🧱 Concrete Calculator

How to Calculate Concrete Volume

Concrete is sold by the cubic yard (27 cubic feet) for truck delivery, or by the bag for smaller projects.

Slab Formula

Column/Post Formula

Converting to Cubic Yards

Pre-Mixed Concrete Bag Coverage

• 40 lb bag: ~0.30 cubic feet
• 60 lb bag: ~0.45 cubic feet
• 80 lb bag: ~0.60 cubic feet

For projects over 1 cubic yard, consider ordering ready-mix concrete delivered by truck. It's often more cost-effective and ensures consistent quality.

Working purely in bags? Our concrete bag calculator handles every bag size, plus mixing water, pallet counts, and the point where ready-mix becomes cheaper. If you're considering skipping the mixing altogether, read our honest assessment on the dry pour concrete page first.

Concrete Coverage Per Cubic Yard

One cubic yard of concrete equals 27 cubic feet. How much area that covers depends entirely on slab thickness:

Slab Thickness	Area Covered by 1 Cubic Yard
2 inches	162 sq ft
3 inches	108 sq ft
4 inches	81 sq ft
5 inches	65 sq ft
6 inches	54 sq ft
8 inches	40 sq ft

Working backwards from area? Enter your square footage and thickness in the calculator above, or start with our Square Footage Calculator.

How to Pour a Concrete Slab: Step by Step

Pouring concrete is straightforward, but the work happens fast once the truck arrives — so preparation is everything. Here's the full sequence for a typical slab.

1. Plan and call 811 first. Check local codes and permits, and have underground utilities marked before you dig — it's free and prevents dangerous mistakes.
2. Excavate and grade. Strip sod and topsoil, then dig to your slab thickness plus about 4 inches for the base. Keep the bottom level and firm.
3. Add and compact the base. Lay 4 inches of gravel or crushed stone and compact it in thin lifts with a plate compactor. A solid, well-drained base is the #1 defense against cracking.
4. Build the forms. Stake and brace 2x4 or 2x6 lumber around the perimeter. Build in a slight slope — about 1/8 to 1/4 inch per foot — so water drains away from structures.
5. Add reinforcement. Set fiber mesh or #3–#4 rebar on chairs at mid-slab depth. For interior slabs, lay a 6-mil vapor barrier over the base first.
6. Order or mix the concrete. Aim for a workable 4–5 inch slump. Don't be tempted to add extra water on site — it weakens the mix (see mistakes below). Use our calculator above to order the right amount plus 10%.
7. Pour and spread. Start at the far corner and work toward your exit. Move concrete with a shovel or come-along and consolidate it into the form corners.
8. Screed. Drag a straight 2x4 across the top of the forms in a sawing motion to strike off the surface level.
9. Float. Run a bull float over the surface to fill voids and bring up the paste. Then wait — do not do any further finishing until the bleed water sheen has evaporated.
10. Edge and cut joints. Round the edges with an edger, then tool or saw control joints (see spacing rule below).
11. Finish. A broom finish drags texture across the surface for slip resistance — ideal for exterior slabs.
12. Cure. Keep the surface damp (or use a curing compound) for at least 7 days. Concrete reaches full strength at 28 days.

7 Concrete Mistakes to Avoid

Most concrete failures trace back to a handful of avoidable errors. Watch out for these:

• Adding too much water. A soupy mix is easier to place but dramatically weaker. Excess water leads to surface cracking, dusting, and scaling. Keep the slump moderate and add water sparingly.
• Skipping base prep and compaction. Pouring on loose or uncompacted soil is the leading cause of settling and cracks. Always compact a gravel base.
• Too few control joints. Concrete will crack — joints decide where. Space them (in feet) no more than 2–3× the slab thickness in inches: a 4-inch slab needs joints every 8–12 feet, cut at least 1/4 of the slab depth deep.
• Finishing over bleed water. Troweling while water is still sitting on the surface traps it and causes scaling and a weak top layer. Wait for the sheen to disappear.
• Pouring in extreme temperatures. Below 40°F concrete may freeze before it cures; above 90°F it sets too fast. Use cold- or hot-weather measures (blankets, retarders, shade, damp subgrade) when needed.
• Not curing. Letting fresh concrete dry out too quickly can cut surface strength roughly in half. Keep it moist for at least 7 days.
• No vapor barrier under interior slabs. Without a 6-mil poly barrier, ground moisture wicks up through the slab and ruins flooring later.

Concrete Mix & Strength (PSI) Guide

Cement vs. concrete: they aren't the same thing. Cement is the powdered binder; concrete is the finished material made from cement, sand, gravel (aggregate), and water. You pour concrete, not cement.

Concrete strength is rated in PSI (pounds per square inch) after 28 days of curing. Match the mix to the job:

Strength	Best For
2,500 PSI	Non-structural fill, some footings
3,000 PSI	Patios, sidewalks, general residential slabs
3,500–4,000 PSI	Driveways, garage floors, freeze-thaw climates
4,000+ PSI	Heavy loads, commercial, structural work

Bagged Mix Types

• Standard mix (e.g. 3,000–4,000 PSI) — the everyday choice for slabs and footings.
• Fast-setting / high-early — sets in 20–40 minutes; great for posts and small repairs.
• Crack-resistant — contains synthetic fibers; good for thin slabs and overlays.
• Sand / topping mix — no large aggregate; for thin layers under 2 inches and resurfacing.

Bagged concrete makes sense up to about 1 cubic yard. Beyond that, ready-mix truck delivery is usually cheaper and far less work.

Curing, Weather & When You Can Use It

Curing — keeping concrete moist as it hardens — is what actually builds strength. Skipping it is one of the most common (and invisible) mistakes.

How Long Until You Can Use It?

