Concrete Calculator - Volume, Bags, Rebar and Ready-Mix
Get the exact volume, cement bags, sand, aggregate and water for slabs, footings, columns, beams, walls and stairs. Includes rebar estimate, formwork area, ready-mix truck comparison and cost - all from real engineering formulas.
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Total Concrete Volume
Material Breakdown
| Material | Volume | Weight | Bags / Notes |
|---|
Additional Estimates
| Item | Estimate |
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Using the Concrete Calculator for Slabs, Footings, Columns, Beams and Walls
Pick the concrete element, enter its dimensions and quantity, and the calculator returns total volume plus every material you need to order.
Typical Concrete Volume per Element
Quick reference for common structural elements. Use this to sanity-check your calculated volumes.
| Element | Typical size | Volume | Notes |
|---|---|---|---|
| House slab (100 m²) | 100 mm thick | 10 m³ | Standard residential floor |
| Driveway slab (50 m²) | 150 mm thick | 7.5 m³ | Car traffic |
| Pad footing | 1.2 × 0.6 × 0.5 m | 0.36 m³ | Per footing |
| Strip footing | 0.6 × 0.6 m | 0.36 m³/m | Per linear metre |
| Column 300 × 300 | 3 m tall | 0.27 m³ | Per column |
| Column 400 × 400 | 3 m tall | 0.48 m³ | Per column |
| Round column 300 mm | 3 m tall | 0.21 m³ | Per column |
| Beam 230 × 450 | 6 m long | 0.62 m³ | Per beam |
| Retaining wall 4 m | 2.4 × 0.2 m | 1.92 m³ | Per wall |
| Concrete stairs (12 steps) | 1.1 m wide | 0.5 m³ | Typical straight flight |
Concrete Mix Grades and Their Uses
Every grade has a defined cement, sand and aggregate ratio and a 28-day compressive strength. Choose the lowest grade that meets your structural needs.
| Grade | Ratio (C:S:A) | Strength | Best for |
|---|---|---|---|
| M5 | 1 : 5 : 10 | 5 MPa | Lean base, backfill, kerbs |
| M7.5 | 1 : 4 : 8 | 7.5 MPa | Mass fill, non-structural |
| M10 | 1 : 3 : 6 | 10 MPa | Light foundations, garden walls |
| M15 | 1 : 2 : 4 | 15 MPa | House slabs, footings |
| M20 | 1 : 1.5 : 3 | 20 MPa | Beams, columns, RC slabs |
| M25 | 1 : 1 : 2 | 25 MPa | Heavy-duty structural |
Site-Mix vs Ready-Mix: Which Is Cheaper?
The break-even point depends on volume, labour cost and access. These are the standard guidelines.
| Volume | Recommendation | Why |
|---|---|---|
| Under 1 m³ | Site mix (bags) | Delivery not economical |
| 1 - 3 m³ | Site mix or small truck | Depends on labour cost |
| 3 - 10 m³ | Ready-mix | Consistency, speed, less waste |
| 10 - 30 m³ | Ready-mix (multiple trucks) | Site mix unrealistic |
| Over 30 m³ | Ready-mix with pump | Best cost and quality |
How the Concrete Calculator Works
Every result uses the standard materials estimation method that quantity surveyors apply on site. Here is the exact process.
- Volume of each element is calculated from your dimensions using the correct geometric formula.
- Total volume multiplies by the quantity of elements you enter (footings, columns, etc).
- Dry volume factor of 1.54 is applied to the wet volume to account for voids and shrinkage.
- Cement, sand and aggregate volumes come from the mix ratio - e.g. for M20 (1:1.5:3), cement is 1/5.5 of the dry volume.
- Cement weight uses a density of 1,440 kg/m³, then converts to your chosen bag size.
- Water is calculated from the water-cement ratio so you get a workable, strong mix.
- Wastage is added on top so you always have spare material on site.
- Rebar estimate uses the reinforcement ratio method - typically 0.5-2% of concrete volume depending on structural demand.
- Formwork area is estimated from the contact surface between concrete and form panels.
Concrete Formula and Worked Example
These are the exact formulas used by the calculator. Bookmark this section if you want to verify the numbers yourself.
- Volume of concrete = area × thickness (slab) or L × W × H (block)
- Dry volume = wet volume × 1.54
- Cement volume = (cement part ÷ total parts) × dry volume
- Cement weight = cement volume × 1,440 kg/m³
- Cement bags = cement weight ÷ bag size
- Sand volume = (sand part ÷ total parts) × dry volume
- Aggregate volume = (aggregate part ÷ total parts) × dry volume
- Water (litres) = cement weight × water-cement ratio
- Rebar weight = volume × reinforcement ratio × 7,850 kg/m³
Worked example: A 5 m × 4 m slab, 150 mm thick, M20 concrete. Volume = 5 × 4 × 0.15 = 3 m³. Dry volume = 3 × 1.54 = 4.62 m³. Total mix parts = 1 + 1.5 + 3 = 5.5. Cement = 4.62 × (1/5.5) = 0.84 m³ = 1,210 kg = 24.2 bags of 50 kg (25 with 5% waste). Sand = 4.62 × (1.5/5.5) = 1.26 m³. Aggregate = 4.62 × (3/5.5) = 2.52 m³. Water at 0.5 w/c = 605 litres.
