Concrete Slab Calculator
Enter your slab dimensions to get the concrete you need — cubic yards, bags, cost breakdown and a printable estimate.
Slab dimensions
= 0.333 ft · 101.6 mm · 10.16 cm
Covers spillage and uneven subgrade. 5–10% is normal.
You will need
Including 10% waste · 1 slab
- Cubic feet
- 44.0
- Cubic metres
- 1.25
- Weight
- 6,600 lb
Order 1.75 yd³ of ready-mix
Or bag it: 74 × 80 lb · 98 × 60 lb · 147 × 40 lb
| Cost breakdown (ready-mix) | Amount |
|---|---|
| Concrete (1.48 yd³) | $245 |
| Waste (10%) | $24 |
| Delivery | $65 |
| Total | $334 |
| Bagged alternative | Cost |
|---|---|
| 74 × 80 lb bags | $481 |
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Is this thickness right?
4″ suits foot traffic, not vehicles
Mixing time (bagged)
7 h 24 m
One person, 74 bags
Material weight
6,600 lb
≈ 2,994 kg · at 150 lb/ft³
The short answer
To work out concrete for a slab, multiply length × width × thickness with every measurement in feet, then divide by 27 for cubic yards. A 10 × 12 ft slab at 4 inches thick is 40 cubic feet, or 1.48 cubic yards. Add 5–10% for waste, then round up to the next quarter yard when you order.
Common slab sizes
Concrete needed for standard slab dimensions, before waste.
| Slab size | 4″ thick | 5″ thick | 6″ thick | 80 lb bags (4″) |
|---|---|---|---|---|
| 8 × 8 ft | 0.79 yd³ | 0.99 yd³ | 1.19 yd³ | 36 |
| 10 × 10 ft | 1.24 yd³ | 1.54 yd³ | 1.85 yd³ | 56 |
| 10 × 12 ft | 1.48 yd³ | 1.85 yd³ | 2.22 yd³ | 67 |
| 12 × 12 ft | 1.78 yd³ | 2.22 yd³ | 2.67 yd³ | 80 |
| 12 × 16 ft | 2.37 yd³ | 2.96 yd³ | 3.56 yd³ | 107 |
| 16 × 20 ft | 3.95 yd³ | 4.94 yd³ | 5.93 yd³ | 178 |
| 20 × 20 ft | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ | 222 |
| 24 × 24 ft (2-car garage) | 7.11 yd³ | 8.89 yd³ | 10.67 yd³ | 320 |
How much does one bag cover?
An 80 lb bag yields about 0.60 cubic feet of mixed concrete. How far that goes depends entirely on thickness.
| Thickness | 80 lb bag | 60 lb bag | 40 lb bag |
|---|---|---|---|
| 2 in | 3.6 sq ft | 2.7 sq ft | 1.8 sq ft |
| 3 in | 2.4 sq ft | 1.8 sq ft | 1.2 sq ft |
| 4 in | 1.8 sq ft | 1.35 sq ft | 0.9 sq ft |
| 5 in | 1.44 sq ft | 1.08 sq ft | 0.72 sq ft |
| 6 in | 1.2 sq ft | 0.9 sq ft | 0.6 sq ft |
How to calculate concrete for a slab
The maths is simple volume — what people get wrong is mixing units. Thickness is quoted in inches while length and width are in feet, so convert before you multiply.
- Measure the slab. Length and width in feet, thickness in inches.
- Convert the thickness to feet. Divide inches by 12. A 4-inch slab is 0.333 ft.
- Multiply for volume. Length × width × thickness gives cubic feet.
- Convert to cubic yards. Divide by 27 — a cubic yard is 3 × 3 × 3 ft.
- Add waste. Add 5–10%, then round up to the next quarter yard.
Worked example
A 10 ft × 12 ft patio slab, poured 4 inches thick:
- Area10 × 12 = 120 sq ft
- Thickness in feet4 ÷ 12 = 0.333 ft
- Volume120 × 0.333 = 40 cu ft
- Cubic yards40 ÷ 27 = 1.48 yd³
- Plus 10% waste1.63 yd³
- Order (rounded up)1.75 yd³
Ready-mix is sold in quarter-yard increments, so always round up. Running short mid-pour leaves a cold joint — the more expensive mistake by far.
What thickness for which project
Thickness is driven by load, not by area. A bigger patio does not need a thicker slab; a car parked on it does.
| Thickness | Suitable for | Not suitable for | Reinforcement |
|---|---|---|---|
| 3 in | Light footpath, stepping areas | Anything load-bearing | Fibre mix |
| 4 in | Patio, walkway, shed base, AC pad | Driveways, vehicles | Wire mesh or fibre |
| 5 in | Light vehicle parking, RV pad edges | Heavy or repeated vehicle loads | Wire mesh or rebar |
| 6 in | Driveway, garage floor, parking pad | Commercial or plant loads | Rebar grid |
| 8 in+ | Heavy vehicle or commercial slab | — | Engineered rebar design |
Before you pour: checklist
- Compact the sub-base4–6 in of compacted gravel. This matters as much as the concrete itself.
- Set and brace the formsCheck level and square. Concrete finds every weakness in a form.
- Lay the vapour barrier6-mil poly under any slab that will be enclosed or heated.
- Place reinforcementMesh or rebar, held up on chairs so it sits mid-slab, not on the ground.
- Plan the expansion jointsControl joints every 8–12 ft, cut to a quarter of the slab depth.
- Check the forecastRain, frost or extreme heat in the first 24 hours will damage the pour.
