Concrete Calculator
Enter the slab’s length and width, its thickness, a waste allowance and the yield printed on one bag of premix. The calculator gives the volume of concrete in cubic yards or cubic metres and the number of whole bags to buy.
Answer
Concrete to order
1.36cu yd
62 bags of premix at 0.6 cu ft each, or 1.36 cu yd of ready-mix.
- cu ft slab volume
- 33.33
- cu ft with 10% waste
- 36.67
- bags of 0.6 cu ft
- 62
- cu ft left over
- 0.533
Check the work
Every line comes from the same calculation as the answer.
volume = length × width × thickness × (1 + waste)
| thickness | 4 ÷ 12 | 0.3333 ft |
|---|---|---|
| volume | 10 × 10 × 0.3333 | 33.3333 cu ft |
| with waste | 33.3333 × (1 + 0.1) | 36.6667 cu ft |
| to order | 36.6667 ÷ 27 | 1.358 cu yd |
| bags | ⌈ 36.6667 ÷ 0.6 ⌉ | 62 |
How it works
A slab is a box of concrete, so its volume is length times width times thickness. The catch is the units: slabs are measured in feet but poured to a thickness in inches, and ready-mix is sold by the cubic yard. In metric, the thickness is usually in centimetres and the volume in cubic metres.
The calculator converts the thickness into the same unit as the length and width before multiplying, then shows the volume in the unit concrete is ordered in. Every conversion appears in the working so you can check it against a supplier’s quote.
For small jobs mixed by hand, bags of premix state how much finished concrete each one makes. Dividing the volume by that yield, and rounding up, gives the number of bags. A waste allowance covers spillage, uneven ground and the concrete left in the mixer.
Formula
volume = length × width × thickness × (1 + waste); bags = ⌈volume ÷ bag yield⌉
The three dimensions are multiplied in one unit to give the slab volume, the waste allowance is added, and the volume is divided by the yield of one bag and rounded up.
Worked examples
- Try these numbers
Shed base, 10 by 10 ft, 4 inches thick
- 4 inches is 4 ÷ 12 = 0.3333 ft, so the slab is 10 × 10 × 0.3333 = 33.33 cu ft.
- Adding 10%: 33.33 × 1.1 = 36.67 cu ft, which is 36.67 ÷ 27 = 1.36 cu yd.
- Bags yielding 0.6 cu ft: 36.67 ÷ 0.6 = 61.1, rounded up to 62 bags.
Order 1.36 cu yd of ready-mix, or buy 62 bags of 0.6 cu ft yield.
- Try these numbers
Garden pad, 3 by 3 m, 10 cm thick
- 10 cm is 0.1 m: 3 × 3 × 0.1 = 0.9 m³.
- With 5% waste: 0.9 × 1.05 = 0.945 m³.
- Bags yielding 12 litres (0.012 m³): 0.945 ÷ 0.012 = 78.75, so 79 bags.
You need 0.945 m³ — 79 bags of 12-litre yield.
- Try these numbers
Step pad that uses whole bags exactly
- A 2 × 1 m pad, 50 cm deep, is 1 m³.
- With no allowance and 500-litre bags, 1 ÷ 0.5 = 2 bags exactly.
2 bags with nothing spare — rarely wise on site, where an allowance is safer.
Inputs and results
- Slab length
- One side of the slab, in feet or metres.
- Slab width
- The other side, in the same unit.
- Thickness
- Depth of the slab, in inches or centimetres.
- Waste allowance
- Extra concrete for spillage and uneven ground, as a percentage you choose.
- Yield per bag
- Volume one bag of premix makes, in cubic feet or litres, from the bag.
- Slab volume
- Length times width times thickness.
- Volume to order
- Slab volume with the waste allowance added, in cubic yards or cubic metres.
- Bags
- Volume to order divided by bag yield, rounded up.
- Left over
- Concrete the last bag makes beyond the volume needed.
When to use it
- Pricing a shed base or patio slab before choosing between bags and a ready-mix delivery.
- Checking a ready-mix order: the volume in cubic yards or cubic metres is what a supplier asks for.
- Seeing how a thicker slab changes the number of bags for the same footprint.
Common mistakes
- Entering the thickness in feet or metres after the field asks for inches or centimetres; 4 inches is 4, not 0.33.
- Using the bag weight instead of its yield. An 80 lb bag is a weight; the yield printed on it is the volume it makes.
- Ordering exactly the calculated volume. Ground is rarely perfectly level, so an allowance avoids running short mid-pour.
What it does not do
- It covers rectangular slabs of even thickness, not footings, columns, steps or circular pads.
- It does not give structural advice on thickness, reinforcement or concrete strength; follow your plans or a qualified engineer.
- It does not know the yield of a particular bag; use the figure printed on the bag you are buying.
Method and assumptions
The thickness is converted to the length unit (12 inches to the foot, 100 centimetres to the metre) and multiplied by length and width. Imperial volumes are shown in cubic feet and converted to cubic yards by dividing by 27.
Bag yields are converted into the same cubic unit (1,000 litres to the cubic metre) before dividing, and the bag count is rounded up to whole bags.
Assumptions
- The slab is rectangular with an even thickness.
- Bag yield is the finished volume printed on the bag.
- Bags are bought whole.
Edge cases
- A thickness of zero has no volume, so no answer is shown.
- A waste allowance of 0% gives the bare slab volume.
- Very large slabs can need hundreds of bags; at that size a ready-mix order is usually more practical.
Sources and corrections
This calculator uses an elementary formula, so it needs no outside data source. The method, the rounding rules and how a reported error is corrected are set out in the site standards.
Result looks wrong?
Send the inputs you used and the answer you expected. Every report is checked against the formula and its tests, and confirmed errors are corrected in public.