Frequently Asked Questions About Concrete Calculation
How much concrete do I need for a 5 m × 4 m slab?
Volume = length × width × thickness. For a 5 × 4 m slab at 100 mm thick, you need 2 m³. At 150 mm thick, you need 3 m³. At 200 mm thick, you need 4 m³. Add 5 - 10% for wastage. The calculator above gives exact numbers for any dimensions.
How many bags of cement in 1 m³ of concrete?
It depends on the mix grade. For M20 (1:1.5:3) you need roughly 8.1 bags of 50 kg. For M15 (1:2:4) it is 6.4 bags. For M25 (1:1:2) it is about 11 bags. Higher grades need more cement because the concrete is stronger.
How do I calculate concrete for footings?
Footing volume = length × width × depth. Multiply by the number of footings. For example, 8 pad footings each 1.2 × 0.6 × 0.5 m = 8 × 0.36 = 2.88 m³. Always add 5 - 10% for wastage and over-excavation. The calculator has a Footing mode with the quantity field built in.
How much does 1 m³ of concrete weigh?
Standard concrete weighs about 2,400 kg per cubic metre (150 lb per cubic foot). Higher grades with more cement are slightly heavier; lightweight aggregate concrete can be 1,600 - 1,900 kg/m³. Water weighs 1,000 kg/m³, so the concrete is 2.4x heavier than the water content.
What is the difference between M15, M20 and M25 concrete?
M stands for mix and the number is the compressive strength in MPa after 28 days. M15 is standard for house slabs and footings. M20 is for structural beams and columns. M25 is heavy-duty and used where high loads are expected. Higher grade = more cement = higher cost but greater strength.
How thick should a concrete slab be?
For footpaths and garden slabs, 75 - 100 mm is fine. For residential floors, 100 - 150 mm is standard. For driveways and car parks, 150 - 200 mm. For industrial floors, 200 - 300 mm. Thicker slabs need more concrete but resist heavier loads. The calculator handles any thickness you enter.
Do I need rebar for my slab?
For any structural slab, yes. Reinforcement ratios typically range from 0.5% of volume (light) to 2% (heavy). For a 3 m³ slab with medium reinforcement, you would need about 3 × 0.01 × 7,850 = 235 kg of steel. The calculator gives you this estimate based on the reinforcement level you choose.
How long does concrete take to cure?
Concrete sets in 24 - 48 hours but needs 28 days for full strength. Keep it moist for the first 7 days - this is when 70% of the strength develops. Foot traffic is OK after 24 hours, vehicles after 7 days, full structural use after 28 days. Cold weather slows curing; hot weather needs more watering.
Can I mix concrete by hand?
Yes for volumes up to 1 m³. Use a clean surface or mixing board. Dry-mix cement and sand until uniform, add aggregate and mix again, form a crater and add water slowly. Hand-mixing is 3 - 4x slower than a mixer but produces the same quality if done carefully. For anything over 1 m³, hire a mixer or order ready-mix.
What is the correct water-cement ratio for concrete?
For structural concrete, use 0.45 to 0.50. This gives workable, strong, durable concrete. Above 0.60 the concrete loses strength rapidly and cracks. Below 0.40 it becomes difficult to place and mix. The calculator defaults to 0.50, which works for most slabs, footings and columns.
Should I order ready-mix or mix on site?
Under 1 m³ - mix on site. Over 3 m³ - ready-mix is usually cheaper and more consistent. Between 1 - 3 m³ depends on your labour cost and site access. Ready-mix guarantees the correct mix design and saves hours of labour, but small orders carry a delivery premium.
How many wheelbarrows in 1 m³ of concrete?
A standard builder's wheelbarrow holds about 65 - 100 litres. So 1 m³ is roughly 10 - 15 wheelbarrow loads. This is important when planning labour - moving 3 m³ of concrete by wheelbarrow takes 2 people about an hour.
Why is concrete measured in cubic metres?
Cubic metres (m³) is the standard unit because concrete is ordered and priced by volume. When you call a ready-mix supplier, you tell them the total volume and the grade - for example "6 m³ of M20". Site-mix concrete is calculated the same way but you buy the ingredients separately.
What is the minimum concrete cover for rebar?
For slabs cast against formwork, 20 mm. For beams and columns, 25 - 40 mm. For footings cast against earth, 50 - 75 mm. The cover protects the steel from corrosion and provides fire resistance. Always follow the reinforcement drawing or your local code.
References & Standards
- IS 456:2000 - Plain and Reinforced Concrete Code of Practice (India)
- BS EN 206 - Concrete: Specification, performance, production and conformity
- ACI 211.1 - Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete
- ACI 318 - Building Code Requirements for Structural Concrete
- BS 8500 - Concrete: Complementary British Standard to BS EN 206
- ASTM C94 / C94M - Standard Specification for Ready-Mixed Concrete
- AS 3600 - Concrete structures (Australia)
- NIS 550 - Nigerian Industrial Standard Building Code
- NBC 2016 - National Building Code of India, Part 6