How to use the concrete & rebar calculator
- Choose the shape: slab, footing, wall, round column, post hole or stairs, and enter its dimensions.
- Set how many of them, the waste allowance (10% is normal) and the bag size you can buy.
- Read the volume in cubic yards and m³, the number of bags, the weight, and for slabs and footings the rebar or mesh you need.
How the volume is calculated
Every shape reduces to length × width × depth. Thickness entered in inches or millimetres is converted to feet or metres first, which is where most hand calculations go wrong (a 4-inch slab is 0.333 ft thick, not 4). A round column is π r² h; a post hole subtracts the post's own volume from the hole's; stairs are summed as a stack of blocks, one per tread, plus the landing. The result is multiplied by the quantity and by the waste allowance.
Cubic yards = (length ft × width ft × thickness in ÷ 12) ÷ 27 · m³ = length m × width m × thickness mm ÷ 1000
Why 10% waste
Sub-base that is not quite flat, formwork that bows, spillage, and the concrete left in the chute or the mixer all eat into the order. Ready-mix suppliers and most contractors add 5–10%; for hand-dug footings in uneven ground use 15–20%, because the trench is never the width you drew. Running short on a pour is far more expensive than a few extra bags, since a cold joint between two batches is a crack waiting to happen.
Bags versus ready-mix
Bagged premix yields a known volume: an 80 lb (36 kg) bag makes 0.60 ft³, a 60 lb bag 0.45 ft³, a 40 lb bag 0.30 ft³; in metric markets a 20 kg bag makes about 9.5 litres and a 25 kg bag 12 litres. A cubic yard is 45 bags of 80 lb, which is 3,600 lb to carry and about an hour of continuous mixing for two people. The practical crossover is roughly one cubic yard (0.75 m³): below that, bags; above it, order ready-mix and have help ready, because a truck gives you about 90 minutes before the load starts to set.
Ready-mix is ordered in quarter-yard (0.25 m³) steps and a standard truck carries 8–10 yd³ (6–8 m³). Short-load fees apply below 4–5 yd³. Tell the supplier the slump and strength you want: 3,000–4,000 psi (20–25 MPa) with a 4-inch slump for residential slabs; 4,000 psi air-entrained in freeze-thaw climates such as Canada, the northern US and the UK.
Rebar and mesh
The rebar estimate places a two-way grid at your spacing with 3 inches (75 mm) of edge cover, adds 5% for laps and cut-offs, and converts to 20 ft (6 m) stock lengths and weight. #4 bar (12 mm, Canadian 10M–15M) weighs 0.668 lb per foot; #3 is 0.376 and #5 is 1.043. Bars must be lapped at least 40 bar diameters where they join (20 inches for #4) and held at mid-depth of the slab on chairs or dobies; rebar lying on the ground does nothing.
Welded wire mesh (6×6-W2.9 in the US, A193 in the UK, SL72 in Australia) comes in sheets of 5 × 10 ft or 2.4 × 4.8 m; the mesh figure allows 10% for overlap of one full square at every joint. Synthetic fibre mixed into the concrete is the third option for light slabs and replaces mesh for crack control but not for structural reinforcement.
Worked example: two-car garage slab
24 × 24 ft at 4 in: 24 × 24 × 0.333 = 192 ft³ = 7.1 yd³; with 10% waste, 7.8 yd³, so order 8 yd³, which is one truck. Reinforcement with #4 on a 12-inch grid: 24 bars each way at 23.5 ft is 1,128 ft plus 5% = 1,185 ft, which is 60 lengths of 20 ft weighing about 790 lb. At $150 per yd³ the concrete is around $1,200 delivered; the rebar adds roughly $600–900 in 2026 prices. If vehicles over 5,000 lb will park on it, make it 5 in, which adds 25% to every figure.
Quantities are for ordering. Slab thickness, reinforcement, footing size and frost depth for structural work are set by your local building code and may need an engineer's or inspector's sign-off.
Questions and answers
How many bags of concrete for a 10×10 slab 4 inches thick?
33.3 cubic feet, which is 1.23 cubic yards. With 10% waste that is 37 ft³: 62 bags of 80 lb (0.60 ft³ each), 82 bags of 60 lb, or 123 of 40 lb. At that size a ready-mix delivery of 1.5 yd³ is usually cheaper than bags and far less work.
How much does a cubic yard of concrete cost?
Ready-mix in the US runs roughly $125–180 per cubic yard delivered in 2026, plus a short-load fee of $50–150 for orders under about 4 yd³. In the UK, £100–150 per m³; Australia A$250–350 per m³; Canada C$180–250 per yd³. An 80 lb bag of premix is $6–8, which works out at $270–360 per cubic yard, so bags cost about twice as much per volume but have no minimum order.
How much does a cubic yard of concrete weigh?
About 4,050 lb (1.84 tons) for normal-weight concrete, or 2,400 kg per cubic metre. A 10 × 10 ft, 4-inch slab therefore weighs around 5,000 lb; this matters for what you are placing it on and for whether a trailer can carry the bags.
How thick should a slab be?
4 inches (100 mm) for a patio, shed base or sidewalk; 4 inches minimum for a residential garage, with 5–6 inches where vehicles over 5,000 lb will park; 6 inches (150 mm) for a workshop with machinery or an RV pad. Thicken the edge to 8–12 inches on a slab that supports a wall.
Do I need rebar or wire mesh in a slab?
A 4-inch patio or shed slab on well-compacted ground is often poured plain with fibre reinforcement or 6×6 welded wire mesh, which limits crack width but adds little strength. Driveways, garage floors and anything on poor soil should have #4 (12 mm) rebar on a 12–18 inch grid, held at mid-depth on chairs. Footings get two or three continuous #4 or #5 bars along their length. Your local building code sets the minimum for structural work.
How many bags of concrete per fence post?
For a 4×4 post in a 10-inch hole 3 feet deep, the annulus around the post is about 1.3 ft³, which is 2–3 bags of 80 lb (or 50 lb fast-setting). A 12-inch hole takes 3–4 bags. The rule of thumb is one third of the post length in the ground, with the hole three times the post width.
Free to use, runs in your browser, nothing uploaded. Last updated 2026-10-07. Made by HandyToolkits; questions to [email protected].