Concrete Calculator
This concrete calculator works out how much concrete you need for a slab, footing, or pad, plus the cement, sand, and gravel. Enter the dimensions and the concrete calculator returns the volume in cubic meters and cubic yards, with the bags of cement to order. Use it as a concrete slab calculator, a concrete estimator, or an online concrete calculator for any pour.
Results
The Concrete Calculator is part of our Concrete collection. Browse more concrete to size, convert, and check your work alongside this tool.
What the concrete calculator does
The concrete calculator tells you how much concrete to order and what it takes to mix it. Order too little and you risk a cold joint; too much and you pay for waste. As a concrete slab calculator and concrete estimator, this tool computes the volume from your dimensions and breaks it into cement, sand, and gravel, so you can plan any slab, pad, or footing with confidence.
Concrete calculation formula
The concrete calculator multiplies the three dimensions, adds waste, then splits the dry materials:
It multiplies the wet volume by a dry factor of 1.54, then splits it 1:2:4 between cement, sand, and gravel. Each 50 kg cement bag is about 0.0347 m³, which is how the concrete calculator bags figure is worked out.
Concrete calculator example
A concrete slab 5 × 4 meters at 0.15 m thick with 5 percent waste, run through the concrete slab calculator:
From that concrete estimation you get roughly 20 bags of cement, 1.4 m³ of sand, and 2.8 m³ of gravel.
How to use the concrete calculator
Enter the length, width, and depth in meters, plus a waste allowance of 5 to 10 percent. The concrete calculator returns the volume including waste, then the cement bags, sand, and gravel. For a 150 mm slab, enter 0.15 as the depth. As an online concrete calculator it works for slabs, pads, floors, and footings alike.
Reading the concrete calculator result
The volume is what you order from a ready-mix supplier, rounded up to their increment. The bags figure from the concrete calculator bags estimate suits an on-site mix. For budgeting, pair the volume with a local price to use it as a cost concrete slab calculator. These are estimates on the 1:2:4 ratio; a structural mix design may differ, so follow the specification for structural work.
When to use the concrete calculator
- Ordering ready-mix for a slab with the concrete slab calculator
- Estimating materials for a hand-mixed pour with the concrete estimator
- Budgeting a pour with a cost concrete slab calculator
- Sizing a shed or floor base with the concrete pad calculator
Common concrete calculation mistakes
- Forgetting waste. Order the bare volume and you come up short; the concrete calculator adds a margin.
- Mixing units. Keep all dimensions in meters; 150 mm is 0.15.
- Skipping the dry factor. On-site mixes need the 1.54 factor the concrete estimator applies.
Tips for accurate concrete results
- Measure the real excavation, which is often larger than the plan.
- Round the concrete calculator volume up to the supplier increment.
- Keep bagged concrete on hand for a small top-up.
Why the concrete calculator adds waste
The waste allowance in the concrete calculator is cheap insurance. Real pours lose concrete to spillage, uneven ground, and flexing formwork, so ordering the bare calculated volume leaves you short. Running short mid-pour is far costlier than a little waste, because a delay while more concrete arrives creates a cold joint, a plane of weakness where fresh meets set concrete. The concrete slab calculator builds in 5 to 10 percent so the pour goes down as one sound mass.
From volume to a full concrete estimate
Two ideas make the concrete estimator reliable. The dry factor of 1.54 accounts for how loose cement, sand, and gravel consolidate when water fills the gaps between particles, so a cubic meter of concrete needs more than a cubic meter of dry material. The 1:2:4 mix ratio then splits that dry volume into the concrete calculator bags figure plus sand and gravel. Change the ratio for a richer structural mix and the quantities shift, which is why matching the ratio to the job matters as much as the volume itself.
Concrete slab quantity chart
Approximate concrete volume and cement bags for common slab sizes at 100 mm thick:
| Slab size | Volume | Cement bags |
|---|---|---|
| 3 × 3 m | 0.9 m³ | ~6 |
| 5 × 4 m | 2.0 m³ | ~13 |
| 6 × 6 m | 3.6 m³ | ~23 |
Concrete Calculator FAQ
How much concrete do I need for a slab?
Multiply length by width by thickness, then add 5 to 10 percent waste. The concrete slab calculator does this: a 5 × 4 m slab at 0.15 m needs about 3.15 m³.
How many bags of concrete do I need?
For a 1:2:4 mix, roughly 6 to 7 bags of 50 kg cement per cubic meter. The concrete calculator bags figure gives the total for your volume automatically.
What does the concrete estimator include?
It gives the concrete volume plus the cement, sand, and gravel to mix it. Pair the volume with a local price and it also works as a cost concrete slab calculator.
Why multiply by 1.54?
Dry materials occupy more space than finished concrete because they compact when mixed. The concrete calculator uses the 1.54 dry factor so you buy enough material.