01 Concrete slab calculator
Enter your slab size, pick your region and get the volume to order, the number of bags and an estimated ready-mix cost. It works for slabs, pads, patios, garage floors and driveways.
Assumes a flat, rectangular slab and normal-weight concrete. Bag yields: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 50 lb ≈ 0.375 ft³, 40 lb ≈ 0.30 ft³.
02 The concrete slab formula
Imperial (US)
yd³ = (L ft × W ft × T in ÷ 12) ÷ 27
Metric (UK · AU · NZ)
m³ = L m × W m × T mm ÷ 1000
Everything else (bags, cost, weight) is derived from that single volume. Convert between systems with 1 yd³ = 0.7646 m³ and 1 m³ = 1.308 yd³.
03 How to calculate concrete step by step
- Measure the slab. Record length and width in feet (or metres) and thickness in inches (or millimetres). Split L-shaped slabs into rectangles and add them together.
- Convert thickness. Divide inches by 12 to get feet (4 in → 0.333 ft), or millimetres by 1,000 to get metres (100 mm → 0.1 m).
- Multiply L × W × T to get cubic feet or cubic metres.
- Convert to cubic yards. Divide cubic feet by 27. (Skip this step if you are working in m³.)
- Add 5–10% waste for uneven subgrade, spillage and form movement.
- Round up to the next quarter yard (or 0.1 m³) when ordering ready-mix, or to the next whole bag if you are mixing yourself.
04 Concrete needed for common slab sizes
| Slab size | Thickness | Net yd³ | Order (+10%) | 80-lb bags | Typical use |
|---|---|---|---|---|---|
| 10 × 10 ft | 4 in | 1.23 | 1.36 yd³ | 62 | Small patio / shed base |
| 12 × 12 ft | 4 in | 1.78 | 1.96 yd³ | 88 | Patio |
| 10 × 20 ft | 4 in | 2.47 | 2.72 yd³ | 123 | Single-car driveway strip |
| 20 × 20 ft | 4 in | 4.94 | 5.43 yd³ | 245 | Two-car garage floor (light use) |
| 20 × 20 ft | 6 in | 7.41 | 8.15 yd³ | 367 | Heavy-duty garage / workshop |
| 24 × 24 ft | 6 in | 10.67 | 11.73 yd³ | 528 | Large garage or workshop |
Square-footage shortcut: one cubic yard covers 81 ft² at 4 in, 65 ft² at 5 in and 54 ft² at 6 in (before waste).
05 How many bags of concrete do you need?
Divide the volume you need (in cubic feet) by the yield printed on the bag. Always round up: a half-filled form is worse than a spare bag.
| Bag size | Yield (ft³) | Bags per yd³ | Bags per 4-in, 100 ft² slab |
|---|---|---|---|
| 40 lb (18 kg) | 0.3 | 90 | 112 |
| 50 lb (22.7 kg) | 0.375 | 72 | 89 |
| 60 lb (27 kg) | 0.45 | 60 | 75 |
| 80 lb (36 kg) | 0.6 | 45 | 56 |
06 Choosing slab thickness
Thickness has the biggest effect on volume: going from 4 to 6 inches adds 50% more concrete. Common guidance:
- 4 in (100 mm): patios, walkways, shed bases and light-duty garage floors.
- 5–6 in (125–150 mm): driveways, workshops and areas with heavier vehicles.
- Over 6 in or reinforced: structural or heavy-load slabs, which should be specified by an engineer.
07 Concrete slab cost
Price depends on your region, mix strength, delivery distance and access. Use your supplier’s quote in the calculator above; as a sanity check for the 10 × 10 ft, 4 in example (1.36 yd³):
- Ready-mix: 1.36 yd³ at an example $165 per yd³ ≈ $224 for the concrete itself, before delivery or short-load fees.
- Bags: 62 bags at roughly $5–$8 per 80-lb bag ≈ $310–$500.
