Introduction
Quick answers to the questions people ask most about estimating and pouring concrete slabs — patios, driveways, garage floors, and shed bases. For the full numbers behind any answer here, run your dimensions through the Concrete Slab Calculator and read the linked guides for depth. If your question isn’t answered, the complete guide covers the estimating process end to end.
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Frequently Asked Questions
How much concrete do I need for a 10×10 patio slab at 4 inches?
10 × 10 × (4 ÷ 12) = 33.33 ft³; with 10% waste that’s 36.67 ft³ = 1.36 yd³, or about 0.94 m³. In bags: 36.67 ÷ 0.6 = 61.1 → 62 eighty-pound bags. At 1.36 yd³ you’re in short-load ready-mix territory — one small truck delivery beats mixing 62 bags by hand.
How thick should my slab be?
4 in (10 cm) for patios, walkways, and shed bases carrying foot traffic; 5–6 in (12.5–15 cm) for driveways and garage floors carrying passenger vehicles; 6–8 in (15–20 cm) for RV parking or heavy vehicles. These are standard residential values — always check local code, which may require more. Thickness selection is covered in the complete slab guide.
How many 80-lb bags do I need for a 12×12 slab at 4 inches?
12 × 12 × (4 ÷ 12) = 48 ft³; × 1.10 waste = 52.8 ft³; ÷ 0.6 ft³ per bag = 88 bags. Metric check: 3.66 m × 3.66 m × 10 cm = 1.36 m³; × 1.10 = 1.50 m³; ÷ 0.017 = 88 bags. Same answer both ways.
Should I use ready-mix or bagged concrete?
Under about 1 yd³ (roughly 45 bags), bag mixing is reasonable. From 1–3 yd³, short-load ready-mix delivery usually wins on cost and sanity. Over 3 yd³, a standard truck is cheapest per yard. The DIY vs pro guide maps the exact thresholds.
How much does a concrete slab cost per square foot?
Contractor-installed plain slabs typically run $6–$10 per sq ft for 4-in patios and walkways, $8–$12 per sq ft for 5–6 in driveways and garage floors — bundling excavation, base, forms, concrete, labor, and a basic finish. DIY materials run roughly $2–$4 per sq ft. Stamped or decorative finishes add $4–$10 per sq ft. Full breakdowns are in the cost guide.
Do I need gravel under a concrete slab?
Yes — 4–6 in (10–15 cm) of compacted gravel is standard. It spreads the load, drains water away from the slab, and gives you a level, stable surface to screed against. Pouring directly on soft soil or topsoil is one of the fastest routes to a cracked, settled slab. Compact the gravel in 2–3 in lifts with a plate compactor, and dampen it before the pour so it doesn’t draw water out of the fresh concrete.
Do I need rebar or wire mesh in a slab?
Reinforcement doesn’t prevent cracks — it holds cracked sections together so they stay tight and level. Wire mesh (6×6) suits most patios and walkways; rebar (typically #3 or #4 at 18-in spacing) suits driveways and garage floors carrying vehicles. The comparison is in the material guide.
How far apart should control joints be?
2–3 ft of spacing per inch of slab thickness: every 8–12 ft (2.4–3.6 m) for a 4-in slab, every 12–18 ft (3.6–5.5 m) for a 6-in slab. Cut one-quarter of the slab depth — 1 in (25 mm) on a 4-in slab — and saw within 6–12 hours of pouring, before shrinkage cracks beat you to it.
How long before I can walk or drive on a new slab?
Foot traffic after 24–48 hours; light vehicles after about 7 days; full loads after 28 days, when concrete reaches its design strength. Keep the surface moist for the first 7 days regardless — curing is what builds surface strength.
How do I estimate an L-shaped or irregular slab?
Split it into rectangles, estimate each one separately, and add the volumes. Never force an irregular plan into a single length × width box — corner overlaps silently inflate or deflate the number. A worked split-rectangle example is in the estimating guide.
What slope should a patio slab have?
1/8–1/4 in of fall per foot (10–20 mm per meter) away from buildings — enough to shed water, subtle enough to walk on. On a 20-ft patio at 1/4 in per foot, that’s 5 in of total fall from house to edge.
Can I pour concrete in hot or cold weather?
Yes, with precautions. Above ~90°F (32°C), pour early morning, dampen the base and forms, and have the full crew ready — concrete sets fast in heat. Near or below freezing, keep fresh concrete above freezing for the first several days with insulated blankets; frozen fresh concrete loses significant strength.
How much waste should I add to my estimate?
Ten percent is the standard default for residential slabs. Use 5% for clean, well-prepared forms; 10–15% for uneven ground, over-excavation, or irregular shapes. Then round the final order up — running short mid-pour is the one estimating error you can’t fix cheaply.
Related Guides
- Complete Slab Guide — the full walkthrough of slab estimating.
- Slab Cost Guide — concrete, base, forms, labor, and finishes priced out.
- Slab Material Guide — mixes, mesh vs rebar, and sealers.
- DIY vs Hiring a Pro — size thresholds for mixing it yourself.
- Estimating Guide — step-by-step slab measurement.
Get Your Number
Still have a question the list didn’t cover? Run your slab through the Concrete Slab Calculator — length, width, thickness, waste — and check the result against the worked examples above. If your number and the calculator’s number agree, you’re ready to call the supplier.
Disclaimer: These answers are general guidance for typical residential slabs. Calculator results are planning estimates based on the dimensions you enter. Actual requirements vary with site conditions and mix design — verify volumes with your supplier, check local building codes for thickness and setback requirements, and consult a licensed professional for structural or code-related decisions.
