Concrete Slab Calculator
Calculate cubic yards and bags of concrete for a slab.
Includes 5% extra for spillage and uneven subgrade. An 80 lb bag yields about 0.60 cu ft; a 60 lb bag about 0.45 cu ft.
Concrete is unforgiving: once the pour starts, you can’t pause to run to the store, and a short load means a cold joint scarring your slab forever. Our concrete calculator above converts your slab dimensions into cubic feet, cubic yards, and the exact number of 80 lb or 60 lb bags — with a 5% safety margin already included.
The Concrete Volume Formula
Volume (cu ft) = Length (ft) × Width (ft) × Thickness (in) ÷ 12
Cubic yards = Volume ÷ 27
A classic example: a 10 × 10 ft shed slab at 4 inches thick is 10 × 10 × 4/12 = 33.3 cu ft, or 1.23 cubic yards. With the 5% allowance for spillage and uneven ground, plan for about 1.3 yd³.
Bags or Ready-Mix Truck?
This is the first decision for any slab project, and the math is mostly about volume:
Bagged concrete makes sense up to roughly 1 cubic yard (about 45 × 80 lb bags). An 80 lb bag yields about 0.60 cu ft of concrete; a 60 lb bag about 0.45 cu ft. Bags are cheap per unit but brutal in quantity — a 10 × 10 slab means mixing nearly 60 bags, which is a serious day of labor even with a rented mixer.
Ready-mix delivery wins for anything over about 1–1.5 cubic yards. You pay a delivery fee and often a “short load” surcharge under 3–4 yards, but the concrete arrives consistent, properly mixed, and all at once — which matters, because a slab should be poured in one continuous operation.
A good rule of thumb: if the calculator says more than 40–50 bags, get quotes from ready-mix suppliers before committing to bags.
How Thick Should Your Slab Be?
- 4 inches — the standard for patios, walkways, and shed bases.
- 5–6 inches — driveways and garage floors that carry vehicles.
- 6+ inches with reinforcement — anything supporting heavy equipment or RVs.
Thickness is the biggest cost lever in the formula: going from 4 to 6 inches increases your concrete volume by 50%. Don’t guess — requirements for driveways are often set by local code.
Why the 5% Extra Matters
Our calculator adds 5% to the raw volume, and experienced concrete finishers consider that the minimum. The extra covers:
- Uneven subgrade — a base that dips half an inch across the slab swallows a surprising volume of concrete.
- Spillage and pump/chute waste — some concrete always stays in the mixer, wheelbarrow, or chute.
- Form deflection — forms that bow slightly outward under the wet load increase actual volume.
Running short is far more expensive than 5% overage: a second batch poured later doesn’t bond properly, creating a weak seam called a cold joint.
Pre-Pour Checklist
- Excavate and compact a 4-inch gravel base for drainage (use our gravel calculator).
- Set and stake forms, then verify they’re square by measuring the diagonals.
- Lay reinforcement: wire mesh or rebar for slabs carrying loads.
- Check the weather — avoid pouring in rain or temperatures below 40°F (4°C).
- Line up help: even a small slab needs two people once the mud starts flowing.
Frequently Asked Questions
How many bags of concrete for a 10×10 slab? At 4 inches thick: about 59 × 80 lb bags or 78 × 60 lb bags, including the 5% margin. At that quantity, price out ready-mix too.
How much does a cubic yard of concrete weigh? Roughly 4,000 lbs (2 tons) when cured. Plan your wheelbarrow trips accordingly — a full contractor barrow holds about 5–6 cu ft.
How long before I can walk or drive on it? Foot traffic after 24–48 hours; vehicles after 7 days; full design strength at 28 days. Keep the slab damp during the first week for stronger curing.
Estimates are for planning purposes. For structural slabs, footings, or code-regulated work, consult a professional.