# Concrete Calculator

> Calculate concrete needed for any project. Get volume, bag count, ready-mix quantity, and cost estimates for slabs, paths, and foundations.

Canonical: https://renoquant.com/concrete-calculator
Prices updated: 2026-03

## At a glance

A 3×3 m patio at 10 cm thick is 0.9 m³ of concrete — about 75 of the 25 kg premix bags (84 bags per m³), or one ready-mix truck delivery. Ready-mix runs EUR 100–140 per m³ and is cheaper and faster above 0.5 m³; below that, premix bags at EUR 5–7 each are practical. Add 10% for waste; minimum slab thickness is 5–8 cm for paths, 10 cm for patios, 12–15 cm for driveways.

## How Much Concrete Do I Need?

Whether you are pouring a patio, laying a garden path, or setting fence posts, knowing the exact amount of concrete saves money and prevents mid-project shortages. If you are setting posts for a new boundary, our [fence calculator](/fence-calculator) works out the panels, posts, and Postcrete you need. This guide covers the calculation formula, bag counts for common project sizes, and how to choose between premix bags and ready-mix delivery.

### The Concrete Volume Formula

**Volume (m3) = Length (m) x Width (m) x Depth (m)**

Always convert depth from centimetres to metres first. A 10 cm slab is 0.10 m. Then add 10% for waste (spillage, uneven subbase, formwork irregularities).

**Example**: A 4m x 3m patio slab at 10 cm depth:
- Volume = 4 x 3 x 0.10 = 1.2 m3
- With 10% waste = 1.32 m3
- Premix bags (25 kg) = 1.32 / 0.012 = 110 bags
- Ready-mix delivery = 1.4 m3 (rounded up to nearest 0.1)

### Premix Bag Count by Slab Size

The table below shows 25 kg bag counts for common slab sizes at three depths. All figures include 10% waste.

| Slab Size | 5 cm Depth | 10 cm Depth | 15 cm Depth |
|-----------|-----------|------------|------------|
| 1m x 1m | 5 bags | 10 bags | 14 bags |
| 2m x 2m | 19 bags | 37 bags | 56 bags |
| 3m x 3m | 42 bags | 83 bags | 124 bags |
| 4m x 3m | 56 bags | 110 bags | 165 bags |
| 5m x 4m | 93 bags | 184 bags | 275 bags |
| 6m x 5m | 138 bags | 275 bags | 413 bags |

For anything over 50 bags, seriously consider ready-mix delivery. Mixing 100+ bags by hand is exhausting and time-consuming.

### Recommended Depth by Project Type

Choosing the right depth ensures your concrete is strong enough for its purpose without wasting material.

| Project | Recommended Depth | Reinforcement |
|---------|------------------|---------------|
| Garden edging / stepping stones | 5 cm | None needed |
| Garden path | 5-8 cm | None or fibre mesh |
| Patio / shed base | 10 cm | Wire mesh recommended |
| Garage floor | 12-15 cm | Wire mesh + rebar at edges |
| Driveway | 12-15 cm | Wire mesh + rebar |
| Deck foundation pads | 10-15 cm | None for small pads |
| House foundation / retaining wall footing | 20-30 cm | Rebar cage |

### Premix vs Ready-Mix: When to Choose Each

| Factor | Premix Bags | Ready-Mix Delivery |
|--------|------------|-------------------|
| Best for | Small jobs under 0.5 m3 | Anything over 0.5 m3 |
| Cost per m3 | EUR 420-580 (bags at EUR 5-7 each) | EUR 100-140 (delivered) |
| Convenience | Buy from any DIY store | Need to schedule delivery |
| Minimum order | 1 bag | Usually 1 m3 minimum |
| Labour | Mixing required (by hand or mixer) | Poured directly from truck |
| Quality | Consistent if mixed correctly | Factory-batched, very consistent |
| Shelf life | 3-6 months (keep dry) | Must be used within 90 minutes |

The crossover point is roughly 0.5 m3 (about 42 bags). Below that, premix is simpler. Above that, ready-mix saves significant time, effort, and money.

