Concrete & Cement Calculator
Calculate concrete volume for slabs, footings, walls, columns, and circular pads. Shows bags needed (25 kg, 40 kg, 60 lb, 80 lb), cubic yards, and optional cost estimate.
Quick presets:
Press Enter to calculate · Esc to reset
What Is the Concrete & Cement Calculator?
Concrete volume calculation is straightforward geometry, but getting the number right before you order saves money, time, and the headache of a second truck delivery. This calculator covers the five most common pour types: slabs, footings, walls, columns/piers, and circular pads. Enter your dimensions in any unit (mm, cm, m, in, or ft), add a waste allowance, and get the volume in m³, yd³, ft³, and litres alongside the exact number of premix bags in four popular sizes.
The waste allowance accounts for spillage, over-pour, uneven sub-base, and slight measurement errors. For regular rectangular slabs with good formwork, 5–8% is typically sufficient. For circular pours, irregular ground, or first-time DIY work, 10% is the safer choice.
Formula
Core Volume Formulas
Slab / Footing: V = L × W × T
Wall: V = L × T_wall × H
Column / Pier: V = π × (D/2)² × H
Circular Slab: V = π × (D/2)² × T
With waste: V_order = V × (1 + waste%/100)
Bags needed: N = ⌈V_order / yield_per_bag⌉
Unit conversion: 1 m³ = 1.308 yd³ = 35.31 ft³ = 1,000 L
| Symbol | Meaning | Notes |
|---|---|---|
| V | Concrete volume | Net volume without waste |
| L | Length | Longest horizontal dimension |
| W | Width (or thickness) | Shorter horizontal dimension |
| T | Thickness (slab/circ.) | Vertical depth of flat pour |
| H | Height | Vertical dimension of walls/columns |
| D | Diameter | Full width of circular section |
| V_order | Volume to order | Includes waste allowance |
| N | Number of bags | Always rounded up (ceiling) |
BAG YIELD REFERENCE
25 kg
0.0124 m³
~81 bags/m³
40 kg
0.0190 m³
~53 bags/m³
60 lb
0.0127 m³
~79 bags/m³
80 lb
0.0170 m³
~59 bags/m³
How to Use
- 1Select pour type: Choose slab, footing, wall, column/pier, or circular slab. The diagram updates to show the relevant dimensions.
- 2Pick your unit: Select mm, cm, m, in, or ft, all inputs use the same unit. Metric by default; switch to feet/inches for US projects.
- 3Use a preset (optional): Tap any quick preset (Driveway, Patio, etc.) to pre-fill typical dimensions. Adjust as needed.
- 4Enter dimensions: Fill in length/width or diameter and depth/thickness depending on the pour type.
- 5Set waste %: 5% for clean, simple pours; 8–10% for irregular ground, first pours, or slabs with cut-outs.
- 6Add ready-mix price (optional): Enter the price per m³ from your supplier to get an instant cost estimate.
- 7Calculate: Press the button or hit Enter. Review m³, yd³, bag counts, and the step-by-step working.
Example Calculation
Example 1, Concrete Driveway Slab
6 m long × 3 m wide × 100 mm thick, 10% waste.
V = 6 × 3 × 0.1 = 1.800 m³
With 10% waste: 1.800 × 1.10 = 1.980 m³
= 2.589 yd³ | 69.93 ft³ | 1,980 litres
40 kg bags: ⌈1.980 / 0.019⌉ = 105 bags
→ Ordering tip: use ready-mix truck (more economical above ~0.5 m³)
Example 2, Circular Concrete Pad
Diameter 1.2 m, 150 mm thick, 8% waste.
V = π × (0.6)² × 0.15 = π × 0.36 × 0.15 = 0.1696 m³
With 8% waste: 0.1696 × 1.08 = 0.1832 m³
40 kg bags: ⌈0.1832 / 0.019⌉ = 10 bags
Example 3, Foundation Wall
8 m long × 200 mm thick × 600 mm deep, 10% waste.
V = 8 × 0.2 × 0.6 = 0.960 m³
With 10% waste: 0.960 × 1.10 = 1.056 m³
40 kg bags: ⌈1.056 / 0.019⌉ = 56 bags
Understanding Concrete & Cement
How Concrete Volume Calculations Work
Every concrete pour, whether a backyard patio or a commercial footing, comes down to the same question: how many cubic metres (or cubic yards) of concrete do I need? The maths is simple geometry, but the consequences of getting it wrong are significant. Too little and you have a cold joint, a structural weak point where fresh and partially-set concrete meet. Too much and you are paying for material you do not use. A 10% overage is the industry-standard buffer.
Pour Type Reference
| Pour type | Formula | Common use | Typical waste |
|---|---|---|---|
| Slab | L × W × T | Driveways, patios, floors | 8–10% |
| Footing | L × W × D | Strip & pad foundations | 10–15% |
| Wall | L × T × H | Retaining walls, basement walls | 8–10% |
| Column / Pier | π(D/2)²× H | Deck piers, fence posts | 10% |
| Circular Slab | π(D/2)²× T | Round pads, manholes, well covers | 8–10% |
Understanding Concrete Mix Ratios
The classic 1:2:3 mix (1 part cement : 2 parts sand : 3 parts gravel by volume) produces a general-purpose concrete of roughly 20–25 MPa compressive strength. Modern bagged premixes use carefully tested ratios that typically yield 25–28 MPa. The water-cement (w/c) ratio is the single most important factor for strength, lower w/c produces stronger concrete, down to the workability limit of about 0.40–0.42.
Reinforced concrete (rebar or mesh) requires adequate cover, minimum 40 mm for slabs exposed to the weather, 20 mm for internal slabs. Without adequate cover, moisture reaches the steel, causing rust expansion and concrete spalling.
