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.
Notes from building this calculator
There is a break-even point where bagged concrete stops making sense, and the arithmetic puts it right around one cubic yard, roughly 45 eighty-pound bags. Past that you are mixing for hours and paying more than a ready-mix truck would cost, so this calculator flags the crossover instead of just returning a bag count.
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Sources and Further Reading
Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. Assembly and dimensional references come from applicable model building codes. See our Methodology page for how figures are selected and verified.
Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.
Last updated: August 7, 2026