• 24–48 hours: light foot traffic is okay.
• 7 days: concrete reaches about 70% strength — fine for normal foot traffic and to remove forms.
• 28 days: full design strength. Wait this long before parking vehicles on a new driveway.

Hot-Weather Pours

In heat, dampen the subgrade, pour early in the day, and cure aggressively — mist the surface or cover it so it doesn't flash-dry and crack. A set retarder buys working time.

Cold-Weather Pours

Only pour when temperatures stay above 40°F, never on frozen ground. Use insulating blankets to hold in the heat of hydration, and consider an accelerator. Protect the slab from freezing for the first few days.

Sealing

After the full 28-day cure, a penetrating or film-forming sealer helps resist stains, de-icing salts, and freeze-thaw damage — reapply every 1–3 years for exterior slabs.

Frequently Asked Questions

Is cement the same as concrete?

No. Cement is one ingredient in concrete. Concrete is the finished building material made by mixing cement with sand, gravel, and water. When people say "cement driveway" they almost always mean concrete.

How much does a yard of concrete weigh?

One cubic yard of concrete weighs roughly 3,700–4,050 pounds — about 2 tons. That's why ready-mix is delivered by heavy trucks and why a solid, compacted base matters so much.

Can I pour new concrete over old concrete?

Yes, if the existing slab is sound (no major cracking or heaving). A bonded overlay should be at least 2 inches thick; thinner resurfacing needs a bonding agent or a sand/topping mix. If the old slab is failing, remove it instead — new concrete will mirror the cracks beneath it.

Do I need rebar or wire mesh?

For a standard 4-inch residential slab, fiber mesh or welded wire mesh is usually enough to control cracking. Use rebar (typically #3–#4 on a grid) for slabs 5 inches or thicker, driveways, or anything carrying heavy loads or built on weak soil.

How long does concrete take to cure?

Concrete is walkable in 24–48 hours, reaches about 70% strength in 7 days, and hits full strength at 28 days. Keep it damp during the first week for the strongest result.

How many cubic feet are in a yard of concrete?

There are 27 cubic feet in one cubic yard of concrete. To convert your project's cubic feet to cubic yards, divide by 27. For example, a 10 × 10 slab at 4 inches thick is 33.3 cubic feet, or about 1.23 cubic yards.

How many yards of concrete are in a truck?

A fully loaded ready-mix concrete truck holds about 10 cubic yards (most carry 8–11). If you order less than a full truck, expect a "short-load" fee, so it's often worth rounding up small pours or combining work.

How many bags of concrete make a cubic yard?

It takes roughly 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags to make one cubic yard. Because that's a lot of mixing, ready-mix delivery is usually more economical for anything over about 1 cubic yard.

How many wheelbarrow loads are in a yard of concrete?

A standard 6-cubic-foot contractor wheelbarrow safely carries about 3 cubic feet of concrete, so one cubic yard is roughly 9 wheelbarrow loads. Smaller 2-cubic-foot barrows take 14 or more loads per yard.

How much extra concrete should I order?

Order 10% extra to account for spillage, uneven ground, and slight measurement errors. Need concrete for fence posts? Try our Fence Post Concrete Calculator.

How thick should a concrete slab be?

4" for sidewalks and patios, 5-6" for driveways, 6-8" for heavy vehicles or RV pads. Try our Patio, Driveway, Sidewalk, or Curb concrete calculators for project-specific estimates.

Can I mix concrete by hand?

For small projects (under 1/3 cubic yard), hand mixing is fine. Larger projects benefit from a mixer or ready-mix delivery.

How much does a yard of concrete cover?

A cubic yard is 27 cubic feet, so the area it covers depends entirely on thickness: about 81 sq ft at 4 inches, 108 sq ft at 3 inches, 65 sq ft at 5 inches and 54 sq ft at 6 inches. The formula is 27 ÷ (thickness in feet).

How many square feet of concrete are in a yard?

There is no single answer, because a cubic yard is a volume and square feet is an area — thickness converts between them. At the usual 4 inch slab depth one yard covers 81 sq ft; double the thickness to 8 inches and the same yard covers only 40 sq ft.

How do I figure out how many yards of concrete I need?

Multiply length × width × thickness with every measurement in feet, then divide by 27. A 4 inch slab is 0.333 ft thick, so a 20 × 20 ft driveway is 20 × 20 × 0.333 = 133.3 cubic feet, or 4.94 cubic yards. Order 5 to 5.5 to allow for subgrade variation.

What is the formula for concrete in cubic yards?

Cubic yards = (length in ft × width in ft × thickness in ft) ÷ 27. To get thickness in feet, divide inches by 12. The only step people usually miss is that last division by 27 — a cubic yard is 27 cubic feet, not 3.

How many yards of concrete do I need for a driveway?

For a typical 20 × 20 ft two-car driveway at 4 inches, 4.94 cubic yards — call it 5. At 5 inches, which is worth considering if a heavy vehicle will use it, the same driveway takes 6.2 cubic yards. Enter your own dimensions above for an exact figure.

How much concrete is in a cubic foot?

A cubic foot of concrete weighs roughly 145 lb. If you are mixing from bags, an 80 lb bag yields about 0.60 cubic feet, so a cubic foot takes a little under 1.7 bags. A 60 lb bag yields 0.45 cu ft and a 40 lb bag 0.30 cu ft.

How much area does 15 cubic yards of concrete cover?

15 cubic yards is 405 cubic feet, which covers about 1,215 sq ft at 4 inches or 810 sq ft at 6 inches. That is well past the point where ready-mix delivery beats mixing bags.

How do I figure square footage for concrete?

Square footage is just length × width in feet — the thickness is not part of it. Break an irregular slab into rectangles, work out each one, and add them together. Our square footage calculator does the splitting for you; bring the total back here with your thickness to get cubic yards.