- Line up enough handsConcrete does not wait. Know who is placing, screeding and finishing.
- Have a curing planPlastic sheeting, curing compound or damp hessian ready before you start.
Common mistakes
- Mixing inches with feetThe single most common error. Thickness in inches must be divided by 12 before multiplying.
- Ordering the exact amountNever order the raw calculation. Subgrade is never perfectly flat, and coming up short mid-pour is far costlier than a little surplus.
- Skipping the sub-baseConcrete poured on soft or uneven ground cracks regardless of its thickness.
- Under-thickening a drivewayFour inches is fine for a patio and will crack under a car. Six inches for vehicles.
- Adding water on site to make it workableExtra water weakens the mix substantially. Order the right slump instead.
- Forgetting control jointsConcrete will crack. Joints decide where.
- Bagging a job that wants ready-mixAbove half a yard, bags cost more in money and far more in labour.
Curing timeline
24–48 hours
Walk on it
Forms can come off. Keep it damp.
7 days
Light loads
About 70% strength. Light vehicles only.
28 days
Full strength
Design strength reached. Heavy loads fine.
30+ days
Seal it
Wait for full cure before sealing or staining.
Concrete gains strength by hydration, not drying. Keeping it damp for the first week matters more than anything else you do after the pour.
Weather
Pour in these conditions
- 50–80 °F (10–27 °C)
- Overcast, light breeze
- No rain forecast for 24 hours
- Ground above freezing, and staying there
Postpone if
- Heavy rain within a few hours of the pour
- Freezing temperatures in the first 24–48 hours
- Above 90 °F (32 °C) — flash setting and surface cracks
- Strong drying winds with full sun
Bags or ready-mix?
Bags make sense up to about half a cubic yard — roughly 22 × 80 lb bags. Past that, mixing labour becomes the real cost and the pour is no longer continuous, which risks cold joints.
The example above needs about 67 bags: a full day of mixing for two people. Ready-mix would cost less and pour in under an hour. For a fuller breakdown, see how much a concrete slab costs.
What this calculator assumes
Every estimate here rests on these assumptions. If your project differs, adjust accordingly.
- The slab is rectangular and of uniform thickness
- Concrete density of 150 lb/ft³ (2,400 kg/m³) for weight
- Bag yields of 0.60, 0.45 and 0.30 cu ft for 80, 60 and 40 lb bags
- No deduction for the volume displaced by reinforcement
- Ready-mix sold and rounded to quarter-yard increments
- Mixing rate of roughly 6 minutes per 80 lb bag, per person
- Prices are whatever you enter — they are not live market rates
Common questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. An 80 lb bag yields about 0.60 cu ft, so you need about 45 bags. A 60 lb bag yields 0.45 cu ft (60 bags), and a 40 lb bag 0.30 cu ft (90 bags).
How thick should a concrete slab be?
Four inches is standard for patios, walkways and shed bases. Go to five or six inches wherever a vehicle will sit.
Is 4 inches enough for a driveway?
No. Four inches handles foot traffic but cracks under vehicle loads. Pour a driveway at six inches, on a compacted sub-base, with rebar.
How much does a yard of concrete weigh?
About 4,050 lb (roughly 1,840 kg), based on 150 lb per cubic foot. Wet concrete is heavier than cured concrete, which matters if you are moving it by barrow.
Do I need gravel under a concrete slab?
Almost always. A compacted gravel sub-base of 4–6 inches gives drainage and even support. A well-bedded 4-inch slab outlasts a badly bedded 6-inch one. Size it with the gravel calculator.
Do I need rebar in a concrete slab?
For patios and walkways, wire mesh or fibre reinforcement is usually enough. Use rebar for driveways, garage floors and any slab taking vehicle loads. See the rebar calculator.
What PSI concrete should I order?
3,000 PSI suits most residential slabs. Choose 4,000 PSI for driveways, garage floors, and anywhere facing freeze-thaw cycles or heavy loads.
Can I pour concrete in the rain?
Light drizzle is survivable if the surface is protected. Heavy rain dilutes the surface paste and ruins the finish. If heavy rain is forecast within a few hours, postpone.
How long before I can drive on it?
Seven days for light vehicles, 28 days for heavy ones. You can usually walk on it after 24–48 hours.
How much waste should I allow?
5–10%. Uneven subgrade, spillage and over-dug edges all consume more than the geometry suggests.
Should I use bags or ready-mix?
Bags up to about half a cubic yard. Beyond that, ready-mix is cheaper, faster and gives one continuous pour.
What is the minimum ready-mix order?
Most suppliers have a one-yard minimum and charge a short-load fee below three or four yards. Ask first — on a small pour that fee can wipe out the saving over bags.
Does the calculator include rebar or sub-base?
No. It calculates concrete volume only. Reinforcement and sub-base are estimated separately.
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—
materialcalculate.com/concrete-slab-calculator/
Project
| Dimensions | — |
| Thickness | — |
| Number of slabs | — |
| Waste allowance | — |
Volume
| Cubic yards (incl. waste) | — |
| Cubic feet | — |
| Cubic metres | — |
| Estimated weight | — |
Order
| Ready-mix (rounded up) | — |
| 80 lb bags | — |
| 60 lb bags | — |
| 40 lb bags | — |
Estimated cost
| Concrete | — |
| Waste | — |
| Delivery | — |
| Total | — |
Notes
Estimate generated by materialcalculate.com. Figures assume a rectangular slab of uniform thickness and a concrete density of 150 lb/ft³. Confirm quantities with your supplier before ordering. Not a substitute for engineering advice.