Concrete slab calculator

Every line shows the quantity to buy and how StudSheet got it: the spacing, the waste and the package size, so you can check it before the supply house or the quote.

In Canada (3.05 m × 3.05 m slab, 102 mm thick)

Set to Canada, the Sheet works the job out again for Ontario: metric sizes, bag sizes, the frost depth and the code notes. It doesn't just convert feet to meters. The first lines:

• 1.25 m³, Ready-mix concrete (delivered): 3.05 m × 3.05 m × 102 mm = 0.94 m³, +10% for an uneven base and spills, rounded up to the next 0.25 m³. Small loads carry a short-load fee at most plants — ask when you order.
• 1.5 m³ delivered, Crushed stone base, bulk: 9.3 m² × 102 mm deep, +20% because gravel compacts = 1.13 m³ loose (80 bags — order it in bulk).
• 3 sheets, Welded wire mesh, 1.52×3.05 m sheet: Covers 2.95 m × 2.95 m (51 mm in from the edges); sheets lap one full square plus 51 mm, +10% for cut-offs.
• 16 chairs, Rebar chairs or dobies: About one every 914 mm each way, so the mesh ends up at mid-depth instead of lying on the gravel.
• 1 roll, Rebar tie wire: Ties the sheet laps together.
• 3 boards, 2×4: Forms on the open sides (3.05 m + 3.05 m + 3.05 m), on edge with the top at the finished height. A 2×4 is 89 mm wide: set its top at the finished height and the concrete fills the last 13 mm under it.

1. Call 811 (NJ One Call) at least 3 full business days ahead and wait for the utility markings.
2. Lay out the slab with stakes and a string line, 10′ × 10′, and square it by measuring the diagonals. Plan the slope: ⅛–¼″ per foot away from the house.
3. Dig out 8″ (4″ of concrete on 4″ of gravel), plus 6″ past the edges for the forms. Strip sod and soft soil down to firm ground and tamp the bottom.
4. Set the forms on the open sides: 2×4s on edge, stakes every 3 ft on the outside, tops at the finished height with the slope built in. Nail through the stakes with duplex nails, and fasten the expansion strip against the house.
5. Spread the gravel in 2″ layers and compact each one; finish it flat, 4″ below the form tops.
6. Lay the mesh 2″ in from the edges, sheets lapped one full square plus 2″ and tied; set it on 2″ chairs about every 3 ft.
7. Pour: have the crew, wheelbarrows, a concrete rake and the screed ready before the truck comes. Start at the far corner and fill to just above the forms.
8. Strike it off with the screed, sawing it along the form tops, then float it once. Wait for the bleed water to soak back in before the next pass.
9. Edge the slab and tool the control joints 5′ apart at most (4 panels), 1″ deep with the groover; then broom it across the slope for grip.
10. Cure: spray the cure-and-seal as soon as the sheen is gone. Keep it from freezing for 48 hours.
11. Strip the forms after 1–2 days and backfill along the edges.

Must-have tools first, then the ones that help and the ones to rent for the day.

Mistakes that cost a callback or a second trip

• Adding water to make it pour easier: Extra water makes weak, dusty concrete that cracks and scales. Use the water the bag or the plant says; rent a mixer instead.
• Finishing over bleed water: Troweling the water back into the surface makes it flake off after the first winter. Wait until the sheen is gone.
• Joints too far apart, or too shallow: Concrete shrinks and cracks: cut joints ¼ of the depth and about 8′ apart at most, so you decide where.
• Pouring on soft, muddy or frozen ground: The slab settles and cracks where the ground gives. Firm, compacted base; never pour on frost.

