Concrete Volume Calculator
Turn slab, footing or post sizes into concrete and bags
Concrete volume and bag count FAQ
Why is there a waste allowance on concrete?
Concrete pours rarely match the sketch exactly. Forms are not perfect, bases aren’t laser flat and you always lose a little to overfilling, spillage and cleaning the mixer or chute. This calculator automatically adds roughly 10% extra volume so the truck or pallet is more likely to cover the actual pour than leave you short on the last corner.
What thickness should I use for a slab?
Common residential slabs (paths, patios) are often poured around 4 in (100 mm), while heavier loads (driveways, shed pads) may need 5–6 in (125–150 mm) or more, depending on soil and reinforcement. Local codes, soil conditions and engineer drawings always override generic rules, so treat the thickness box here as a math helper, not a design tool.
How accurate are the bag counts?
Bag yields are based on typical coverage for common premix sizes (40/60/80 lb and 20/25/30 kg). Different products and mix designs can vary slightly, so use the counts as an estimate and cross-check against the coverage chart on the bag or your supplier’s website before ordering.
When should I switch from bags to a truck?
Mixing a few bags on site is fine for small slabs or a handful of posts, but becomes slow and hard work once you get into multiple yards of concrete. As volume climbs, ready-mix from a truck often becomes cheaper per yard and easier to place in one continuous pour. Use the yardage figure here as a talking point with your concrete supplier.
Does reinforcement change the concrete volume?
Rebar and mesh take up a little space but in most small jobs they do not change the concrete volume enough to affect the order. What they do change is strength and crack control, which is why you still need to follow local reinforcement guidance even though this calculator focuses on volume only.
How do I use this for multiple sections or shapes?
For complex pours, split the job into simple pieces: one calc for each slab, footing run or set of posts. Add the yardage and bag counts together at the end. This mirrors how many contractors estimate concrete for jobs with steps, pads and thickened edges.
Can I use this for walls, stairs or beams?
The formulas here are set up for flat slabs, rectangular footings and round post holes. You can still use them as a sanity check for walls or beams by treating them as length × width × thickness, but for structural elements always rely on engineering drawings and your supplier’s calculations.
Do I need to adjust for very wet or very stiff mixes?
Mix slump affects workability more than volume. A wetter mix may behave differently on site, but the volume of concrete from a given bag or truck is broadly the same. What does change is how well you can place and finish it, so follow mix and water limits from your ready-mix plant rather than diluting concrete on site.
How to use this concrete volume calculator
This calculator turns the dimensions of a slab, footing or group of post holes into concrete volume, a built-in waste allowance and typical bag counts so ordering feels less like a guess and more like a plan.
1. Pick pour type and units
Start by choosing the pour type: slab/pad, footing/strip or post holes. Then choose whether you’re working in US units (feet, inches, cubic yards) or metric (metres, centimetres, cubic metres). The labels and hints update automatically when you switch.
2. Enter the three key dimensions
Depending on the pour type you chose, the three inputs change meaning slightly:
- Slab — length, width and slab thickness.
- Footing — total run length, footing width and depth.
- Post holes — hole diameter, hole depth and number of posts.
Round up slightly if the ground is uneven or forms are not perfectly set; the built-in waste allowance will help cover small variations.
3. Read volume, waste and bag counts
Tap Estimate concrete to see:
- Base concrete volume in cubic yards and cubic feet (or m³ with a yard conversion in metric mode).
- Volume with a ~10% waste allowance applied.
- Approximate bag counts for common sizes (40/60/80 lb and 20/25/30 kg).
The bag numbers are rounded up so you have whole bags and a small buffer instead of running out at the end of the pour.
4. Copy and share the plan
Use Copy summary to paste the result into your notes, a site diary or an email to your supplier. For trucks, the key figure is the cubic yards with waste; for small jobs, the bag counts help you build a pallet list.
Think of this tool as your quick back-of-site helper. Final quantities should always respect structural drawings, local codes and your concrete supplier’s advice, but this gives you a fast, transparent starting point.
How the concrete volume calculator math works
Under the hood, the calculator uses standard volume formulas, unit conversions and typical bag yields similar to those used in concrete yardage charts and big-box calculators.
1. Volume from simple shapes
Everything is converted into metres for internal math, then back out to yards or feet:
- Slab / footing:
Volume = length × width × thickness - Post holes:
Volume = number × π × (diameter ÷ 2)² × depth
In US mode, feet and inches are converted to metres; in metric mode, metres and centimetres are used directly. The final numbers come out in cubic yards and cubic feet (or m³ with a yard equivalent).
2. Waste and safety margin
Once base volume is calculated, the tool multiplies it by about 1.10 to add a simple waste and safety factor. This lines up with the common “add 10%” rule you see on many concrete calculators and bag charts.
3. Bag yields and counts
To estimate bag counts, the calculator converts volume to cubic feet and divides by typical yields per bag:
- 40 lb ≈ 0.30 ft³
- 60 lb ≈ 0.45 ft³
- 80 lb ≈ 0.60 ft³
- 20 kg ≈ 0.29 ft³
- 25 kg ≈ 0.37 ft³
- 30 kg ≈ 0.44 ft³
The resulting counts are rounded up to whole bags so you never under-order based on fractions.
4. Transparent, checkable formulas
All formulas are kept simple enough that you can spot-check with a handheld calculator: multiply your dimensions, convert to yards by dividing cubic feet by 27, then compare with the numbers on screen. If you prefer a bigger or smaller waste factor, you can scale the yardage up or down accordingly before placing your order.
References and further reading on concrete volume and bag estimates
These resources follow the same measurement and conversion logic as this calculator:
- QUIKRETE — Concrete Calculator — slab and footing volume with 40/50/60/80 lb bag counts.
- ConcreteNetwork.com — Concrete Slab and Footing Calculators — length × width × thickness formulas and yardage estimates.
- Lowe’s — Concrete Buying Guide — explanation of converting slab dimensions to cubic yards before ordering.
Use them alongside this tool, your drawings and your supplier’s advice to fine-tune final volumes and bag or truck counts.