Concrete Calculator

How concrete quantities and slab pours actually work

Concrete is the most expensive thing on this site to over-order and the most painful to under-order. A truck on-site costs you whether or not your form is ready; running short means a cold joint that becomes a permanent crack line. The calculator above does the math; here's the context that turns a number into a confident pour.

What a "yard" of concrete means

A cubic yard is a cube 3 ft × 3 ft × 3 ft = 27 cubic feet. One cubic yard of concrete weighs about 4,000 lbs (1,800 kg). A standard ready-mix truck holds 8–10 cubic yards. Your typical residential pour is 1–5 yards.

The leap most homeowners struggle with: it takes ~60 bags of 60-lb concrete or 45 bags of 80-lb concrete to make one cubic yard. That's a pallet's worth. Mixing that by hand is a full Saturday with help; mixing it alone is two days and you'll regret it.

Coverage: how far one cubic yard goes

Bags vs. ready-mix truck — the real break-even

The crossover happens at about 0.5 cubic yards (0.4 m³) — that's a 6×6 ft × 4 in slab. Below that line, bagged concrete from Home Depot is the right call: no minimum delivery fee, no time pressure, no Saturday-morning truck driver waiting for your form to be ready. Above 0.5 yd, the math goes upside-down fast:

Short-load fees ($75–$150) kick in below 3 yards on most ready-mix trucks. They're still cheaper than bagging once you cross 1 yd.

The math, walked through

For a 12 ft × 10 ft × 4 in slab:

1. Area: 12 × 10 = 120 sq ft.
2. Convert thickness to feet: 4 ÷ 12 = 0.333 ft.
3. Volume: 120 × 0.333 = 40 cu ft.
4. To cubic yards: 40 ÷ 27 = 1.48 cu yd.
5. Add 10% waste: 1.48 × 1.10 = 1.63 cu yd.
6. Order in 0.25-yd increments: 1.75 cu yd.

For pros: most ready-mix yards order in 0.25 yd, but some round to 0.5. Confirm before you commit.

Slab thickness — choose by load, climate, and base

• 3.5 in (89 mm) — walkways, light patios, AC condenser pads. Foot traffic only. Will crack under repeated vehicle weight.
• 4 in (100 mm) — the residential default. Driveways, patios, single-car garage floors in mild climates (zones 7+). Adequate with a 4-in compacted base.
• 5 in (125 mm) — driveways in freeze-thaw climates (zones 4–6), garage floors with workshop equipment, RV pads. The thickness most pros default to in cold-winter regions.
• 6 in (150 mm) — heavy vehicle pads, commercial-grade driveways, slabs that'll see point loads (jacks, tripod stands, lifts). Required by some municipalities for new home garages.
• 8 in+ (200 mm+) — structural slabs, footings, slab-on-grade with frost heave concerns. Engineering territory; consult a local code reference.

Reinforcement: rebar vs. wire mesh vs. fiber

Concrete is strong in compression and weak in tension. Reinforcement keeps cracks from opening into structural failures. The right choice scales with thickness:

• 4-in slab, light use: 6×6 #10 wire mesh OR fiber-mesh additive in the mix. Mesh costs more but performs better.
• 4-in slab, driveway: #3 (3/8″) rebar in a 16″ grid is the modern standard. Holds the slab together if a control joint fails.
• 5–6 in slab: #4 (1/2″) rebar in a 12–16″ grid. Place on chairs at mid-depth — rebar lying on the base is decorative, not structural.
• Footings, retaining walls, structural: engineered rebar plan, often #4 or #5. Code-driven; get a local engineer's spec.

Pro tip: fiber-mesh in the mix is great for limiting plastic-shrinkage cracking but does not replace structural rebar in a driveway. Use both for cold climates.

Joints: control joints, expansion joints, and isolation joints

Concrete will crack — your job is to control where. Three joint types, three purposes:

• Control joints (saw-cut or tooled). Pre-planned weak lines that crack invisibly. Cut every 8–12 ft for a 4-in slab; rule of thumb is depth = 1/4 of slab thickness, spacing in feet ≤ 2.5 × thickness in inches. Cut within 4–24 hours of pouring (sooner in hot weather).
• Expansion joints. Foam strips between slabs and against fixed structures (foundations, footings, columns). Allow thermal movement without cracking. Required where slabs abut a different structure or every 30 ft on long pours.
• Isolation joints. Same idea as expansion joints, but specifically separate two slabs that are pouring at different times. Without one, the new pour bonds to the old, and the old slab's settling cracks the new one.

Base preparation — the failure point most DIYers miss

A perfect concrete pour on a bad base will crack. A mediocre pour on a great base will outlast you. Steps:

• Excavate below grade to allow for the slab plus 4 in of base gravel (so 4″ slab + 4″ base = 8″ excavation depth from finished grade).
• Compact the subgrade. Plate-tamp the native soil or rent a jumping-jack for clay-heavy soils. Soft subgrade = settling cracks.
• Add 4 in of compacted base gravel (3/4″ minus, road base, or "crusher run"). See the gravel calculator for tonnage.
• Compact in 2-in lifts. Don't dump 4 in and run the compactor once — you'll get a fluffy bottom layer.
• Vapor barrier (optional but recommended) for slabs under heated buildings — 6-mil poly between gravel and concrete.

Cure time and load schedule

Keep the slab moist for 7 days (curing blanket, sprinkler, or sheet plastic) — not soaking, just damp. Concrete that dries too fast loses 30%+ of its strength.

Common mistakes that ruin a pour

• Pouring on hot dry afternoons. Concrete sets too fast, loses water, develops surface crazing. Pour in the morning when ambient is under 80°F (27°C).
• Adding water on-site to "make it workable." Every gallon added drops final strength by 5–10%. The truck mix is correct; let it be.
• Pouring on frozen ground. When the ground thaws, the slab settles and cracks. Wait or thaw with tarps + heaters.
• Rebar lying on the base. Steel must be in the middle of the slab depth (or upper third) to do its job. Use 2-in rebar chairs.
• Forgetting the slope. Slabs need 1/8″–1/4″ per foot of slope away from buildings. Flat slabs pond water and fail at every joint in freeze-thaw.
• Tooling the surface too early. Wait until the bleed water disappears. Tool while wet and you trap water below the surface — that's how you get scaling and pop-outs in 2 winters.

Pro additions: admixtures, mix designs, and PSI

Beyond DIY, the spec gets richer. Key terms:

• 3000 / 3500 / 4000 PSI mix. The compressive strength after 28 days. 3000 is residential default; 4000 is required for most freeze-thaw exposed driveways and pool decks.
• Air entrainment (4–6%). Microbubbles in the mix that absorb expansion when water freezes inside. Required for any exterior slab in zones 5 and below. Ask the supplier — air-entrained mix costs ~$5/yd more.
• Slump (4–6 in). How "wet" the mix is. Higher slump = easier to place, lower strength. Driveways: 4 in. Pumped pours / hard-to-reach forms: up to 6 in. Anything over 6 means too much water.
• Calcium chloride / accelerators. Speed cure in cold weather. Don't use near rebar (corrosion).
• Retarders. Slow set time on hot days or for stamped/decorative work that needs more workability.

Frequently asked questions