### Concrete Mixing Ratios

If you are mixing concrete from raw materials (cement, sand, gravel) rather than using premix bags, use these standard ratios by volume:

| Application | Cement | Sand | Gravel | Strength |
|-------------|--------|------|--------|----------|
| General purpose | 1 | 2 | 3 | C20 — suitable for most domestic work |
| High strength (driveways, garage floors) | 1 | 1.5 | 2.5 | C30 — heavier loads |
| Foundation / footings | 1 | 2.5 | 3.5 | C15 — buried, low exposure |
| Fence posts / bedding | 1 | 3 | 6 | C7.5 — non-structural |

The water-to-cement ratio should be around 0.45-0.55 by weight. Too much water weakens the concrete. The mix should hold its shape when squeezed, with a slight sheen of moisture on the surface.

### Step-by-Step: Pouring a Concrete Slab

1. **Excavate** — Dig to the required depth plus 10-15 cm for a compacted sub-base (gravel or hardcore).
2. **Compact the sub-base** — Use a plate compactor or hand tamper. A solid base prevents cracking from settlement.
3. **Build formwork** — Use timber boards (25 mm thick) staked every 60-90 cm. Check levels and add a slight fall (1-2%) for drainage on outdoor slabs.
4. **Lay reinforcement** — Place wire mesh on spacers (chairs) so it sits in the lower third of the slab, not resting on the ground.
5. **Pour and spread** — Work in sections. Pour concrete into the formwork and spread with a rake or shovel.
6. **Level and screed** — Drag a straight edge (screed board) across the formwork rails to level the surface.
7. **Finish** — Use a bull float or hand trowel for a smooth surface. For a non-slip outdoor finish, drag a broom across the surface.
8. **Cure** — Cover with plastic sheeting or spray with curing compound. Keep moist for at least 7 days.

If you are pouring foundation pads for a raised deck, use our [decking calculator](/decking-calculator) to work out the boards, joists, and screws for the structure above. For an indoor floor slab that will be finished with laminate or vinyl, our [flooring calculator](/flooring-calculator) helps estimate the covering material.

### Common Mistakes to Avoid

- **Adding too much water** — Makes mixing easier but dramatically weakens the concrete. A stiff mix is stronger.
- **Pouring on hot days without shade** — Concrete that dries too fast cracks. Pour in the morning or erect shade cloth.
- **Skipping the sub-base** — Concrete poured directly on soil will crack from uneven settlement within 1-2 years.
- **Forgetting expansion joints** — Slabs wider than 3 metres need expansion joints every 2.5-3 metres to control cracking.
- **Underestimating quantity** — Running out mid-pour creates a cold joint (weak seam). Always round up and add 10% waste.

## Frequently Asked Questions

### How much concrete do I need for a slab?

Multiply the length x width x depth in metres to get the volume in cubic metres. For example, a 3m x 3m patio slab at 10cm deep needs 0.9 m3 of concrete, which is about 75 premix bags (25 kg each). Always add 10% for waste.

### How many bags of concrete do I need per cubic metre?

A standard 25 kg premix bag yields approximately 0.012 m3 of mixed concrete. You need about 84 bags per cubic metre. For volumes over 0.5 m3, ready-mix delivery is usually more practical and cost-effective.

### What is the difference between premix bags and ready-mix concrete?

Premix bags are pre-blended dry concrete you mix with water on site. They cost EUR 5-7 per bag but are labour-intensive. Ready-mix is wet concrete delivered by truck at EUR 100-140 per m3. Ready-mix is cheaper and faster for anything over 0.5 m3.

### How long does concrete take to cure?

Concrete reaches initial set in 24-48 hours and can bear light foot traffic after 48 hours. It reaches about 70% strength at 7 days and full design strength at 28 days. Keep it moist during the first 7 days for proper curing.

### Can I mix concrete by hand or do I need a mixer?

You can mix by hand with a shovel on a mixing board for small jobs (up to 10-15 bags). For anything larger, rent an electric or petrol concrete mixer (EUR 25-50 per day) to save time and ensure consistent quality.

### How thick should my concrete be?

Paths and garden edges need 5-8 cm. Patios and shed bases need 10 cm. Garage floors and driveways need 12-15 cm. Structural foundations need 20 cm or more. Thicker slabs need reinforcement mesh.

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Interactive version with calculator: https://renoquant.com/concrete-calculator. Figures are estimates; final quotes vary by site conditions, materials, and region.