How Much Concrete Do I Need? A Slab-by-Slab Guide

Published 2026-05-05 · By the CalculatorHive editorial team

Concrete is unforgiving about quantity in a way most building materials are not. Run short on deck boards and you drive back to the store; run short mid-pour and you get a cold joint, a visible seam, and a weak line through the slab. Here is the arithmetic — volume, conversion, bag yields, waste — and where the bag-versus-truck line falls.

The formula: length × width × thickness ÷ 27

Concrete is priced and delivered in cubic yards, but you measure a slab in feet and inches. So the whole job is one unit conversion.

Work in feet for all three dimensions. Convert thickness by dividing inches by 12:

• 4 inches = 4 ÷ 12 = 0.3333 ft
• 5 inches = 5 ÷ 12 = 0.4167 ft
• 6 inches = 6 ÷ 12 = 0.5 ft

Multiply the three together and you have cubic feet. Because a cubic yard is 3 × 3 × 3 feet, divide cubic feet by 27 to get cubic yards.

A 10 × 12 ft patio at 4 inches: 10 × 12 × 0.3333 = 40 cubic feet, and 40 ÷ 27 = 1.48 cubic yards. That is the entire calculation. Our concrete calculator handles slabs, footings, and cylindrical piers, and our square footage calculator is useful first if your slab is L-shaped and you need to break it into rectangles.

For round footings — deck piers poured in a tube form — the formula is π × radius² × depth. A 12-inch-diameter tube 4 feet deep is 3.1416 × 0.5² × 4 = 3.14 cubic feet, or about five and a quarter 80 lb bags per pier. Six piers is 18.8 cubic feet, still comfortably bag territory.

How many bags is that?

Bag yields are printed on the sack and are consistent across the major brands, because they are set by the mix density rather than the marketing:

To go from volume to bags: cubic feet ÷ bag yield, then round up. Always round up — a partial bag is not a thing that exists once the mixer is running.

Common slab sizes and what they take

The table below assumes a 10% waste allowance and rounds bags up. Thicknesses are the usual choices: 4 inches for foot traffic, 5 inches once cars are involved.

Those last three bag counts are printed to make a point rather than to be followed.

Worked example: a 10 × 12 patio

Say you want a 10 × 12 ft patio, 4 inches thick, on a 4-inch compacted gravel base.

1. Area: 10 × 12 = 120 sq ft.
2. Volume: 120 × 0.3333 = 40 cubic feet = 1.48 cubic yards.
3. With 10% waste: 44 cubic feet = 1.63 cubic yards. You would order 1.75 yards, since trucks sell in quarter-yard increments.
4. Base: 4 inches of gravel over the same 120 sq ft is another 40 cubic feet, about 1.5 cubic yards or roughly 2 tons of compacted crushed stone — run it through the gravel calculator, which converts volume to tons at the right density.

Now the bag comparison. 44 cubic feet ÷ 0.6 = 74 bags of 80 lb mix — 5,920 lb, nearly three tons, to buy, haul, cut open, and mix a wheelbarrow at a time. At typical bag prices that is $370–$520 in material. A 1.75-yard ready-mix delivery usually runs $250–$400 plus a short-load fee of $50–$150 under the truck minimum (commonly 3 yards). Cost is roughly a wash; the difference is that the truck arrives at one water-cement ratio and empties in fifteen minutes.

When do you switch from bags to ready-mix?

The practical threshold is about 1 cubic yard — 45 bags. Below that, bags are genuinely easier: you can work alone and pour a pier today and another next weekend. Above it, three things break down at once. You need help you probably do not have; each hand-mixed batch has a slightly different water content, which means different strength and colour; and the first section starts curing before the last is placed.

Two factors push you toward a truck earlier than the arithmetic suggests. If the pour has to be monolithic — a garage floor, a driveway, anything structural — the strength penalty of cold joints matters. And above about 85 °F, mix stiffens fast enough that the bag route becomes a race.

