Concrete Calculator
✓ Free online calculator · No signup · Instant results
Ordering too little concrete means a costly second delivery; ordering too much wastes money and creates disposal problems. This professional concrete calculator gives you an accurate volume estimate with a 10% waste buffer, then converts it to bags (25 kg, 40 kg) or ready-mix cubic meters — and shows which option is cheaper for your project. Supports 6 structure types: slabs, circular slabs, rectangular and circular columns, footings, and stairs. Plan your project dimensions with our Area Calculator or Volume Calculator.
Units:
You need 0 m³
Volume (ft³) 0 ft³
Volume (liters) 0 L
Material Quantity (with 10% waste allowance)
25kg Bags 0 bags
40kg Bags 0 bags
Ready-Mix 0 m³
Note: Results include 10% waste allowance. Actual usage depends on conditions
and pouring technique. For large projects, consult with a contractor.
Worked Examples
Scenario 1 — Patio Slab (4 m × 5 m × 10 cm)
- Volume: 4 × 5 × 0.10 = 2.0 m³
- With 10% waste: 2.2 m³
- 25kg bags needed: 2.2 ÷ 0.012 ≈ 184 bags
- Cost (bags @ $6 each): 184 × $6 = $1,104
- Ready-mix @ $130/m³ + $50 delivery: 2.2 × $130 + $50 = $336 — Save $768!
Verdict: Ready-mix wins decisively for a 2 m³ patio — bags would cost 3× more.
Scenario 2 — Fence Posts (8 posts, 30 cm × 30 cm × 80 cm each)
- Volume per post: 0.30 × 0.30 × 0.80 = 0.072 m³
- Total 8 posts: 8 × 0.072 = 0.576 m³
- With 10% waste: 0.634 m³
- 25kg bags needed: 0.634 ÷ 0.012 ≈ 53 bags
- Cost (bags @ $6 each): 53 × $6 = $318
- Ready-mix: minimum order ~1 m³ + delivery ≈ $180+ — not cost-effective
Verdict: Bags are the right choice for fence posts — small volume, flexible timing.
Scenario 3 — Driveway (6 m × 3 m × 15 cm)
- Volume: 6 × 3 × 0.15 = 2.7 m³
- With 10% waste: 2.97 m³
- 25kg bags needed: 2.97 ÷ 0.012 ≈ 248 bags
- Cost (bags @ $6 each): 248 × $6 = $1,488
- Ready-mix @ $130/m³ + $50 delivery: 2.97 × $130 + $50 = $436
Verdict: Ready-mix saves over $1,000. Order 3.0 m³ (next 0.25 increment) — standard residential driveway order.
Concrete Mix Ratios by Application
Choosing the right mix ratio is as important as calculating the volume. Below are standard mix ratios for common applications — use these when mixing bagged concrete manually.
| Application | Strength Grade | Cement : Sand : Gravel | Water-Cement Ratio |
|---|---|---|---|
| Footings / Foundations | C20/25 | 1 : 2 : 3 | 0.50 |
| Driveways / Patios | C25/30 | 1 : 1.5 : 2.5 | 0.45 |
| Structural Elements | C30/37 | 1 : 1 : 2 | 0.40 |
| Fence Posts (general) | C20/25 | 1 : 2 : 3 | 0.50 |
Note: Ratios are by weight. Pre-bagged concrete mixes are pre-proportioned — just add water per bag instructions. For ready-mix, specify the strength grade (e.g., C25/30) and the supplier handles the mix design.
Formulas & Calculations
The calculator computes the raw volume of your structure using standard geometric formulas, adds a 10% waste buffer, and converts to material quantities:
- Slab / Patio / Driveway: Length × Width × Thickness (e.g., 3m × 4m × 0.1m = 1.2 m³)
- Circular Slab: π × (Diameter ÷ 2)² × Thickness
- Rectangular Column: Length × Width × Height
- Circular Column: π × (Diameter ÷ 2)² × Height
- Footing / Foundation: Length × Width × Depth
- Stairs: Sum of each step's volume (width × rise × run) + 15% for structural support and landings
Why add 10% for waste? The margin covers spillage during pouring, uneven ground that absorbs extra volume, residue left in the mixer, and measurement inaccuracies. Skipping this buffer is a common DIY mistake that causes a project to run short mid-pour — when it's too late to order more.
Bags vs Ready-Mix — when does each win?
- Bagged concrete makes sense for projects under 0.5 m³ (about 40 bags of 25 kg), difficult-access areas, or staged work where you pour in multiple sessions. You control the timing and don't pay for truck delivery fees.
- Ready-mix concrete wins for projects above 1 m³ — the per-m³ cost is much lower, quality is factory-controlled, and you can pour the entire slab in one session (important for structural integrity). Most suppliers have a minimum of 0.5-1 m³ and charge a short-load fee below that.
- The break-even zone is 0.5–1.0 m³: enter your bag and ready-mix prices to see which is cheaper for your specific project.