What the calculator assumes

• Concrete: 100 sq ft × 4″, +10% for an uneven base and spills. 80 lb bag ≈ 0.6 cu ft (Quikrete); ready-mix rounded up to the next quarter yard.
• Base: 4″ of crushed stone with fines, +20% loose because it compacts.
• Control joints: no farther apart than 24–36 times the thickness or 15 ft, panels no longer than 1.5 × their width, cut ¼ of the depth (at least 1″), sawn 4–12 hours after finishing (NRMCA CIP 6). This Sheet plans 24 times: 8′ for 4″.
• Mesh: 6×6 W1.4×W1.4 sheets lapped one square plus 2″, held at mid-depth on chairs (IRC R506: from the middle to the upper third). It keeps cracks tight; it doesn’t stop them.
• Outdoors: no vapor retarder (the IRC exempts patios, walks, driveways and unheated buildings).
• Curing: keep it wet or sealed for 5 days in warm weather, 7 when it’s 50–70°F, and from freezing for 48 hours (Quikrete). Walk on it after 1–2 days, light use after a week, full strength at 28 days.

The Sheet sets its code notes for the job's state or province. These are New Jersey's and Ontario's; the code reference has more. Always confirm with your building department.

• Outdoor concrete in freezing winters: Where winters freeze, porches, steps and garage floors take 3500 psi air-entrained concrete, 5–7% air (IRC Table R402.2); order the same for driveways and patios. Bagged mixes usually aren’t air-entrained: seal them and keep de-icing salt off the first winter.
• Pour day is a crew job: A truck brings 1.5 cu yd at once, and in warm weather you have an hour or two to place and finish it. Plan at least 3 people for the first yard and one more for each one after, or hire a finisher for pour day.
• Call 811 first — it’s the law: In New Jersey, call 811 (NJ One Call) at least 3 full business days before digging. Free. Wait for the utility markings.
• Ask your town about zoning: Patios and walks on the ground usually need no construction permit in New Jersey, but many towns want a zoning permit for new paving (lot coverage, drainage). Ask your town.
• Click Before You Dig: Before you dig, request a free utility locate from Ontario Dig Authority (ontariodigauthority.ca) at least 5 business days ahead, and wait for the markings.
• Usually no permit: A patio or walk at grade usually needs no building permit in Canadian cities. Zoning may limit paved area and water draining toward neighbours: ask your municipality’s building department if unsure.

What materials do I need to pour a concrete slab?

For the worked example, a 10′ × 10′ slab, 4″ thick, the Sheet counts 10 lines with waste included, starting with ready-mix concrete (delivered) (1.5 cu yd), crushed stone base, bulk (1.5 cu yd delivered), welded wire mesh, 5×10 ft sheet (3 sheets), rebar chairs or dobies (16 chairs). Each line's math is in the table above. Put in your own sizes in the concrete calculator and every line is counted again.

How long does it take to pour a concrete slab, and can I do it myself?

About 17 to 26 hours for the worked example, in 11 steps. StudSheet stamps it PRO STEP: dIY with a pro for one step.

What tools do I need to pour a concrete slab?

25 in all. The must-haves start with rebar cutter-bender, pump sprayer, tape measure (25 ft), mason's line + stakes, line level, 4-ft level. On the Sheet you mark each one as owned, to buy or to rent.

Stuck while you pour a concrete slab? Ask in the Concrete slabs and footings group of the StudSheet community with this Sheet attached. Homeowners who've done it and verified pros answer, and anyone can read it.

• Concrete and masonry for contractors: run this takeoff at your own prices, send the estimate, and get it signed and the deposit paid.
• Build a garden or retaining wall (16′ wall, 2′ tall): Stack a block wall up to 4 ft: base, drainage, courses, caps.
• Pour concrete steps (brick or stone facing optional) (3 concrete steps, 1′ 9″ rise): Forms, base, concrete (bags or a truck), rebar and the finish for a set of outdoor steps and their landing — with the frost and handrail rules where you live.
• Build a paver patio (16′ × 12′ paver patio): Excavation, compacted gravel base, 1″ of bedding sand, pavers, edge restraint and polymeric sand for a patio or walk — counted in bags or by the yard.
• All 45 Sheets, across 16 trades.
• Code reference: frost depths, guard heights, stairs, retaining walls.