How to Calculate Materials for a Concrete Slab

A finished slab rests on four groups of materials, and you need all four on your list before you order anything: the concrete itself, a compacted gravel base underneath it, steel reinforcement inside it, and the formwork that holds it to shape while it sets. Each one has a simple formula based on the slab’s length, width and thickness. Work through them in order and you will have a complete order list, not just a concrete quantity. If you would rather check your figures as you go, our Concrete Calculator does the concrete maths instantly.

Throughout this guide the worked example is the slab people build most often: a 10 × 10 ft patio slab, 4 in thick, poured on a 4 in compacted gravel base. Every number below belongs to that slab, so you can follow the whole calculation from the first measurement to the final checklist and then swap in your own dimensions.

1. Concrete

Concrete is sold by volume, so start with the volume of the slab: volume = length × width × thickness, with everything in feet. Thickness is given in inches, so divide it by 12 first — 4 in is 0.333 ft. For the example slab that is 10 × 10 ft × 4 in, which gives 33.3 cu ft. Divide by 27 to convert cubic feet to cubic yards and you get 1.23 yd³ net.

Never order the net figure. Forms bow a little, the ground is never perfectly level, and some concrete stays on tools, so add 10% for waste: 1.23 yd³ becomes 1.36 yd³ to order. In bags, the volume with waste is 36.7 cu ft, and at 0.60 cu ft per 80 lb bag that is 62 × 80 lb bags. That count is why ready-mix makes sense at roughly a cubic yard and up: mixing 62 bags is slow, heavy work, while a truck places the same concrete in minutes. Our guide on how much concrete do I need explains the conversions in more depth.

2. Gravel base

A slab should never sit directly on soil. A base of compacted crushed stone spreads the load, drains water away from the underside of the concrete and gives you a level surface to pour onto. Allow 4 in of compacted crushed stone under the slab. The base covers the same footprint and is the same thickness in this example, so the volume is the same: 1.23 yd³.

Gravel is usually ordered by weight, and 1.23 yd³ of crushed stone is about 1.7 tons — order 2 tons to be safe, so you are not left short at the far corner. Tip it inside the forms, rake it level and compact it in lifts, a layer at a time; a thin layer compacts hard, while a deep loose layer stays spongy however long you run the compactor over it. You can check your own footprint and depth in the Gravel Calculator.

3. Steel — rebar or mesh

Concrete is strong in compression but weak when it bends or the ground moves, which is why a slab needs steel in it. For the example slab, use #4 rebar (½ in) laid in a grid at 18 in spacing both ways. Across a 10 ft slab that spacing takes 8 bars each way, and 8 bars × 10 ft in each direction gives 160 linear ft of bar in total. The tidy way to buy it is eight 20 ft bars and cut them in half, giving sixteen 10 ft bars with no awkward offcuts.

Position matters as much as quantity. Keep the steel near mid-depth on chairs, so it sits inside the concrete rather than underneath it, and keep it 2 in clear of the edges so no bar end sits exposed at the face. If you prefer mesh, the alternative is welded wire mesh: two 5 × 10 ft sheets cover a 10 × 10 slab, and add a third sheet if you lap the joints properly, because lapped overlaps eat into each sheet’s coverage.

4. Formwork

Formwork is the timber frame that holds wet concrete to the right shape and level until it sets. For a square slab it is simply the perimeter: 10 + 10 + 10 + 10, so the perimeter is 40 ft — four 10 ft lengths of 2×6 lumber. The depth of a 2×6 comfortably contains a 4 in slab and leaves room to screed across the top.

Boards on their own will bow outward under wet concrete, so hold them with stakes: about 14 stakes, one every ~3 ft around the perimeter, driven tight against the boards, plus screws to fix the boards to the stakes and at the corners. Check the frame is square by measuring both diagonals before you pour — equal diagonals mean square corners.

Materials checklist for the example slab

Here is the whole example in one list. Run your own dimensions through the Concrete Calculator and scale the other items from the same footprint.

Item	Amount for 10×10 ft × 4 in	Notes
Concrete	1.36 yd³ ready-mix or 62 × 80 lb bags	Includes 10% waste
Gravel base	1.23 yd³ ≈ 1.7 tons (order 2 tons)	4 in compacted crushed stone
Rebar	160 ft of #4 (eight 20 ft bars) or 2–3 mesh sheets	Grid at 18 in spacing, or mesh sheets 5 × 10 ft
Form boards	40 ft of 2×6 + ~14 stakes	Four 10 ft lengths, stakes every ~3 ft, plus screws
Plastic sheeting (optional)	100+ sq ft	Vapour barrier under the slab

How thick should a slab be?

Thickness drives every number above, so settle it before you calculate. The usual guide is:

• Patio / walkway: 4 in
• Shed base: 4 in
• Driveway: 4–5 in (more for heavy vehicles)
• Garage floor: 4–5 in

Common mistakes

• Ordering with no waste allowance. The neat geometry is always less than the real pour, so a net order runs short near the finish, where a shortfall hurts most.
• Booking ready-mix before the base and forms are ready. A truck runs on a clock; the gravel, steel and formwork must all be finished before it arrives, not while it waits.
• Steel left lying on the ground instead of mid-slab. Bar or mesh at the bottom of the pour does little work and can rust against damp soil; lift it onto chairs.
• Skipping base compaction. Uncompacted stone settles unevenly after the pour, and the slab above it can crack along the hollow spots.

Put the order together in the order you will use it: timber and stakes first, then gravel, then steel, and the concrete last, booked for the day everything else is ready. A final run through the Concrete Calculator with your measured — not planned — dimensions catches most slips before they become a short load.

Frequently asked questions

Estimates only — for structural slabs or anything load-bearing, have the design checked by a professional.