Concrete Calculator
Enter your pour dimensions and get the volume in cubic yards and cubic feet, the number of 60 lb or 80 lb bags to buy, and a material cost estimate. Built for slabs, footings, columns and stairs, with a wastage allowance so a short load never stops your pour.
Concrete needed (incl. wastage)
| Volume (cubic feet) | |
| Volume (cubic metres) | |
| 60 lb bags (rounded up) | |
| 80 lb bags (rounded up) | |
| Ready-mix cost | — |
| Bagged cost (80 lb bags) | — |
Rounding: volumes are rounded to two decimals for display only — bag counts always round up to whole bags, and costs are computed from the unrounded volume.
What a concrete estimate really measures
Concrete is sold by volume, but poured against reality. The geometric volume of a slab — length times width times thickness — is only the starting point, because the ground under it is never perfectly level and the forms never perfectly tight. A practical estimate therefore has two layers: the shape's true volume, and an allowance for everything the drawing cannot see. Get the first layer wrong and the error multiplies — thickness entered in feet instead of inches is the classic 12× mistake — while skipping the second layer is how pours come up short with the truck already gone.
Quantity also decides how you buy. Ready-mix trucks price by the cubic yard and carry batch-plant consistency, but charge minimum-load fees that punish small orders. Bagged pre-mix flips that: cheap and flexible for a fence post or a small pad, but slow and physically brutal at volume — a single cubic yard means mixing 45 bags of 80 lb material. Knowing your cubic yards, your bag counts and both costs side by side, before phoning a supplier, is exactly the decision this tool is built to inform.
How to use the concrete calculator
- Choose your unit system. Pick feet and inches, or metres and centimetres. Every label on the form updates to match, and prices switch between per cubic yard and per cubic metre.
- Select the shape. Choose slab, footing, column or stairs. Each shape asks only for the dimensions that drive its volume.
- Enter the dimensions. Thickness and small cross-sections are taken in inches or centimetres because that is how plans state them; converting them for you removes the most common source of a 12x error.
- Set a wastage percentage. Spillage, over-excavation and uneven sub-grade always consume more than the geometric volume — 10% is a sensible default for slabs, and rough ground deserves more.
- Add prices (optional). Enter your ready-mix price per cubic yard and a bag price, so the same inputs return a budget figure alongside the quantity.
- Read the result. Total cubic yards for ordering ready-mix, or 60 lb and 80 lb bag counts — always rounded up to whole bags — for smaller pours.
Key formulas
All shapes reduce to volume in cubic feet, then convert. With dimensions in feet (small dimensions entered in inches are divided by 12 first):
- Slab / footing: V = length × width × thickness
- Round column: V = π × (diameter ÷ 2)² × height × count
- Stairs (solid pour): V = rise × run × width × n(n+1) ÷ 2, where n is the number of steps
- Ordering volume: Vorder = V × (1 + wastage%)
- Cubic yards: yd³ = ft³ ÷ 27 · Bags: count = ⌈ft³ ÷ yield⌉ (0.45 ft³ per 60 lb bag, 0.60 ft³ per 80 lb bag — manufacturer product data)
Worked example
A patio slab 12 ft long, 10 ft wide, 4 in thick, with 10% wastage: volume = 12 × 10 × (4 ÷ 12) = 40.00 ft³; with wastage, 40 × 1.10 = 44.00 ft³ = 44 ÷ 27 = 1.63 yd³. Bags: 44 ÷ 0.60 = 73.3 → 74 × 80 lb bags, or 44 ÷ 0.45 = 97.8 → 98 × 60 lb bags. At $170 per cubic yard, ready-mix costs 1.6296 × 170 = $277.04; at $6.80 per 80 lb bag, bagged material costs 74 × 6.80 = $503.20 — a clear ready-mix win at this size. Enter these numbers above and the tool reproduces every figure.
