Concrete Calculator
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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.
Whether you're pouring a backyard patio, setting fence posts, or building a garage foundation, getting the concrete quantity right the first time saves hundreds of dollars and hours of frustration. Enter your measurements below and the calculator instantly shows how many bags to buy or how much ready-mix to order — with real-time cost comparison when you add local prices.
Material Quantity (with 10% waste allowance)
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.
Concrete Prices in the US & UK (2026 Reference)
Prices vary by region, grade, and order volume. Below are typical ranges for residential projects — use the cost section above to enter your local prices for an accurate estimate.
| Grade | Ready-Mix (US, $/yd³) | Ready-Mix (UK, £/m³) | Bagged 25 kg (UK, £) | Bagged 80 lb (US, $) |
|---|---|---|---|---|
| C20/25 (general) | $125–155 | £95–115 | £5.40–7.00 | $5.28–6.98 |
| C25/30 (driveways) | $140–175 | £115–135 | £6.00–8.00 | $6.48–8.48 |
| C30/37 (structural) | $160–195 | £125–155 | £7.00–9.00 | $7.50–9.50 |
| Delivery | $55–180/load | £200–300 min | — | — |
Note: US prices per cubic yard (1 yd³ = 0.765 m³). Short-load surcharge applies below 10 yd³ ($40–60/yd³ extra). UK pump hire £250–800. Prices as of early 2026.
Concrete Estimation Tips by Project Type
Different projects have different challenges when estimating concrete. Use these practical tips to avoid costly mistakes on common residential and small commercial jobs.
Driveways & Garage Floors
A standard single-car driveway (3 m × 6 m × 150 mm) needs about 2.7 m³ before waste allowance. Always use a minimum thickness of 100 mm (4 in) for passenger cars and 150 mm (6 in) if heavy vehicles will park on it. Prepare a compacted gravel sub-base of at least 100 mm to prevent settling and cracking. Use C25/30 grade concrete and consider adding fiber mesh reinforcement for extra durability. Order ready-mix for any driveway — the volume is always too large for bags to be cost-effective.
Patios & Garden Slabs
Patios typically require 100 mm (4 in) thickness. For a standard 4 m × 5 m patio, expect about 2.0 m³ of concrete. Ensure proper drainage by sloping the slab 1–2% away from the house (roughly 10–20 mm per meter). This prevents water pooling against the foundation. C20/25 is usually sufficient for patios unless you plan to park vehicles on them.
Fence Posts & Mailbox Posts
Each fence post hole typically measures 300 mm × 300 mm × 800 mm (or 300 mm diameter × 800 mm deep for round holes). For a standard garden fence with 8–10 posts, total concrete is usually 0.5–0.7 m³ — firmly in "bags" territory. Mix each post individually and allow it to set before attaching fence panels. Use fast-setting concrete for posts so you can attach rails the same day.
Foundations & Footings
Strip foundations for garden walls and extensions are typically 400 mm wide × 300 mm deep. A 10 m perimeter foundation requires about 1.2 m³. For structural foundations, always get an engineer's specification — they will specify the exact concrete grade (usually C25/30 or higher), reinforcement requirements, and minimum frost depth. In the US, footings must extend below the frost line (varies by region: 300 mm in the South to 1,500 mm in northern states).
Concrete Stairs
Outdoor stairs are one of the trickiest concrete projects to estimate because the volume is not a simple box shape. The calculator sums each step individually and adds 15% for the structural slab beneath the steps and any landings. For safety, ensure each step has a consistent rise height (170 mm is standard) and a run depth of at least 250 mm. Building codes typically require a handrail for more than 3 steps.
How Much Does Concrete Weigh?
Wet concrete weighs approximately 2,400 kg per cubic meter (150 lb per cubic foot). This is important for planning delivery access and structural load. A single 25 kg bag produces only 12 liters of concrete but the mixed result still weighs about 29 kg (the water adds weight). For ready-mix deliveries, a standard truck carries 6–8 m³, weighing 14,400–19,200 kg — ensure your access route can support this.
Concrete Weight & Coverage Reference
Use this reference table to estimate how much your concrete project will weigh and how far your materials will go. Useful for planning transport, sub-base requirements, and structural loads.
| Concrete Type | Density (kg/m³) | Coverage per 25 kg bag | Typical Use |
|---|---|---|---|
| Standard (C20–C30) | 2,300–2,400 | 0.012 m³ (12 L) | Most residential projects |
| Lightweight aggregate | 1,400–1,800 | 0.015–0.018 m³ | Roof decks, insulating fills |
| High-strength (C40+) | 2,400–2,500 | 0.011 m³ | Commercial structures, bridges |
| Fiber-reinforced | 2,300–2,400 | 0.012 m³ | Slabs on grade, pavements |
| Fast-setting | 2,200–2,300 | 0.012 m³ | Post holes, repairs, cold weather |
Tip: When buying bags from a home improvement store (Home Depot, Lowe's, B&Q, Wickes), always check if the bag is pre-mixed concrete (just add water) or cement-only (you need to add sand and gravel separately). Pre-mixed bags are more expensive per m³ but far more convenient for small projects.
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.