About the Concrete / Construction Calculator
A concrete calculator works out the volume needed for a slab, footing, or column, in cubic yards or cubic metres, with a waste allowance added. Ordering concrete accurately matters because it is delivered in fixed truck loads and cannot be topped up once a pour has started.
The formula
Volume = length · width · thickness, converted to cubic yardsOne cubic yard is 27 cubic feet. Thickness entered in inches is divided by 12 to convert to feet before multiplying. A waste factor of 5 to 10 percent covers spillage and uneven subgrade.
How to use this calculator
- 1Enter your Length (feet). The field starts at
20, which you can overwrite. - 2Enter your Width (feet). The field starts at
10, which you can overwrite. - 3Enter your Thickness (inches). The field starts at
4, which you can overwrite. - 4Enter your Waste Factor (%). The field starts at
10, which you can overwrite. - 5Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Length (feet) | 20 |
| Width (feet) | 10 |
| Thickness (inches) | 4 |
| Waste Factor (%) | 10 |
Result
Volume: 2.47 cu yd
With 10% Waste: 2.72 cu yd
60-lb bags needed: 163
80-lb bags needed: 123
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
The waste allowance is not optional. Subgrade is never perfectly level, forms flex under the weight of wet concrete, and some material is lost in handling. Ordering the exact calculated volume reliably results in running short, and a partial second delivery costs far more per yard than the original order.
Running short mid-pour is the outcome to avoid at almost any cost. A cold joint forms where fresh concrete meets concrete that has begun to set, creating a structural weakness and a visible line. Most contractors order 10 percent over for this reason alone — surplus concrete is cheap compared with a compromised slab.
Things worth knowing
- Order 5 to 10 percent extra. Running short mid-pour creates a cold joint that permanently weakens the slab.
- Ready-mix is typically sold in full or half cubic yards, so round up to the next increment.
- A 4-inch slab suits patios and walkways; driveways generally need 5 to 6 inches, more for heavy vehicles.
- Standard mixes reach around 3,000 psi. Higher strengths are available and cost more.
- Concrete reaches most of its strength in 28 days but can usually take foot traffic after 24 to 48 hours.
Frequently asked questions
How much concrete do I need for a slab?+
Multiply length by width by thickness in consistent units, then convert to cubic yards by dividing cubic feet by 27. A 20 by 10 foot slab at 4 inches needs about 2.5 cubic yards before waste.
How much extra should I order?+
5 to 10 percent above the calculated volume. Uneven subgrade, form flex, and spillage all consume more than the geometry suggests, and running short mid-pour is far more costly than surplus.
How thick should a concrete slab be?+
Four inches for patios, walkways, and shed bases; five to six for driveways carrying cars; more with reinforcement for heavy vehicles. Local building codes may specify minimums.
How long does concrete take to cure?+
It reaches roughly 70 percent of design strength in seven days and most of it by 28 days. Foot traffic is usually fine after 24 to 48 hours and vehicles after a week.