Concrete questions, answered

Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 for cubic yards — the unit concrete is sold in. Add 5–10% for waste. The slab calculator does it for you.

Exactly 27. A cubic yard is a 3 ft × 3 ft × 3 ft cube, and 3 × 3 × 3 = 27 cubic feet.

Divide cubic feet by 27. For example, 54 cubic feet ÷ 27 = 2 cubic yards.

At 4 inches thick, a 10×10 ft slab is about 1.23 cubic yards (1.36 with 10% waste) — roughly 62 eighty-pound bags. Thicker slabs need proportionally more.

At 4 inches, about 4.94 cubic yards (5.43 with waste); at 6 inches, about 8.15 cubic yards with waste. Use the slab calculator for your exact thickness.

Add 5–10% for uneven subgrade, spillage and over-dig, then round up to the nearest half yard. Ten percent is a safe default for most slabs.

Use π × radius² × depth ÷ 27. For a round slab or a Sonotube column, the column calculator handles it from the diameter.

One cubic yard covers about 81 square feet at 4 inches thick, 65 sq ft at 5 inches, or 54 sq ft at 6 inches. Coverage drops as thickness rises.

About 45 eighty-pound bags, 60 sixty-pound bags, 72 fifty-pound bags, or 90 forty-pound bags. An 80-lb bag yields about 0.60 cubic feet.

An 80-lb bag yields about 0.60 cubic feet — roughly 1.8 square feet at 4 inches thick, or 3.6 sq ft at 2 inches.

Twenty-seven cubic feet ÷ 0.60 cubic feet per bag = 45 bags of 80-lb concrete mix per cubic yard.

About 3 quarts (0.75 gallon) for an 80-lb bag, 2.5 quarts for a 60-lb bag. Aim for a thick-oatmeal consistency and don't over-water — extra water weakens the concrete.

Bags win under about 1 cubic yard; ready-mix wins above roughly 2 yards. Between the two it's close. Compare your size on the bags-vs-ready-mix calculator.

A 10-inch-wide hole 2 ft deep takes about two to three 60-lb bags per post. The post hole calculator sizes it per post.

Standard mix sets in a few hours and is walkable in 24–48 hours. Fast-setting (red-bag) mix sets in 20–40 minutes, which is why it's used for posts.

Material-only ready-mix runs roughly $125–$165 per cubic yard delivered in 2026, before delivery and short-load fees. Prices vary 30%+ by region.

Installed and finished, concrete commonly runs $5–$10+ per square foot including labor, forms and reinforcement — well above the material-only cost.

A surcharge ready-mix plants add for small orders, typically under about 4 cubic yards, on top of the delivery fee. It's a big reason bags win for small jobs.

Beyond the per-yard rate, expect a delivery fee (often $60+) and possibly a short-load fee, fuel surcharge, or after-hours fee. Confirm with your local plant.

At about $6–$7 per 80-lb bag, a cubic yard of bagged mix costs around $270 in material — far more than truck concrete — but with no delivery minimum, so it wins on small jobs.

Four inches for patios, walkways and shed floors; 5–6 inches for driveways and garage floors; 6 inches or more for heavy vehicles. Follow local code for structural work.

Four to five inches for cars over a compacted gravel base; thicker for trucks or RVs. Reinforce with rebar for vehicle loads.

Five to six inches for cars and light trucks, more if you'll use a lift or park heavy equipment, over a 4-inch compacted base.

PSI is the compressive strength at 28 days. Residential concrete is usually 2,500–4,500 PSI: 3,000 for footings, 3,500–4,000 for driveways and exterior slabs.

Typically 3,500–4,000 PSI, with air entrainment in freeze-thaw climates. Just specify the PSI when ordering ready-mix.

Thicker concrete adds load-bearing capacity, but strength (PSI) comes from the mix and the water-cement ratio, not thickness. Both matter — a thick slab made from a weak mix can still fail.

Slabs that carry real load — driveways, garage floors and footings — need rebar. Light-duty patios and walkways are usually fine with welded-wire mesh instead. See rebar vs wire mesh.

Rebar carries structural load (vehicles, walls); mesh controls shrinkage cracking in light slabs. Use rebar where loads are real, mesh for crack control on patios.

#4 (½-inch) bar at 12–18 inches on center is typical for residential slabs-on-grade. Size and spacing should follow your design or local code.

