Concrete calculator — volume, bags and rebar
A 5×4 m slab at 15 cm thick takes 3.3 m³ of concrete with 10% waste — 264 bags of 25 kg premix, or one ready-mix order. Enter your slab or footing below for volume, bags, rebar, mesh and gravel — free, no signup.
Your quantities
How the concrete quantities are worked out
Volume is length × width × thickness for a slab, or π × radius² × depth for a footing, with your waste percentage added once. Bags are the volume divided by each bag's yield, rounded up — a 25 kg bag yields 0.0125 m³, an 80 lb bag 0.60 cubic feet.
The rebar grid counts bars at your spacing in both directions — bars = floor(span ÷ spacing) + 1 — and adds 10% for laps. Mesh and vapour barrier add 10% for overlap, gravel is weighed at 1.6 tonnes per m³, and formwork timber is counted in board feet around the slab's edge.
Running these jobs? Masonry & Plaster
How to use the concrete calculator
- Choose the pour shape — a rectangular slab or a round footing.
- Type the measurements and the thickness, then set the waste: 5–10% is the usual allowance.
- Tick the extras the job includes: rebar grid, welded mesh, a gravel bed, vapour barrier and formwork.
- Read the full site list: volume, every bag size, rebar length and weight, gravel tonnes and formwork.
How much concrete does a 5×4 m slab, 15 cm thick, need?
Volume is 5 × 4 × 0.15 = 3 m³ before waste, and 3.3 m³ with the 10% waste the calculator opens on. In premix bags that is 264 × 25 kg bags or 165 × 40 kg bags — the point where a ready-mix truck usually wins.
The same slab on a 20 cm rebar grid takes 229.9 m of 12 mm bar — about 204 kg — including 10% for laps. Mesh and vapour barrier take 22 m² each with overlap, the 10 cm gravel bed is 2 m³ (3.2 tonnes), and formwork comes to 65 board feet.
Bag yields and assumptions the calculator uses
Premix bag yields
| Bag | Yield | Bags per unit volume |
|---|---|---|
| 25 kg bag | 0.0125 m³ | 80 per m³ |
| 40 kg bag | 0.020 m³ | 50 per m³ |
| 80 lb bag | 0.60 ft³ | 45 per yd³ |
| 60 lb bag | 0.45 ft³ | 60 per yd³ |
Allowances built into the sums
| Item | Allowance |
|---|---|
| Waste | Your percentage, added once to the volume |
| Rebar laps | +10% on bar length |
| Mesh and vapour barrier | +10% for overlap |
| Gravel bed | 1.6 t/m³ (metric) · 1.4 tons/yd³ (imperial) |
Pouring concrete in Gulf heat
- Pour early in the morning or late in the afternoon. Above about 29 °C, evaporation outruns curing and the surface cracks while it dries — plastic-shrinkage cracking is the classic Gulf summer failure.
- Keep the concrete at or below 32 °C at placing, the ACI hot-weather practice limit; batch plants cool the mix with chilled water or shaved ice when the day demands it.
- Pre-wet the formwork and subgrade so they do not steal mix water, and start curing as soon as finishing allows — under damp curing, concrete reaches about 70% of its design strength in 7 days.
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How many 25 kg bags of concrete make 1 m³?+
80 bags. One 25 kg premix bag yields 0.0125 m³ of placed concrete, so a cubic metre takes 1 ÷ 0.0125 = 80 bags — which is why anything past a small slab is cheaper as ready-mix.
How much concrete do I need for a 5×4 m slab, 15 cm thick?+
3 m³ before waste, 3.3 m³ with 10% waste — that is 264 bags of 25 kg premix, or one ready-mix order of 3.3 m³. The same slab needs about 230 m of 12 mm rebar on a 20 cm grid.
How much rebar goes in a slab?+
It follows the grid: at 20 cm spacing each way, a 5×4 m slab takes 21 bars one way and 26 the other — 229.9 m of bar with 10% added for laps, about 204 kg in 12 mm steel.
How many 40 kg bags of concrete make 1 m³?+
50 bags. A 40 kg premix bag yields 0.020 m³ of placed concrete, so a cubic metre takes 1 ÷ 0.020 = 50 bags. The 3.3 m³ slab above takes 165 of them.
How much waste allowance should I add to a concrete order?+
Add 5–10%. It covers spillage, an uneven subgrade and formwork that bulges slightly under the pour. The calculator adds your percentage once, to the volume, before it counts bags.
How do you calculate concrete for a round footing?+
Volume = π × radius² × depth. A footing 40 cm across and 1.2 m deep is 0.15 m³ of concrete — 13 bags of 25 kg premix before waste. The calculator’s round-footing shape does this sum directly.
How much gravel goes under a slab?+
The calculator multiplies the footprint by the bed depth and weighs gravel at 1.6 tonnes per m³. A 20 m² slab on a 10 cm bed needs 2 m³ of gravel — 3.2 tonnes. Compacted, clean 20 mm crushed aggregate is the standard subbase.
What is the standard thickness for a concrete slab?+
100 mm for a driveway taking standard passenger cars. Go to 125–150 mm, reinforced, for heavy vehicles or weak ground. The calculator takes any thickness — the slab on this page opens at 150 mm.
What is concrete slump, and why does it matter?+
Slump measures how workable and fluid the fresh concrete is. A higher slump places and levels more easily, but the extra water that creates it reduces the final compressive strength — order the slump the job needs, not the wettest mix.
How long does concrete take to reach full strength?+
About 70% of its design strength at 7 days, and its nominal design strength at 28 days of curing. Keep the pour damp while it cures — strength comes from hydration, and a dried-out surface never catches up.
How much does a cubic metre of concrete weigh?+
About 2,400 kg per m³ for normal placed concrete. That weight is why slab thickness and subgrade compaction matter: the ground carries the full load from the first day.
What does this calculator not include?+
No prices, no labour and no pump or truck charges — quantities only. It counts the pour and its usual companions (rebar, mesh, gravel, barrier, formwork); finishing and curing materials are separate. The quantities do not carry into InstaJack automatically yet.