Concrete Calculator (Slabs, Footings & Posts)

Measure Length × Width in feet to calculate total square footage, then multiply by the depth in feet (depth in inches divided by 12) to get cubic feet. Finally, divide cubic feet by 27 to calculate cubic yards. Formula: Volume (Cu Yds) = (Length ft × Width ft × [Depth in ÷ 12]) ÷ 27. Always add a 10% safety margin for sub-base variance.

One cubic yard equals 27 cubic feet, requiring 45 bags of 80 lb concrete (0.60 cu ft yield), 60 bags of 60 lb concrete (0.45 cu ft yield), or 72 bags of 50 lb concrete (0.375 cu ft yield). In metric standards, one cubic meter (1 m³) requires approximately 108 bags of 20 kg mix (~0.00925 m³ yield) or 54 trade bags of 40 kg mix.

A 10×10 ft slab (3.05×3.05 m) at 4" (100 mm) depth requires 1.23 cubic yards (33.33 cubic feet), or 0.94 m³ net. That equals exactly 56 bags of 80 lb concrete (or 74 bags of 60 lb). With an industry-standard 10% waste buffer, purchase 62 bags of 80 lb (or 82 bags of 60 lb). In metric, this direct unit conversion requires 1.04 m³ with waste (112 bags of 20 kg, or 56 trade bags of 40 kg). If pouring a rounded 3×3 m metric slab, volume is 0.99 m³ with waste (108 bags of 20 kg).

No. Pouring directly on dirt or organic soil causes concrete to sink, crack, and soak up ground water like a sponge. Always excavate topsoil, tamp the native dirt, and lay a compacted 4-inch base of #57 crushed stone or road base (crusher run) to ensure proper drainage and load distribution.

A 4-inch slab with rebar or wire reinforcement is standard for pedestrian patios, sidewalks, shed floors, and hot tubs under 1,500 lbs. Step up to a 6-inch slab for driveways, vehicle parking pads, heavy machinery, or structures exceeding 5,000 lbs.

As a contractor rule of thumb, projects requiring more than 1.5 to 2.0 cubic yards (65+ bags of 80 lb or ~5,200 lbs) are exhausting to hand-mix in a wheelbarrow and risk weak seams where one dry batch fails to stick to the next wet batch (cold joints). For large pours, ready-mix transit trucks deliver freshly mixed concrete in minutes.

Standard mixed concrete weighs approximately 4,000 lbs (2 short tons) per cubic yard, which equals roughly 148 lbs per cubic foot. When fully cured, concrete maintains this mass.

Use 3,000 to 3,500 PSI mix (standard Quikrete or Sakrete in yellow bags) for walkways, patio pads, and footings. Use 4,000 to 5,000 PSI (often designated "Crack-Resistant" or "5000 Plus") for driveways, garage floors, and freeze-thaw northern climates.

You can walk on new concrete after 24 to 48 hours. Light vehicle parking is safe after 7 days (when concrete reaches ~70% of its design strength). Full structural curing and 100% rated PSI strength is reached at 28 days.

Add approximately 3 quarts (6 pints) of clean water per 80 lb bag. Add water gradually. The mixed concrete should resemble thick, moldable oatmeal—not soup. Adding excessive water makes concrete easy to pour but weakens final compressive strength by up to 40%.

Cut control joints at a spacing (in feet) of 2 to 2.5 times the slab thickness in inches. For a standard 4-inch slab, cut joints every 8 to 10 feet in both directions to create roughly square panels. Control joints should be cut to a depth of 1/4 the slab thickness (1 inch deep on a 4-inch slab) within 6 to 18 hours of pouring, before concrete shrinks as it dries, which pulls it apart and creates random surface cracks.

Bleed water is the excess water that naturally rises to the surface as heavy rock and sand particles settle after smoothing the surface with a wide board or float tool (bull floating). Never work or trowel concrete while bleed water is present. Working bleed water back into the surface washes away the glue on top, leaving a weak, powdery surface that will peel, blister, and flake off later. Wait until the surface sheen disappears completely and a footprint indents no more than 1/4 inch before finishing.

Use a broom finish (dragging a soft-bristle masonry broom lightly across the freshly floated surface) for all outdoor slabs, patios, sidewalks, and driveways to create slip-resistant traction when wet. Use a hard steel trowel finish only for covered indoor areas like garage floors, basements, and workshops where a smooth, non-porous, dust-free surface is needed.

Proper curing retains moisture so cement can fully hydrate chemically. Start curing as soon as the surface is hard enough not to mar (usually 4 to 6 hours after finishing). Keep the slab continuously damp by misting with a garden hose and covering with plastic sheeting or wet burlap for at least 5 to 7 days, or spray a liquid membrane-forming curing compound. Moist-curing increases final compressive strength by up to 50% and prevents drying shrinkage cracks.

US concrete is rated by 28-day cylinder compressive strength in PSI (ASTM C94). Australia and New Zealand specify characteristic compressive strength in Megapascals (MPa) per AS 1379 / AS 3600 (e.g., N20, N25, N32). The UK and European Union specify dual cylinder/cube strength classes per BS EN 206 (e.g., C20/25, C25/30). A standard US 3,000 PSI mix corresponds to N20–N25 (20–25 MPa) in Australia or C20/25 in the UK/EU. A high-strength 4,000 PSI driveway mix corresponds to N28–N32 (28–32 MPa) or C25/30.

For standard 3.5" to 4" pedestrian slabs (patios and walkways), use straight #2 or better 2×4 lumber (90×45mm in metric). For 5" to 6" structural or driveway slabs, use 2×6 lumber (140×45mm). Drive 2×2 or 1×3 wooden stakes (or steel form pins) every 3 to 4 feet (1 meter) along the perimeter formwork, and install 45-degree diagonal kickers at every corner and splice joint to prevent wet concrete hydraulic pressure from bowing the boards outward.

A sub-base vapor retarder (heavy-duty 10-mil or 15-mil puncture-resistant poly meeting ASTM E1745) is essential under interior living spaces, attached garages, and enclosed workshop or shed slabs where flooring (epoxy, tile, vinyl, or carpet) will be installed. Concrete is naturally porous and wicks groundwater upwards via capillary action, which will delaminate epoxy coatings and rot subfloors. Overlap sheets by at least 6 inches and seal with vapor-tape. Outdoor sidewalks and open patios do not require a vapor barrier.