Lay out a grid at your spacing, count the bars each direction, and total the linear feet. The rebar calculator gives bar count, length and weight.

In the middle third of the slab thickness, held up on chairs — not lying on the ground, where it can't do its job and may rust.

By the foot: #3 = 0.376 lb, #4 = 0.668 lb, #5 = 1.043 lb, #6 = 1.502 lb. Multiply by total linear feet for the order weight.

Concrete is walkable in 24–48 hours, ready for vehicles in about 7 days, and reaches full design strength at 28 days. Keep it moist for the first several days for the strongest result.

Usually 24–48 hours for foot traffic, depending on the mix and weather. Cold slows it down.

About 7 days for cars; wait longer for heavy vehicles. Concrete is only ~70% cured at one week.

Concrete cures rather than dries — water is part of the hardening reaction, not just something that evaporates. That's why keeping concrete moist makes it stronger, not weaker.

Curing is a chemical reaction that needs water. Covering, misting or using a curing compound for the first days produces stronger, less crack-prone concrete.

You can pour concrete in cold weather with precautions below about 40°F: use warm mix water, insulating blankets, and never pour on frozen ground. If fresh concrete freezes before it sets, it can be permanently weakened.

You can pour concrete in hot weather, but above about 90°F pour early or late in the day, dampen the subgrade, and start curing immediately so the surface doesn't dry and crack before it sets.

Concrete cracks because it shrinks as it cures and moves with temperature, creating tension that has to go somewhere. You can't prevent cracking entirely — only control where it happens, using joints.

Deliberate grooves cut about ¼ of the slab depth so the slab cracks along them — straight and hidden — instead of randomly. Cut them within a day of pouring.

In feet, no more than about 2–3 times the slab thickness in inches: a 4-inch slab gets joints every 8–12 ft, kept in roughly square panels.

Rebar does not stop concrete from cracking; it holds the pieces tightly together so cracks stay hairline rather than spreading. Control joints decide where cracks form, while steel controls their width.

Almost any permanent slab needs a compacted gravel base. The crushed stone spreads load, drains water away from the slab, and gives a stable surface to pour on — skipping it invites cracking and settling.

Four inches of compacted base is standard; 6 inches or more over soft or poorly draining soil, often built in layers.

Crushed stone weighs about 1.5 tons per cubic yard (roughly 3,000 lb), though it varies 1.4–1.7 with moisture and gradation.

At 2 inches deep, one cubic yard covers about 160 square feet; at 4 inches, about 80 sq ft. The gravel calculator works it out.

Roughly: topsoil 1.1 tons, sand 1.3 (dry), mulch 0.4, asphalt 2.0 per cubic yard. The materials calculators give both yards and tons.

Gravel is sold both by the cubic yard and by the ton, depending on the supplier, which is why these calculators show both. Use tons for ordering if that's how your supplier prices it.

Cement is the powder binder; concrete is the finished material — cement plus sand, gravel and water. You pour concrete, not cement.

A common ratio is 1 part cement : 2 parts sand : 3 parts aggregate, plus water. The mix ratio calculator converts it to quantities.

The weight of water divided by the weight of cement — the biggest control on strength. Lower (0.40–0.45) is stronger; higher (0.50–0.60) is weaker but easier to pour.

A cubic yard of concrete needs roughly 6 bags of 94-lb Portland cement for a typical 1:2:3 mix. A 94-lb bag is about 1 cubic foot of cement loose.

No, you should not add water to concrete that has started setting — it weakens the concrete and ruins the surface. Only adjust water during mixing, and add it sparingly even then.

About 1.125 standard 8×16 inch blocks per square foot of wall face, including the 3/8-inch mortar joint. The block wall calculator totals them.

About 0.30 cubic feet per 8-inch block when filling every cell. The block fill calculator handles partial fills too.

Roughly 6 cubic feet per 100 standard blocks (about one cubic foot per 16 blocks). The mortar calculator estimates bags.

About 4,050 pounds — roughly 2 tons — based on a density of 150 lb per cubic foot times 27 cubic feet per yard.

An 80-lb bag of dry mix yields about 0.60 cubic feet of concrete weighing roughly 90 lb once water is added.

At 4 inches thick, about 50 lb per square foot; at 6 inches, about 75 lb. That's based on 150 lb per cubic foot.