FAQ
- How much concrete does one 25kg bag yield?
- A standard 25 kg bag yields approximately 0.012 m³ (12 liters) of mixed concrete. A 40 kg bag yields about 0.019 m³, and a 50 kg bag gives roughly 0.024 m³. Actual yield varies slightly with water content and mix design.
- When should I use bags vs ready-mix concrete?
- Use bags for projects under 0.5 m³, work in hard-to-access areas, or jobs done in multiple sessions. Choose ready-mix for projects over 1 m³ when you need consistent quality and a single continuous pour. The economic break-even point is typically 0.5–1.0 m³.
- Why add 10% extra concrete for waste?
- The 10% buffer accounts for spillage during pouring, absorption into uneven or dry ground, concrete left in the mixer drum, and minor measurement errors. Running short mid-pour is one of the most costly DIY mistakes — always over-order slightly.
- What is the water-to-cement ratio for concrete?
- A typical water-to-cement (w/c) ratio is 0.45–0.55. Lower ratios (e.g., 0.40) produce stronger, denser concrete but are harder to work with. Higher ratios improve workability but reduce strength. For driveways use w/c ≈ 0.45; for foundations 0.50 is common.
- What are the different concrete strength grades (C20, C25, C30)?
- Concrete grades denote compressive strength. C20/25 (characteristic strength 20 MPa, test cylinder 25 MPa) suits footings and fence posts. C25/30 is standard for driveways and patios. C30/37 is used for structural beams and columns. The mix ratio shifts from 1:2:3 (C20) to 1:1:2 (C30) by cement:sand:aggregate weight.
- How do I calculate concrete for a driveway slab?
- Multiply Length × Width × Thickness. Example: a 6 m × 3 m driveway at 150 mm thick = 6 × 3 × 0.15 = 2.7 m³. Add 10% waste → 2.97 m³. At 1 m³ ready-mix costing $120 + $50 delivery, total cost ≈ $407. Use this calculator's Slab type for instant results.
- How do I calculate concrete for fence posts?
- Use the Rectangular Column type for each post hole. Example: 8 posts × (0.3 m × 0.3 m × 0.8 m) = 0.576 m³. With 10% waste that is 0.63 m³ — well within bagged-concrete territory. Use C20/25 mix (1:2:3) for post holes.
- How do I calculate concrete for a concrete wall?
- Treat it as a rectangular slab on its side: Length × Height × Thickness. A 5 m long, 1.2 m tall, 0.2 m thick wall = 1.2 m³. Add 10% → 1.32 m³. For retaining or structural walls always consult an engineer and use at least C25/30.
- How do I estimate the cost of a concrete project?
- Open the "Add Prices" section in the calculator. Enter your local bag price ($5–8 per 60 lb bag), ready-mix price ($100–150 per cubic yard), and delivery cost. The calculator automatically recommends the cheaper option and shows the total cost for each material type.
- How long does concrete take to cure and set?
- Concrete is walkable after 24–48 hours and reaches about 70% strength in 7 days. Full design strength (28-day strength) is reached in 28 days. Keep the surface moist for at least 7 days (wet curing) to prevent premature drying and cracking.
- What is the difference between concrete and cement?
- Cement is an ingredient in concrete — it is the binding powder (Portland cement) that reacts with water. Concrete is the final mix of cement, sand (fine aggregate), gravel (coarse aggregate), and water. Mortar uses cement + sand (no gravel) and is for joints, not structural pours.
- How do I prevent concrete from cracking?
- Use the correct water-cement ratio (not too much water), add control joints every 1–1.5 m in slabs, cure properly (keep moist for 7 days), avoid pouring in freezing temperatures, and use reinforcement (rebar or fiber mesh) in structural elements. Hairline cracks are normal; wide cracks indicate structural problems.
- How do I calculate concrete for stairs?
- The calculator sums each step's volume (width × rise height × run depth) and adds 15% for landings and structural support. Example: 12 steps, 1 m wide, 0.17 m rise, 0.28 m run → 12 × 1 × 0.17 × 0.28 = 0.571 m³ + 15% = 0.657 m³. For complex or public stairs always get an engineer's review.
- Can I mix concrete in cold or hot weather?
- Avoid pouring when temperature is below 5 °C (41 °F) — hydration slows dramatically and frost can destroy fresh concrete. In hot weather above 32 °C (90 °F) use chilled water or ice and pour early morning. Add a concrete accelerator in cold conditions or a retarder in hot conditions if needed.
- What is the standard thickness for a concrete driveway or patio?
- Residential patios: 100 mm (4 in). Driveways for passenger cars: 100–150 mm (4–6 in). Driveways for heavy vehicles or RVs: 150–200 mm (6–8 in). Footpaths: 75–100 mm (3–4 in). Always use a compacted gravel sub-base (100–150 mm) beneath any slab to prevent settling.