Cement Bags
Count 25 kg or 40 kg cement bags for any concrete volume and mix type.
OpenConcrete Calculator
Calculate the concrete volume, cement bags, sand and gravel you need for a slab, footing or any rectangular pour.
What this calculates
Calculates concrete volume (net and with waste allowance), estimated cement bags for hand mixing, sand and gravel quantities for a standard mix.
Formula used
Volume = length × width × thickness. Volume with waste = volume × (1 + waste %). Cement bags = volume × 300 kg/m³ ÷ 25 kg per bag (rounded up).
Worked example
A slab of 5 m × 4 m × 10 cm (4 in) has a net volume of 2.00 m³. With 8% waste: 2.16 m³. That requires about 26 bags of cement (25 kg each), 1.19 m³ of sand and 1.73 m³ of gravel.
Typical concrete thicknesses
Residential floor slabs: 10–12 cm (4–5 in). Driveways: 12–15 cm (5–6 in). Sidewalks: 10 cm (4 in). Always consult an engineer for structural applications.
When not to use this calculator
Do not use for structural slabs, elevated slabs, retaining walls, or any application requiring engineered mix design or reinforcement detailing.
How to use this calculator
- Enter the slab length and width in meters (or switch to feet using the Metric / Imperial toggle above the calculate button).
- Enter the thickness in centimeters — 10 cm (4 in) is standard for light residential; use 12–15 cm (5–6 in) for driveways and garage floors.
- Set the waste factor — 8% covers typical spillage and overpour. Raise it to 10–12% for irregular forms or curved areas.
- Optionally enter a local price per m³ to get a preliminary cost estimate alongside the quantities.
- Click "Calculate concrete" — results update in real time as you change any input.
Mix design and international standards
This calculator uses a standard 1:2:3 mix ratio (cement : sand : coarse aggregate by volume), which corresponds to approximately 300 kg of cement per m³ of finished concrete. This is a general-purpose mix suitable for non-structural pours, patios and light residential slabs. It is broadly comparable to:
- ACI 318 (USA): f'c ≥ 2500 psi (17 MPa) — minimum for slabs on grade in frost-free areas. Most driveways use 3000–4000 psi.
- EN 206 / BS 8500 (Europe/UK): Class C20/25 or designation GEN 3 — standard for domestic ground-bearing slabs.
- AS 3600 (Australia / NZ): N20 concrete (20 MPa characteristic strength) for non-structural residential slabs.
- NCh 170 (Chile): Grade H-20 (200 kg/cm² ≈ 20 MPa) — typical for radiers and light residential slabs.
The approximate material breakdown per m³ at this mix ratio: Cement 300 kg · Sand 825 kg (0.55 m³) · Gravel 1 200 kg (0.80 m³) · Water 150–180 liters (water-to-cement ratio 0.50–0.60). For engineered projects, always use a mix design specified by a structural engineer and verified by a certified materials laboratory.
When to consult a structural engineer
Use this calculator for preliminary planning only. Always involve a licensed structural engineer or civil engineer when:
- The slab will carry vehicles, forklifts, heavy equipment or live loads above standard residential norms.
- The pour forms part of a foundation, footing, column base, retaining wall or any load-bearing element.
- A building or construction permit is required — an engineer's sealed drawings are typically mandatory.
- Soil conditions are uncertain: expansive clay, fill material, high water table, freeze-thaw cycles or poor bearing capacity.
- The project uses rebar, post-tension, pre-cast elements or any reinforced concrete structural system.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length × width × thickness and add 8–10% for waste. The result is the volume to order from your ready-mix supplier.
How many bags of cement per m³?
A standard mix uses about 300 kg of cement per m³, which equals about 12 bags of 25 kg. This calculator uses that ratio for hand-mixed concrete.
What thickness is standard for a floor slab?
10 cm (4 in) for light residential use. Driveways usually require 12–15 cm (5–6 in).
Can I use this for a ready-mix concrete order?
Yes. Use the volume with waste as your order quantity. Round up to the nearest 0.25 m³.
How many bags of concrete for a 10×10 slab at 4 inches thick?
A 10 ft × 10 ft × 4 in slab is about 1.23 cubic yards (0.94 m³). For hand-mixed concrete using bulk cement at 300 kg/m³, you need approximately 11 bags of 25 kg (55 lb) cement — plus sand and gravel. If using 80 lb pre-mix bags (like Quikrete), you need about 45–50 bags since each covers roughly 0.6 cubic feet of finished concrete.
How thick should a concrete slab be?
4 inches (10 cm) is standard for residential patios, shed floors and walkways. Driveways require 5–6 inches (12–15 cm). Slabs supporting heavy equipment or vehicles need engineering. The calculator's thickness field accepts any value — change it to match your project.
Assumptions
- Cement: 300 kg/m³ (standard mix, suitable for H-20 grade).
- Sand: 0.55 m³ per m³ of concrete.
- Gravel: 0.80 m³ per m³ of concrete.
- Bag size: 25 kg (adjust the bag count if your bags differ).
- Does not include reinforcement (rebar, mesh), formwork or curing compounds.
Technical warning
Driveways, structural elements and heavily loaded slabs require engineered mix design, proper compaction and professional oversight.
Common mistakes
- Not compacting the sub-base before pouring.
- Adding too much water, which weakens the mix.
- Forgetting expansion joints for large slabs.
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