FREE TOOL · CONSTRUCTION

Concrete Calculator

A bag of concrete covers less than you think. Enter your slab or tube dimensions and a waste factor — the calculator returns cubic feet, cubic yards, and the exact number of 60-lb and 80-lb bags to buy, with a live diagram of your pour.

123

How much concrete do you need?

Pick the shape and enter your dimensions — the calculator shows cubic feet, cubic yards, and the bag count with your waste factor baked in. The diagram below redraws to your numbers as you type.

Live diagram — it redraws to your dimensions as you type.

How it works: slab volume = length × width × (thickness ÷ 12); tube volume = π × (diameter ÷ 24)² × depth. Cubic yards = cubic feet ÷ 27. Bags = the volume with waste, divided by the bag yield and rounded up — 0.45 ft³ per 60-lb bag, 0.60 ft³ per 80-lb bag. On load with the defaults (10 × 10 ft slab, 4 in thick, 10% waste): 33.33 ft³ = 1.23 yd³ → 82 sixty-pound or 62 eighty-pound bags. Nothing you type leaves your device.

Volume first, bags second

Every concrete question is really two questions: how much volume does the hole or form hold, and how many bags fill that volume. The volume is pure geometry — a slab is a box, a post hole is a cylinder. The bag count is the volume plus a waste factor, divided by what one bag yields, always rounded up (you cannot buy 0.4 of a bag, and running short mid-pour is the classic expensive mistake).

Output · formula · meaning

Output	Formula	What it means
Cubic feet	slab: L × W × (T ÷ 12) · tube: π × (D ÷ 24)² × H	The raw volume of your form — 33.33 ft³ for the default 10 × 10 ft, 4-in slab.
Cubic yards	ft³ ÷ 27	The unit concrete is ordered in — 1.23 yd³ on the defaults.
Volume with waste	ft³ × (1 + waste %)	What you actually buy material for — 36.67 ft³ at 10% waste.
60-lb bags	round up (waste ft³ ÷ 0.45)	82 bags on the defaults.
80-lb bags	round up (waste ft³ ÷ 0.60)	62 bags on the defaults.

Worked example: the 10 × 10 patio slab

A 10 × 10 ft patio, 4 in thick. Volume = 10 × 10 × (4 ÷ 12) = 33.33 ft³, which is 33.33 ÷ 27 = 1.23 cubic yards. Add the standard 10% waste: 33.33 × 1.10 = 36.67 ft³. Sixty-pound bags: 36.67 ÷ 0.45 = 81.5 → 82 bags. Eighty-pound bags: 36.67 ÷ 0.60 = 61.1 → 62 bags. At 62 bags (nearly 2½ tons of mix), price a short-load ready-mix truck before committing — past about a yard, the truck usually wins on both cost and your back.

Who uses this

DIY patio pourers: sizing the Saturday project before the store run — and discovering whether it is a bag job or a truck job.
Contractor estimators: fast material takeoffs for quotes, with the waste factor baked in instead of guessed.
Fence & deck builders: tube mode does one post hole — multiply by the hole count for the whole run.
Trade students: watching the volume formulas work on real dimensions, with the diagram showing what each number measures.

Accuracy & limitations

Bag yields (0.45 ft³ for 60-lb, 0.60 ft³ for 80-lb) are the manufacturers’ nominal figures — check your bag, since fast-setting and high-strength mixes can differ slightly. The waste factor covers spillage, form bulge, and uneven subgrade; it does not cover measuring wrong, so measure twice. The math assumes level forms and uniform thickness. Concrete is heavy (about 4,000 lb per cubic yard) — hauling is often the real constraint, not the arithmetic. Always confirm a ready-mix order with the supplier, and get an engineer for anything structural like load-bearing footings.

Comparing bag prices? The unit price calculator finds the cheaper bag per cubic foot. Browse the full construction hub for more project math.

Cite this calculatorBright Side Kit. “Concrete Calculator.” brightoolbox.com, 2026.Bright Side Kit. "Concrete Calculator." https://brightoolbox.com/concrete-calculator/ Accessed October 21, 2026.

Embed this calculator

<iframe src="https://brightoolbox.com/concrete-calculator/" width="100%" height="640" style="border:0;border-radius:12px" title="Concrete Calculator &mdash; Bright Side Kit" loading="lazy"></iframe><p style="font-size:.8rem"><a href="https://brightoolbox.com/concrete-calculator/">Concrete Calculator</a> by Bright Side Kit</p>

Paste this into any web page to embed the live calculator. The “Bright Side Kit” link below it must stay intact.

Frequently asked questions

How many bags of concrete do I need for a 10×10 slab?

At 4 in thick with the standard 10% waste: 33.33 ft³ (1.23 yd³), which is 82 sixty-pound bags or 62 eighty-pound bags. That is nearly 2½ tons of mix — at this size, get a quote for a short-load ready-mix truck before buying bags.

Slab mode or tube mode — which do I use?

Slab mode for anything flat and rectangular: patios, sidewalks, shed floors, footings. Tube mode for round holes and columns: fence and deck posts, piers, sonotubes. For a run of post holes, calculate one hole in tube mode and multiply the bag count by the number of holes.

Why does the calculator add a waste factor?

Because real pours are never perfect: forms bulge, subgrade dips, and some mix always ends up on the ground instead of in the form. 10% is the trade standard. Use 5% if your forms are dead level and you are careful, 15% for rough or hand-dug ground. Running short mid-pour creates a cold joint — a visible weak seam — so err high.

Can I mix a whole slab by hand in one pour?

Up to about a yard, yes, with help — but bag concrete gives you roughly an hour of working time, and a 10×10 slab is 62 bags to mix, haul, screed, and finish before it sets. Past about 1 cubic yard, a ready-mix truck is usually cheaper than bags and arrives mixed. Never stop halfway across a slab: the seam where fresh concrete meets setting concrete is a permanent weak line.

Should I buy 60-lb or 80-lb bags?

An 80-lb bag yields 0.60 ft³ versus 0.45 ft³ for a 60-lb bag, so you buy fewer bags and the price per cubic foot is usually lower. The trade-off is handling: 80-lb bags are genuinely heavy to lift and pour one at a time. Solo DIYers often prefer 60-lb bags despite the higher count. Compare the shelf prices per cubic foot with the unit price calculator — do not assume the bigger bag is always the better deal.

The result says 1.23 cubic yards — how do I order that?

Ready-mix suppliers sell by the yard and most have a 1-yard minimum plus a short-load fee under a few yards, so you would order 1¼ yards (round up, never down). Alternatively, buy it in bags: 62 eighty-pound bags at 10% waste. Either way, confirm the final number with the supplier — they will sanity-check your dimensions.