How Many Bags of Concrete for a Slab?

Slab volume is length × width × thickness in consistent units. A 4 in (100 mm) slab is one-third of a foot thick, so a 10×10 ft pad is 10 × 10 × (4/12) = about 33.3 cu ft, or roughly 1.23 cubic yards before waste. Scale that up and you see why bag counts climb fast.

The Concrete Calculator gives cubic yards or cubic metres from your footprint. The Concrete Bag Calculator then divides that volume by the yield printed on a 40, 60, or 80 lb bag (US) or a typical bagged mix in the UK. Do not skip the volume step and jump straight to “I need fifty bags” from memory.

Add waste — often around 5–10% for a simple rectangle with tidy forms. Uneven subgrade, thick edges, or a first-time pour deserve the higher end. Running out mid-pour with mix setting in the barrow is worse than returning one unused bag.

An 8×8 ft slab at 4 in thick is about 21.3 cu ft, or roughly 0.79 yd³ before waste. With 10% waste you are near 0.87 yd³. An 80 lb bag that yields about 0.6 cu ft needs on the order of 45 bags; a 60 lb bag at about 0.45 cu ft needs more trips. Exact yields vary by brand — always read the bag.

A 10×10 ft pad at the same 4 in depth is about 1.23 yd³ before waste. That is where many homeowners still buy bags and then regret the labour. A 12×12 ft slab at 4 in is about 1.78 yd³ before waste — bag mixing that volume is a long day even with a rented mixer.

Post holes use the same idea with a cylinder: π × radius² × depth, then bags. Three 12 in diameter holes 3 ft deep are still bag territory. A whole patio slab usually is not.

Eighty-pound bags cover more ground per walk from the truck to the mixer. Sixty-pound bags are easier on your back if you are working alone. Forty-pound bags exist for small repairs; they are rarely economical for a full slab.

UK DIY bags are often sold in kilograms (for example 20 kg or 25 kg) with coverage stated in litres or as “bags per m³.” Convert your slab to cubic metres first (length × width × depth in metres), then divide by the stated yield. A 2.4×2.4 m pad at 100 mm is 0.576 m³ before waste — check the bag chart rather than importing a US bag count.

Round up to whole bags. Mix one strength and one product type for the whole pour. Do not blend fast-set and standard mix in the same slab, and do not assume two brands with the same bag weight have identical yields.

Past about one cubic yard, compare bag cost plus mixer hire plus your time against a ready-mix quote that includes a short-load fee. Many yards charge extra for loads under a few yards — that fee can still beat fifty hand-mixed bags when you value a continuous pour.

Hand-mixing fifty or more bags is not just heavy; it is hard to keep a consistent water ratio. Inconsistent mix shows up as soft spots and colour variation. Ready-mix arrives at a specified strength if you order it that way.

Use the Concrete Bag Calculator for your exact footprint, then line the bag total against a truck price with the Concrete Cost Calculator or a local quote. The Cost Estimator helps if you also want labour and waste removal on the same sheet.

Walkway and shed pads are often planned around 4 in. Driveways and garage slabs that take cars are typically thicker and may need mesh or rebar — that is a specification choice, not something a bag count invents for you. Going from 4 in to 6 in raises volume by half for the same footprint.

Reinforcement, control joints, and finish (broom, float, stamped) change labour and materials but not the basic cubic volume of concrete. Ask quotes to state thickness and mix separately so bag math and truck math stay comparable.

If the ground is soft or holds water, fix the sub-base before you count bags. More concrete on a bad base still cracks.

Sub-base gravel is separate. Compacted crushed stone under the slab is estimated with the Gravel Calculator using the base depth in the plan — it never appears inside the concrete bag total.

Forms, poly vapour barrier, isolation joint foam, curing compound, and tools are separate line items. So is demolition of an old pad. A bag count answers “how much mix,” not “what does the whole project cost.”

Weather matters. Hot days shorten working time; cold days slow set. Plan enough help to place and finish the pour in one go once mixing starts, whether bags or truck.

Confirm length, width, and thickness in one unit system. Run the Concrete Bag Calculator with your bag size and waste percent. Round up. Check that you can return unopened bags if you overshoot slightly.

Stage water, mixer, wheelbarrows, and a finishing plan before the first bag opens. For anything near or above a cubic yard, get a ready-mix price the same day so the comparison is real, not theoretical.

If you only need a few post holes or a tiny equipment pad, bags win. If you are pouring a patio you will sit on for years, price the truck — even when the bag math looks cheaper on paper.