The Fundamental Concrete Formula
For any concrete project, the starting calculation is surprisingly simple:
Volume = Length × Width × Depth (Thickness)
For a 4 m × 3 m slab with 100 mm (0.1 m) thickness: 4 × 3 × 0.1 = 1.2 cubic meters of concrete.
But this is just the wet concrete volume you pour. To know how much raw material to buy, you need to account for:
- Dry volume — dry ingredients about 1.54 times the wet volume due to voids between particles.
- Mix ratio — the proportion of cement, sand, and aggregate for your required grade.
- Waste factor — add 5–10% for spillage, uneven subgrades, and formwork variance.
Our Concrete Calculator handles all of this automatically for slabs, footings, columns, steps, and round shapes.
Understanding Concrete Grades: M10 Through M30
Concrete grade (M10, M15, M20, M25, etc.) indicates compressive strength in N/mm² (MPa) after 28 days of curing:
| Grade | Mix Ratio | Strength (MPa) | Typical Use |
|---|---|---|---|
| M10 | 1:3:6 | 10 | Plain concrete, non-structural, leveling courses |
| M15 | 1:2:4 | 15 | Paths, floor slabs, light residential, boundary walls |
| M20 | 1:1.5:3 | 20 | General RCC — slabs, beams, columns. Minimum for residential buildings |
| M25 | 1:1:2 | 25 | Heavy structures, commercial buildings, bridges |
| M30+ | Custom mix | 30+ | High-rise buildings, heavy infrastructure |
Nepal Building Code (NBC) minimum: M20 for all reinforced concrete in residential construction.
How Many Bags of Cement Per Cubic Meter?
For the most common grades (1 bag = 50 kg):
| Grade | Cement Bags/m³ | Sand (m³) | Aggregate/Gravel (m³) | Water (liters) |
|---|---|---|---|---|
| M15 (1:2:4) | ~5.0 | 0.44 | 0.88 | 140–160 |
| M20 (1:1.5:3) | ~6.3 | 0.44 | 0.88 | 140–160 |
| M25 (1:1:2) | ~8.0 | 0.36 | 0.72 | 130–150 |
Example for a 3 m³ M20 slab:
- Cement: 3 × 6.3 = 19 bags (19 × 50 kg = 950 kg)
- Sand: 3 × 0.44 = 1.32 m³
- Aggregate: 3 × 0.88 = 2.64 m³
- Water: ~420–480 liters
Add 5% waste on all quantities for safety.
Concrete Volume by Shape
Slab / Floor
Volume = Length × Width × Thickness
- 5 m × 4 m slab, 150 mm thick: 5 × 4 × 0.15 = 3.0 m³
- 10 ft × 12 ft slab, 6 in thick: 10 × 12 × 0.5 = 60 ft³ → 1.7 m³
Footing / Wall
Volume = Length × Width × Height
- Wall 8 m long, 0.3 m wide, 2.5 m tall: 8 × 0.3 × 2.5 = 6.0 m³
- Strip footing 20 m long, 0.6 m wide, 0.3 m deep: 20 × 0.6 × 0.3 = 3.6 m³
Round Column / Cylinder
Volume = π × r² × h
- 300 mm diameter, 3 m tall column: π × 0.15² × 3 = 0.212 m³ per column
- 10 such columns: 2.12 m³ total
Steps / Stairs
Steps are calculated as individual rectangular prisms. For a standard stair with tread 250 mm, riser 175 mm, width 1 m, and 15 steps:
- Each step: 0.25 × 0.175 × 1.0 = 0.04375 m³
- 15 steps: 0.656 m³ (plus the waist slab)
Our calculator handles all five shapes and sums the total automatically.
Water-Cement Ratio: The Key to Strength
The water-cement (W/C) ratio is the ratio of water weight to cement weight. Lower W/C = stronger concrete, but harder to work with.
| Grade | Recommended W/C Ratio |
|---|---|
| M15 | 0.50 – 0.60 |
| M20 | 0.45 – 0.55 |
| M25 | 0.40 – 0.50 |
| M30 | 0.35 – 0.45 |
- Never exceed 0.65 for structural concrete — strength drops sharply.
- Use only clean, potable water. Impurities (salts, organic matter) weaken concrete.
- In hot climates (common in South Asia), reduce W/C further since water evaporates faster.
Practical check: The mix should be workable but not soupy. If water pools on the surface during compaction, the mix is too wet.
Curing: Why 28 Days Matters
Concrete gains strength through a chemical reaction between cement and water (hydration). This is not instant:
- After 24 hours: ~16% strength
- After 3 days: ~40% strength
- After 7 days: ~65% strength
- After 14 days: ~90% strength
- After 28 days: ~99% strength — this is when concrete reaches its design grade.
Curing Methods
- Water curing: Keep surfaces continuously wet for at least 7 days (14 days for heavy structures). Use wet burlap sacks, ponding water on slabs, or sprinklers.
- Membrane curing: Apply a curing compound that seals moisture in.
- Polythene sheeting: Cover concrete with plastic to prevent evaporation.
In Nepal’s hot summer months, curing is especially critical — rapid drying causes cracking and can reduce final strength by 20–30%.
Concrete Costs in Nepal (Approximate)
Ready-mix concrete (RMC) prices in the Kathmandu Valley:
| Grade | Price per m³ (RMC) | Notes |
|---|---|---|
| M20 | Rs. 8,000 – 12,000 | Most common for residential |
| M25 | Rs. 10,000 – 14,000 | Commercial and heavier structures |
| M30 | Rs. 12,000 – 16,000 | High-rise and special structures |
Site-mixing yourself is cheaper for small projects but less consistent in quality for large volumes. For pours above 5 m³, RMC is usually more economical and reliable.
Cement cost: Standard OPC (Ordinary Portland Cement) 50 kg bags range from Rs. 700–900 depending on the brand (Shivam, Udayapur, Hetauda, etc.) and market conditions.
Unit conversion: 1 cubic meter = 35.315 cubic feet. RMC in Nepal is ordered in m³. In the USA, concrete is ordered in cubic yards (1 yd³ = 0.765 m³).
Quick Checklist Before Ordering Concrete
- Measure all dimensions twice — mistakes multiply to the third power.
- Add 5–10% for waste, over-excavation, and uneven formwork.
- Specify the correct grade (M20 minimum for structural RCC).
- Confirm the mix ratio with your contractor or RMC supplier.
- Have curing materials ready before the pour starts.
- Never add extra water on-site to “make it flow better” — this weakens the concrete.
Concrete Calculator — Free, instant, private. Calculate cement, sand, and aggregate for any shape in any unit system. Supports all standard grades plus custom mix ratios.