Don’t forget the rest of the job: gravel base, formwork, rebar or mesh, finishing tools and curing. Labour is usually the largest line item if you hire a crew—see our concrete driveway cost calculator for full turnkey installed budgets.
08 Bags or ready-mix: which should you choose?
Bagged concrete
- Best below ~½ yd³ (≈ 22 bags of 80 lb)
- No delivery or short-load fee
- Pour at your own pace
- Heavy: 62 bags is about 5,000 lb to move and mix
Ready-mix truck
- Best above ~1 yd³ (≈ 45 bags of 80 lb)
- Consistent mix and strength
- Fast: you need helpers and forms ready first
- Suppliers set minimum loads and delivery windows
09 Common estimating mistakes
- Mixing units. Feet with inches is the classic error. Convert thickness to feet first.
- Forgetting waste. Subgrades are never perfectly flat and forms flex under load.
- Ignoring thickened edges. Slabs with a thicker perimeter or footing need extra volume. Calculate them separately.
- Rounding down. Running short mid-pour creates a cold joint, the weakest point in the slab.
- Trusting generic yields. Bag yield varies slightly by brand. Check the label.
10 Footings, columns and other shapes
The same volume logic applies to any shape. For round footings, tube forms and post holes use the cylinder formula:
V = π × (diameter ÷ 2)² × depth
Example: a 12 in diameter tube, 4 ft deep, is π × 0.5² × 4 = 3.14 ft³, or 6 bags of 80-lb mix after rounding up. Use our dedicated footing & Sonotube calculator for round piers, post holes and strip trenches, or our concrete block wall calculator for CMU foundations.
11 Frequently asked questions
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab that is 4 inches thick needs 1.23 cubic yards (33.3 cubic feet) of concrete. Add 10% for waste and order about 1.36 cubic yards, or roughly 62 bags of 80-lb pre-mixed concrete.
How many 80-lb bags of concrete make a cubic yard?
It takes about 45 bags of 80-lb concrete to make one cubic yard, because each bag yields roughly 0.6 cubic feet and a cubic yard holds 27 cubic feet. A 60-lb bag needs 60 bags per yard and a 40-lb bag needs 90.
How do you calculate cubic yards of concrete?
Multiply length × width × thickness in feet to get cubic feet (convert inches to feet by dividing by 12), then divide by 27 to get cubic yards. Add 5–10% extra to cover uneven ground, spillage and over-excavation.
How far does one cubic yard of concrete go?
One cubic yard of concrete covers about 81 square feet at 4 inches thick, 65 square feet at 5 inches and 54 square feet at 6 inches. These figures are before waste, so plan on slightly less coverage in practice.
How many cubic metres of concrete do I need for a slab?
Multiply length (m) × width (m) × thickness (m). A 5 m × 4 m slab that is 100 mm (0.1 m) thick needs 2.0 m³; with 10% waste, order about 2.2 m³. This is the standard method in the UK, Australia and New Zealand.
How much extra concrete should I order?
Order 5–10% more than your calculated volume. Use the higher end for hand-dug or uneven subgrades, forms that may bow, or when running out would leave a cold joint. Ready-mix suppliers generally prefer you slightly over-order.
How much does a concrete slab weigh?
Normal-weight concrete weighs about 150 pounds per cubic foot (2,400 kg per cubic metre). That is roughly 4,050 pounds, or just over 2 tons, per cubic yard. A 10×10×4 inch slab weighs around 5,000 pounds.
Is it cheaper to mix bags or order ready-mix concrete?
For small jobs under about half a cubic yard, bags are usually cheaper because ready-mix trucks charge delivery and short-load fees. Above roughly one cubic yard, ready-mix is normally cheaper per yard, faster and gives a more consistent finish.
How thick should a concrete slab be?
A 4-inch (100 mm) slab suits patios, walkways, sheds and light-duty garage floors. Driveways and slabs carrying heavier vehicles are commonly poured 5–6 inches (125–150 mm) thick. Check local building codes or ask an engineer for structural slabs.
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