Concrete calculator
Concrete volume
calculator
Calculate concrete for a rectangular slab or circular pad in cubic yards or cubic metres. Compare ready-mix bag quantities and estimate gravel base, reinforcement and waste.
Enter measurements ↓
Based on your result
What you'll need
Quantities update after every calculation. For larger pours, compare bags against a ready-mix truck delivery.
- Ready-mix concrete
- Bags to reach the ordered volume
- Calculate above
- Gravel sub-base
- 4-inch compacted base under the slab
- —
- Steel rebar
- Reinforcing grid at 24-inch centres
- —
- Formwork & tools
- Edge boards, a float and a screed
- As needed
The maths
How we work out your concrete
We turn your slab into a volume — footprint multiplied by thickness and the number of pads — then pad it for spillage and uneven ground. That ordered volume is shown in yards and metres and divided by the yield of your chosen bag. A gravel sub-base and a rebar grid are sized from the same footprint.
volume = footprint × thickness × quantity
ordered = volume × waste
bags = ⌈ ordered ÷ bag yield ⌉
- 5–15%
- waste (you choose)
- 0.60 ft³
- per 80 lb bag
- 4 in
- gravel sub-base
- 24 in
- rebar grid centres
- 27 ft³
- = 1 cubic yard
- ft ↔ m
- converted automatically
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01
Pick a shape
A rectangular slab uses length × width; a circular pad uses π × radius², so patios and footings both work.
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02
Set thickness and count
Depth is entered in inches (or centimetres) and multiplied by how many identical pads you are pouring.
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03
Add a waste margin
Choose 5–15% — uneven subgrade, spillage and over-dig all eat into a pour, so it is padded before ordering.
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04
Split into bags or yards
The ordered volume is shown in cubic yards and metres, and divided by your bag yield for a bag count.
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05
Base and reinforcement
A 4-inch gravel sub-base and a 24-inch rebar grid are added to the list whenever you tick them.
Good to know
Concrete questions, answered
Bags suit small jobs — a few post footings, a shed base or a small pad. Once you are past roughly a cubic yard (about 45 to 60 bags) it is usually cheaper, faster and stronger to order a ready-mix truck. The calculator gives you both the volume and the bag count so you can compare.
Five percent covers a tidy pour onto solid formwork. Go to 10–15% for uneven or soft ground, hand-dug footings, or where the sub-base is rough — the extra means you are not left short with concrete already going off.
Four inches (100 mm) is typical for a patio, shed base or footpath. Driveways, garages and anything taking vehicles usually want five to six inches over a good sub-base, often with reinforcement. Enter the depth your project calls for.
For most ground-bearing slabs, yes. A compacted 4-inch layer of gravel spreads the load, improves drainage and reduces frost movement, which helps the slab resist cracking. The calculator adds it when the box is ticked.
Light-duty slabs often use steel mesh, while thicker or load-bearing slabs use a rebar grid. Either controls cracking rather than stopping it entirely. This tool estimates a rebar grid at 24-inch centres; scale it to your engineer’s spec for structural work.
An 80 lb bag yields about 0.6 cubic feet, and a cubic yard is 27 cubic feet — so roughly 45 bags per yard. 60 lb bags need about 60, and 40 lb bags about 90, which is why big pours favour ready-mix.
Yes. Switch to metres and thickness is entered in centimetres, with the result shown in cubic metres alongside the cubic-yard equivalent. Every input converts automatically.