Bag yield reference
| Bag size | Yield per bag | Bags per ft³ | Bags per yd³ |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 3.33 | 90 |
| 50 lb | 0.375 ft³ | 2.67 | 72 |
| 60 lb | 0.45 ft³ | 2.22 | 60 |
| 80 lb | 0.60 ft³ | 1.67 | 45 |
Yields per manufacturer product data for standard high-strength pre-mix (QUIKRETE / Sakrete). Specialty mixes — fast-setting, fibre-reinforced, 5000 psi — can yield slightly differently; check the bag.
Things to keep in mind
- Thickness drives everything. On a 600 ft² driveway, moving from 4 in to 5 in adds roughly 0.9 yd³ — order against the thickness you will actually strike, not the one on the sketch.
- Sub-grade steals concrete. Low spots, over-dig and soft fill absorb volume invisibly. Wastage covers it on paper; compacting and fine-grading before the pour covers it in practice.
- Cold joints are the real cost of under-ordering. A pour that stops mid-slab bonds poorly and telegraphs a crack line. Over-ordering by a part-yard is nearly always cheaper than a short-load call-out.
- Check local frost depth for footings. Footing depth is a code minimum tied to frost line, not a preference — the depth field here should carry your jurisdiction's number, verified before excavation.
- Metric plans, imperial supplier (or vice versa). The unit toggle exists because Gulf and international projects routinely mix systems; convert in the tool, not in your head at the batching plant.
Frequently asked questions
How many 80 lb bags of concrete make one cubic yard?
An 80 lb bag of pre-mixed concrete yields about 0.60 cubic feet once mixed, so one cubic yard — 27 cubic feet — takes 45 bags. The 60 lb size yields roughly 0.45 cubic feet, which works out to 60 bags per yard. Those yields come from manufacturer product data, and this calculator always rounds bag counts up, because a pour that comes up half a bag short is a cold joint waiting to happen.
Should I order ready-mix or buy bags?
The usual break-even sits around one cubic yard. Below that, bagged mix is cheaper once you count a ready-mix truck's minimum-load and short-load fees; above it, ready-mix wins on both cost and consistency, and it spares you mixing fatigue — a yard is 45 large bags, which is heavy work. For structural pours, ready-mix also gives you a batch ticket stating the design strength, something hand-mixed bags cannot.
How much extra concrete should I order for wastage?
For a formed slab on a well-compacted, level base, 5–10% covers spillage and minor grade variation. Excavated footings and trenches deserve 10–15%, because soil never breaks along the neat lines on the drawing. If the sub-grade is soft, rutted or poorly struck, go higher still. Running short mid-pour forces either a cold joint or an expensive emergency short-load — an extra part-yard is the cheaper insurance.
What slab thickness do I actually need?
Common practice for residential work: 4 inches for patios, walkways and shed bases on decent soil; 5–6 inches for driveways that will see cars and light trucks. Anything carrying heavier loads, poor soil, or vehicle traffic at the thicker end should be confirmed by a local engineer, and many jurisdictions set minimums in code. Thickness is the input your total is most sensitive to — half an inch on a large slab is a whole extra yard.
How is stair concrete volume calculated?
Solid-poured stairs stack like prisms: the first step is one rise tall, the second two rises, and so on. Adding those up gives volume = rise x run x width x n(n+1)/2, where n is the number of steps. This calculator uses exactly that series. If your stairs land on a separate platform or sit over a hollow-formed void, calculate the landing as a small slab and add it — the formula here covers the solid steps themselves.
Does the calculator account for rebar displacing concrete?
No — and in practice neither do estimators. Reinforcing steel typically occupies well under 1% of a residential pour's volume, far smaller than the wastage allowance, so it is ignored in quantity take-offs. What rebar does change is your base preparation and cost lines, not the concrete order. Keep the wastage percentage doing its job and the steel's displacement disappears inside it.
How many square feet does a yard of concrete cover?
It depends entirely on thickness: one cubic yard is 27 cubic feet, so at 4 inches thick it covers 81 square feet, at 5 inches about 65, and at 6 inches 54. That inverse relationship is why this calculator asks for thickness in inches rather than a coverage guess — a driveway quoted "about 80 square feet per yard" at 6 inches would arrive one-third short.
Last updated: 24 July 2026