Concrete Calculator
Concrete is ordered in awkward increments, and running short mid-pour is expensive, so this calculator plans the pour before it starts. Add the sections you are placing — slabs by length, width and thickness, footings by length, width and depth, post holes by diameter, depth and count — and the engine totals their volumes, applies your waste allowance, and reports cubic feet, cubic yards, an approximate weight, and a ready-mix order figure rounded up in the supplier increment. Bag counts for 40, 50, 60 and 80 lb bags appear alongside, each using its own yield, so the bags-versus-ready-mix decision is made on numbers rather than assumption. Multiple sections combine into one pour, which is how real slabs with steps, pads and footings are actually planned. The page is for anyone ordering a slab, a footing or a batch of post holes who wants the volume, the order size and the bag count to agree before the truck arrives.
Calculator input
Concrete sections
Mix slabs, footings and post holes in one estimate.
What this calculator calculates
- Per-section volumes in cubic feet for slabs, footings and holes
- Total pour volume before waste, in cubic feet and cubic yards
- Order volume with waste, plus ready-mix rounded to the supplier increment
- Bag counts at 40, 50, 60 and 80 lb using each bag yield
- Approximate weight of the pour from the concrete density
- Purchase advice comparing bags with ready-mix for your volume
Projects this calculator covers
- Slabs and padsPatios, shed bases and workshop floors entered by length, width and thickness.
- Continuous footingsStrip footings measured by run, width and depth as their own sections in the pour.
- Post holesFence and structural holes by diameter, depth and count, calculated as cylinders.
- Combined poursSlab plus step plus footing added together so one order covers the whole placement.
How the calculation works
Each concrete section is converted to cubic feet, summed, and then increased by your waste allowance. The engine reports bag counts for the configured bag sizes.
Formula: Slab volume = length × width × thickness; footing volume = length × width × depth; post-hole volume = cylinder volume × count.
Worked example
20 × 30 ft slab at 4 in
The engine can also combine multiple parts in one calculation.
Reading the results
- Calculated volume versus ready-mix order. The volume is what the geometry holds. The ready-mix figure adds waste and rounds up in the supplier increment, so the number you give the dispatch office is the one to order.
- Bags have their own yields. A 60 lb bag does not hold the same volume as an 80 lb bag. Bag counts use each bag yield rather than weight alone, which is why the four bag options give different totals.
- Weight is approximate. The weight figure uses a planning density for concrete. Reinforcement, aggregate blend and entrained air all move the real number, so do not treat it as a weighbridge result.
- Running short is the real risk. Ordering the exact finished volume leaves nothing for spillage, uneven subgrade or a slightly deeper patch. The waste allowance exists so the pour can be finished, not just started.
Assumptions
Central engine assumptions can be inspected in the methodology view. User-entered prices never get replaced with invented market prices.
Practical planning guidance
- Measure thickness from the finished slab and remember the sub-base and any reinforcement sit outside this volume.
- Confirm the yield printed on the actual bag rather than assuming every 60 lb bag behaves the same.
- Ask about ready-mix minimums and short-load fees before committing to a bag-versus-truck decision.
- Enter every section of the pour, including steps and thickened edges, so one order covers the whole placement.
- Have help and a plan for the day: concrete punishes hesitation far more than it punishes a slightly generous order.
Common mistakes
- Ordering by finished volume with no waste allowance and running short halfway through the pour.
- Assuming every concrete bag has the same yield when the counts are calculated per bag size.
- Mixing post-hole cylinders up with slab areas in one entry instead of adding them as separate sections.
- Treating the calculator as structural design — thickness and reinforcement are engineering decisions.
- Comparing a bag total with a ready-mix quote without checking the supplier minimum and delivery charge.
Important limitations
Frequently asked questions
How much concrete do I need for a slab?
Length times width times thickness gives cubic feet; divide by 27 for cubic yards. Enter the slab as a section and the calculator adds waste and rounds the order figure for you.
How many bags of concrete equal a cubic yard?
It depends on bag yield. The page counts 40, 50, 60 and 80 lb bags separately using their volumes, so you can see the real bag count for your order.
Should I order ready-mix or bags?
The purchase advice compares your volume with practical bag and truck thresholds. Small jobs favour bags; larger pours usually work out better as ready-mix once minimums are considered.
Does it calculate post holes?
Yes. Enter the hole diameter, depth and count and the engine treats each hole as a cylinder, mixing the result into the same pour total.
Why is the order figure larger than my volume?
The waste allowance is included and ready-mix quantities are rounded up in the supplier increment, because neither material nor truck loads arrive in exact fractions.
Is the weight estimate accurate enough to plan with?
It uses a planning density for concrete and is fine for transport thinking. It is not a substitute for the supplier figure on a weight-based order.
Planning guides
Related calculators
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Enter your dimensions, then calculate. Results stay tied to the tested engine.