to the way that dimension is normally specified, so there is nothing to convert by hand.
Cubic yards, cubic metres and cubic feet
All three describe the same volume. One cubic yard is exactly 27 cubic feet, and one cubic metre is about 35.31 cubic feet, so a cubic metre is roughly 1.31 cubic yards. US suppliers quote and deliver by the cubic yard; almost everywhere else uses the cubic metre. Both are always shown so you can order from either.
| Pour | Typical thickness | Why |
|---|---|---|
| Footpath or patio | 4 in / 100 mm | Foot traffic and furniture only |
| Domestic driveway | 4 to 6 in / 100 to 150 mm | Cars; 6 in if a van or truck parks on it |
| Garage or workshop floor | 5 to 6 in / 125 to 150 mm | Point loads from jacks and racking |
| Shed or garden-room base | 4 in / 100 mm | Light, evenly spread load |
| Strip footing | Set by the engineer | Depends on soil and the load above |
Those are common practice, not a structural specification. Thickness, reinforcement, sub-base and mix strength are engineering decisions, and this calculator only measures the space you are filling.
Why the waste allowance is not optional
- 1The ground is never flat
A sub-base that dips half an inch under a 20 × 10 ft slab swallows an extra 8 cubic feet without anyone noticing. Excavations are always slightly over-dug, never under.
- 2Formwork moves
Timber shuttering bows outward under the weight of wet concrete, so the finished pour is fractionally wider than the drawing everywhere.
- 3Spillage and the last barrow
Concrete is lost to the chute, the barrow and the mixer drum. Some of every load never reaches the form.
- 4Running short is far worse than over-ordering
A second load arriving an hour later creates a cold joint, a permanent weak seam through the slab. 10% spare costs a little money; a cold joint costs the slab.
Bags or ready-mix?
Bagged concrete makes sense for post holes and small repairs. The maths turns against it quickly: an 80 lb bag yields only 0.6 cubic feet, so a single cubic yard takes 45 bags, about 3,600 lb to carry and mix. Once the calculator shows a cubic yard or more it says so, because at that point ready-mix delivery is cheaper, faster and produces one continuous pour instead of forty separate batches.
Weight, and why it matters before delivery
Concrete weighs roughly 150 lb per cubic foot, or 4,050 lb per cubic yard, or about 2,400 kg per cubic metre. A modest 2-cubic-yard pour is over four tons. That figure decides whether a trailer can legally carry it, whether a balcony or suspended floor can take it, and how much barrow work the day involves.
Odd shapes
Break an irregular pour into rectangles and circles, calculate each one, and add them up. An L-shaped patio is just two slabs. For a shape this page does not list, the volume calculator handles cubes, spheres, cones and pyramids, and the cylinder volume calculator covers tubes in any unit. Convert the result to cubic feet and divide by 27 for cubic yards.
Frequently Asked Questions
How do I work out how much concrete I need?
Multiply length by width by thickness with every dimension in the same unit, then divide the cubic feet by 27 for cubic yards. A 20 × 10 ft slab at 4 inches is 20 × 10 × 0.333 = 66.7 cubic feet, which is 2.47 cubic yards before any waste allowance. This calculator does the unit conversion for you, so you can enter the thickness in inches and the rest in feet.
How many cubic feet are in a cubic yard of concrete?
Exactly 27, because a yard is 3 feet and 3 × 3 × 3 = 27. A cubic metre is about 35.31 cubic feet, which makes it roughly 1.31 cubic yards. The calculator shows cubic yards, cubic metres and cubic feet together so you can order from any supplier without converting anything.
How many 80 lb bags of concrete make a cubic yard?
About 45. An 80 lb bag yields roughly 0.60 cubic feet, and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45 bags weighing 3,600 lb in total. A 60 lb bag yields about 0.45 cubic feet, so the same yard takes 60 bags. Once you need a full yard, ready-mix delivery is almost always cheaper and far less work.
How much extra concrete should I order for waste?
Ten percent is the usual allowance and the default here. Use 5% only when the sub-base is flat and the formwork is rigid, and 15% on rough or sloping ground where the excavation is likely to be over-dug. Running short mid-pour creates a cold joint, which is a permanent weak seam, so the spare concrete is cheap insurance.
How thick should a concrete slab be?
Four inches, or 100 mm, is standard for a patio, footpath or shed base. A domestic driveway is usually 4 to 6 inches depending on what parks on it, and a garage or workshop floor is typically 5 to 6 inches because of point loads from jacks and racking. These are common practice rather than a structural specification, and anything load-bearing should be sized by an engineer.
How much does concrete weigh?
Normal-weight concrete is about 150 lb per cubic foot, which works out at roughly 4,050 lb per cubic yard or 2,400 kg per cubic metre. The weight tile shows the total for your pour in both pounds and kilograms, which matters when you are checking a trailer limit or the loading on a suspended floor.
How do I calculate concrete for stairs or a post hole?
Choose the matching shape and the right fields appear. For stairs, enter the number of steps, the rise and run in inches and the width in feet. A poured flight is a triangular stack, so the volume is width × run × rise × n(n+1) ÷ 2. For post holes, choose the round column shape, enter the diameter in inches and the depth in feet, and set "How many identical pours" to the number of holes.
Can I use the calculator for an irregular shape?
Yes. Split it into simple pieces, calculate each one and add the results. An L-shaped patio is two rectangles, and a slab with a circular cut-out is a rectangle minus a round slab. Keep the same waste allowance on the total rather than on each piece, and confirm the final figure with your supplier before ordering.