Slab Thickness Guidelines
| Application | Thickness (mm) | Thickness (inches) | Notes |
|---|---|---|---|
| Garden path, non-load | 75 | 3 | Light foot traffic only |
| Patio / outdoor area | 100 | 4 | Standard residential |
| Driveway (cars) | 100–125 | 4–5 | Mesh reinforcement recommended |
| Driveway (vans, SUVs) | 150 | 6 | Rebar often required |
| Garage floor | 100–125 | 4–5 | Include a vapour barrier |
| Shed floor | 75–100 | 3–4 | Depends on load |
Control Joints and Curing
Concrete shrinks by roughly 0.04–0.08% as it cures. For slabs larger than about 3 m × 3 m, control joints (saw cuts made 6–24 hours after pour, to a depth of one-quarter the slab thickness) direct shrinkage cracking to predetermined lines rather than random surface cracks.
Proper curing is essential for full strength development. Keep the slab moist for a minimum of 7 days (wet hessian, plastic sheeting, or a curing compound). Concrete reaches approximately 70% of its 28-day strength after 7 days, and nearly 100% at 28 days. Avoid pouring below 5 °C or above 35 °C without special measures.
Bagged vs Ready-Mix Concrete
- ›Bagged premix: ideal for small jobs under 0.5 m³, no minimum order, mix at your own pace
- ›Ready-mix truck: economical above 0.5–1 m³, consistent quality, one continuous pour (no cold joints)
- ›Mini-mix or volumetric mixers: good for 0.3–3 m³, mix-on-site, charge by volume used
- ›Self-mix from raw materials: cost-effective for large projects with a hired mixer
Ready-mix suppliers typically have a minimum charge (often 1 m³) and add surcharges for small loads, short-load, or Saturday delivery. Get quotes from at least two suppliers, prices per cubic metre vary considerably by region and season.
Ordering Recommendations
- ›Always round your final volume up to the nearest 0.25 m³ for ready-mix orders
- ›For bagged concrete, always buy one or two extra bags as a contingency
- ›Order same-day delivery if possible to avoid premature setting in the truck
- ›Confirm the truck can access your site before booking, a pump hire may be needed
Frequently Asked Questions
How many bags of concrete per cubic metre?
It depends on the bag size and the mix yield:
- ›25 kg bag → approx. 0.0124 m³ → about 81 bags per m³
- ›40 kg bag → approx. 0.0190 m³ → about 53 bags per m³
- ›60 lb (27 kg) bag → approx. 0.0127 m³ → about 79 bags per m³
- ›80 lb (36 kg) bag → approx. 0.0170 m³ → about 59 bags per m³
Yields vary slightly between brands. Always check the specific product label, this calculator uses typical average values.
How thick should a concrete slab be?
Recommended slab thickness by use:
- ›Foot traffic only (paths, patios): 75–100 mm (3–4 inches)
- ›Residential driveway (cars): 100–125 mm (4–5 inches)
- ›Driveway with vans/trucks: 150 mm (6 inches)
- ›Garage floor: 100–125 mm (4–5 inches)
- ›Structural slabs: engineer-specified, do not rely on this calculator
Thicker slabs resist cracking better, especially on unstable or expansive soils.
How much waste percentage should I add?
- ›5%, simple rectangular slab, good formwork, experienced pour
- ›8%, standard recommendation for most residential slabs
- ›10%, irregular shapes, uneven sub-base, DIY first pour
- ›10–15%, footings in trenches (hard to measure accurately)
Running short mid-pour and needing to add a cold joint is a serious structural issue, always add a buffer rather than ordering too tight.
Should I use bagged concrete or ready-mix?
- ›Bagged concrete: best for small pours under 0.3–0.5 m³, no truck access, DIY repairs and posts
- ›Ready-mix (truck delivery): more economical above 0.5 m³, consistent quality, faster pours
- ›Consider minimum load charges, many suppliers charge for a full m³ even if you order less
Ready-mix eliminates the labour of mixing bags and gives a more uniform result for large pours.
What concrete mix strength do I need?
- ›C20 / 20 MPa / 3000 psi, light-duty garden paths, non-structural fills
- ›C25 / 25 MPa / 3500 psi, residential slabs, driveways, patios (most common)
- ›C30 / 30 MPa / 4000 psi, driveways with heavy vehicles, garage floors
- ›C35+, structural elements; specify with an engineer
Standard bagged "ready-mix" or "general purpose" concrete is typically equivalent to C25–C28.
What is a concrete footing and how is it calculated?
A footing (or foundation) is a concrete pad placed below the frost line to transfer structural loads to stable soil. Volume is the same as a rectangular slab, length × width × depth, but footings are typically deeper and narrower than floor slabs. They often sit in trenches, which makes accurate measurement harder; use 10–15% waste allowance for trench footings to account for overdig and irregular sides.
How do I calculate concrete for a round column or circular pad?
Use the cylinder formula: V = π × (D/2)² × H, where D is the diameter and H is the height or thickness.
- ›Column diameter 300 mm, height 500 mm: V = π × 0.15² × 0.5 = 0.0353 m³
- ›Circular pad diameter 1.2 m, thickness 150 mm: V = π × 0.6² × 0.15 = 0.1696 m³
Select "Column / Pier" or "Circular Slab" in the calculator and enter the diameter and height/thickness.
Why does the calculator always round bags up?
Bags are discrete units, you cannot buy a fraction of a bag. The calculator always rounds up using the ceiling function (⌈N⌉) so you are guaranteed enough material. For example, if the exact requirement is 52.3 bags, the calculator returns 53. Combined with your waste allowance, this ensures you have sufficient concrete without running short mid-pour.