Concrete Slab Calculator
How much concrete you need for a slab, driveway, or patio — in cubic yards and bags.
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A concrete slab — a patio, shed floor, or walkway — is one of the most achievable DIY concrete projects. Success comes down to a solid compacted base, the right thickness, and not rushing the pour or the cure.
DifficultyIntermediate
TimeA weekend for a small slab — forms and base one day, pour and finish the next
SkillsComfortable with formwork and able to work fast and coordinated once the concrete arrives
PermitsMany areas require a permit for slabs over a certain size or close to property lines. Check with your local building department first.
Tools you'll need
- Tape measure, string line & marking paint
- Shovel and a plate compactor (rent it)
- 2× lumber and stakes for forms
- Wheelbarrow (and a strong helper or two)
- Screed board (a straight 2×4)
- Bull float and an edging/grooving trowel
- Rubber gloves and boots
Step by step
- Lay out and build the forms Square your layout (check the diagonals are equal), then set 2× form boards to your slab height, staked every 3–4 ft. Slope exterior slabs about 1/8" per foot away from the house for drainage.
- Excavate and compact a gravel base Dig out sod and soft soil, then add and compact ~4" of gravel base. A solid, level base is what keeps a slab from cracking — never pour on loose dirt.
- Add vapor barrier and reinforcement For floors and slabs that matter, lay 6-mil poly as a vapor barrier, then wire mesh or #3/#4 rebar on chairs so the steel sits in the middle of the slab, not on the ground.
- Order or mix the concrete Use the headline quantity, which already includes your selected allowance — you cannot run short mid-pour. Ready-mix is worth it above roughly 1 cubic yard; below that, bagged mix is fine.
- Pour, screed, and float Place concrete starting at the far corner, rake it roughly level, then screed off the forms with a straight 2×4 in a sawing motion. Bull-float once the sheen starts to leave.
- Edge, joint, and finish Round the edges, then cut control joints every 8–10 ft (about 2.5× the slab thickness in feet) so cracks form where you want them. Finish with a broom for a non-slip surface.
- Cure it slowly Keep the slab damp (or covered with plastic) for several days. Concrete reaches walking strength in ~24–48 hours but full strength at 28 days — slow curing is stronger curing.
Common mistakes
Skipping or under-compacting the gravel base
A compacted base is the single biggest factor in a crack-free slab. Always compact 4" of gravel — don't pour on bare or loose soil.
Adding water to make concrete easier to work
Extra water weakens concrete and causes surface cracking and dusting. Order a workable slump instead and resist the urge to "soup it up."
No control joints
Concrete will crack — control joints decide where. Cut them every 8–10 ft and at any inside corner.
Confusing geometric volume with the order quantity
The calculator includes 8% by default for spillage, uneven base, and over-dig. Adjust that allowance for the site, and do not add it twice. Running out mid-pour leaves a cold joint you can't undo.
Pouring on frozen or saturated ground
Wait for the base to be firm and above freezing. Frozen ground heaves; saturated ground settles.
Safety
- Wet concrete is caustic — wear rubber gloves and boots and rinse skin promptly.
- Bags are heavy (60–80 lb). Lift with your legs and pace yourself.
- Have enough helpers lined up before the truck arrives — you can't pause a pour.
Frequently asked questions
How thick should a concrete slab be?
4 inches is standard for patios, walkways, and shed floors. Go to 5–6 inches for driveways or anything that holds vehicles or heavy equipment.
Do I need rebar or wire mesh?
For a 4" patio on a good base, wire mesh is enough. Add #3–#4 rebar on a grid for driveways, thicker slabs, or poor soil.
How long before I can use it?
You can walk on it in 24–48 hours, but wait ~7 days before heavy loads and 28 days for full strength.
How much concrete do I need?
Enter your length, width, and thickness in the calculator above — it gives cubic yards (with an editable allowance, 8% by default) and the equivalent in 60/80-lb bags.
Can I pour concrete in cold weather?
Below about 40°F you need cold-weather measures (heated mix, blankets). It's far easier to wait for mild, dry weather.
Next steps for this project
Pool Deck
Deck area around a pool, plus concrete or paver ordering allowance.
Gravel
How much gravel you need for a driveway, path, or base — in tons and cubic yards.
Vapor Barrier
Rolls of 6-mil poly sheeting for under a slab, deck, or crawlspace.
Post-Hole Concrete
Bags of concrete to set fence or deck posts.
Paver
Pavers (any size), gravel base and sand for a patio or walkway.