Concrete Calculator
Calculate concrete volume and bags needed
Concrete Calculator tool
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Concrete Calculator: key facts
- What it does
- Calculate concrete volume and bags needed
- Category
- Other Calculators
- Cost
- Free, with no account, sign-up, or install.
- Your data
- Runs entirely in your browser — the files and text you enter are never uploaded to a server.
- Last reviewed
- . Report an incorrect result.
What this tool is for
Order too little concrete and a pour stalls; order too much and you have paid for waste. This calculator estimates the volume your project needs and converts it into the number of bags to buy, handling slabs, footings, columns, and tubes.
Contractors and serious DIYers use it before pouring a patio, setting fence posts, or filling form tubes. It works out the volume in cubic yards, adds a waste factor, and tells you how many 40, 60, or 80-pound bags that comes to — with an optional cost estimate so you can budget the trip to the supplier.
How to use the Concrete Calculator
- Choose your measurement units and the bag size you plan to buy (40, 60, or 80 lb).
- Set a waste factor (5% by default) and, optionally, the price per bag.
- Add each pour, choosing its shape — slab, footing, column, or tube — and entering the dimensions.
- Read the total volume, cubic yards, bags needed for each bag size, and the estimated cost.
Volume first, then bags
The tool computes the volume of each element you add — slabs and footings as length by width by depth, columns as pi times radius squared times height, and tubes as the difference between an outer and inner cylinder — then sums them. Volumes are converted to cubic yards, since that is how ready-mix concrete is ordered and how bag yields are published.
A waste factor is then applied, defaulting to 5%. This is not padding: concrete is lost to spillage, to uneven subgrade that takes more depth than planned, and to the simple fact that a pour cannot be left short while someone fetches more. Running out mid-pour produces a cold joint, which is a structural weakness rather than a cosmetic one.
Bag counts come from published yields — a 60 lb bag makes about 0.017 cubic yards and a 40 lb bag about 0.011 — and are rounded up, because part bags are not useful.
- The default slab — 20 ft by 10 ft at 0.5 ft deep — is 100 cubic feet, or 3.70 cubic yards.
- With the 5% waste factor that becomes 3.89 cubic yards.
- At 0.017 cubic yards per 60 lb bag that is 229 bags — which is the point at which ordering ready-mix becomes far cheaper and easier than mixing by hand.
What the Concrete Calculator gets right
- It handles four pour types — slabs, footings, columns, and tubes — including the hollow-tube calculation that subtracts the inner void.
- It converts volume into bag counts for all three common bag weights and applies an adjustable waste factor.
- It estimates total cost when you enter a price per bag, so you can budget before buying.
- It computes everything in your browser with no sign-up.
When to stop bagging and order a truck
Bags make sense for small jobs — post holes, a few fence footings, a small pad. Beyond roughly half a cubic yard the labour becomes punishing and the cost per unit volume works against you, and past a cubic yard ready-mix delivery is almost always the right answer. Suppliers usually have a minimum order and may charge a short-load fee below it, so it is worth asking before deciding.
Volume is only part of the specification. Concrete is also specified by strength, typically as a compressive figure at 28 days, and the right mix depends on the application — a driveway carrying vehicles needs more than a garden path. Reinforcement, joint spacing, and slab thickness for a given load are engineering decisions, and anything structural should be designed rather than estimated.
Practical points that affect quantity: compact and level the subgrade first, since a dip costs more concrete than expected; use forms that will not bow under the pour; and remember that concrete is heavy at roughly 4,000 lb per cubic yard, which matters for access and for what a barrow can carry. Working in hot or freezing conditions needs specific measures.
Frequently Asked Questions
How much concrete do I need?
Multiply the length, width, and depth of a slab to get its volume, then convert to cubic yards (dividing cubic feet by 27). Columns and tubes use circular formulas. This calculator does the conversion and adds a waste margin automatically.
Why add a waste factor?
Some concrete is always lost to spillage, uneven subgrade, and over-excavation, so ordering exactly the calculated volume risks coming up short. A 5–10% waste factor — adjustable here — gives a practical buffer.
How many bags of concrete are in a cubic yard?
It depends on bag size: it takes more small bags than large ones to fill the same volume. The calculator uses standard coverage for 40, 60, and 80-pound bags and shows the count for each so you can choose.
When should I order ready-mix instead of bags?
Bags suit small jobs like post holes and small slabs, but for large pours the number of bags becomes impractical and ready-mix delivery is usually cheaper and faster. The bag count here is a good gauge — if it runs into the dozens, price up ready-mix too.