Why you add 5–10% waste

Waste on a concrete order is not spillage, it is geometry. The subgrade under your forms is never perfectly level, so average thickness always exceeds nominal thickness; a half-inch of extra depth across 120 sq ft is 5 cubic feet on its own. Forms bow outward under the weight of wet concrete, and edges get over-filled deliberately so you have material to screed off.

Use 5% for a small, well-formed pour on a compacted flat base and 10% for rough ground or irregular edges. Have a plan for the surplus — an extra footing, a splash block, a form for a step — so nobody is scrambling.

Thickness, reinforcement, and joints

These four decisions determine whether the slab cracks:

• Thickness. 4 inches for patios, walkways, shed floors, and pads. 5 inches for a car driveway, 6 inches if a truck, RV, or dumpster will ever sit on it. Going from 4 to 5 inches adds 25% to the concrete cost and far more than 25% to the load capacity — it is the cheapest strength you can buy.
• Base. 4 inches of compacted crushed stone, not sand and not native soil. The base drains water away from the underside and gives a uniform bearing surface, which prevents the differential settlement that cracks slabs.
• Reinforcement. Welded wire mesh (6 × 6 W1.4/W1.4) is adequate for most patios. #3 (3/8 in) or #4 (1/2 in) rebar on an 18-inch grid is the usual choice for driveways and anything carrying vehicles. Steel belongs in the middle third of the slab's depth, held there on chairs — not laid on the gravel and hooked up with a rake, which leaves it in the wrong place half the time. Keep 3 inches of cover between steel and earth. For our 10 × 12 patio, #4 bar at 18 inches each way works out to about 8 bars at 12 ft plus 9 bars at 10 ft, roughly 186 linear feet before laps; the rebar calculator does this grid math including overlap.
• Control joints. Concrete will crack; joints decide where. Cut or tool them to at least one quarter of the slab depth, spaced in feet at roughly two to three times the thickness in inches — so an 8–12 ft grid on a 4-inch slab. Saw-cut joints go in within 6–12 hours of the pour.

Cure it wet. Concrete gains strength through hydration, not drying: keep the surface damp or covered for 3–7 days. It reaches roughly 70% of design strength at 7 days and the specified strength at 28 days. Do not pour below 40 °F without cold-weather measures.

Safety. Wet concrete is strongly alkaline and causes chemical burns that you often do not feel until hours later — wear waterproof gloves and boots, and never kneel in it in ordinary clothes. Dry mix contains crystalline silica; wear an N95 or better when cutting bags or saw-cutting joints, and cut wet where possible. An 80 lb bag is a two-hand lift.

Common questions

How much does a cubic yard of concrete weigh?

About 4,050 lb — roughly two tons — based on a typical density of 150 lb per cubic foot. This matters for two reasons: a half-ton pickup cannot legally or safely carry a yard of mixed concrete, and the load a slab imposes on backfilled or sloping ground is larger than people expect when they build near a retaining wall.

Can I pour a slab in sections over several weekends?

You can, but plan the joint deliberately rather than letting one happen. Form each section as its own slab with a clean edge, and either install a keyway or drill and epoxy short dowels across the joint so the two sections move together vertically. What you should not do is pour half a slab, stop for a day, and butt fresh concrete against a cured face with nothing tying them — that seam becomes the crack.

Is bagged concrete as strong as ready-mix?

The bagged product is formulated to reach a comparable compressive strength — most general-purpose mixes are rated around 4,000 psi at 28 days — but only if mixed to the water ratio on the bag. Adding extra water to make it easier to work is the single most common failure, and it costs strength quickly. Ready-mix is more consistent mostly because a batch plant measures water and a homeowner with a hose does not.

Do I need a permit for a concrete slab?

It depends on the jurisdiction and the use. Many places exempt a patio or walkway at grade under a size threshold but require permits and inspections for anything structural, anything a building sits on, and anything that changes drainage or the impervious-surface percentage of your lot. Driveway aprons in the public right-of-way almost always need approval. Call the building department before you form, not after you pour.