How much concrete do I need for a slab?

Multiply length by width by thickness with thickness in feet, then add about 10 percent waste. A 12 by 20 foot patio at 4 inches is 80 cubic feet, or 3.26 cubic yards with waste. The calculator returns yards, 80 pound bag count, and a delivered-cost range. For anything structural, get the dimensions from a stamped drawing first.

Do I need rebar in a concrete slab?

A 4 inch non-structural slab (patio, shed floor, walkway) can use welded wire mesh or fiber for crack control, or careful jointing on a small pad. A driveway at 6 inches wants #3 or #4 rebar on a grid, typically 18 inches on center. Anything carrying real load is engineered. Reinforcement controls cracking and load transfer; it does not change the concrete volume the calculator returns.

How thick should a concrete slab be?

Four inches for a patio, shed floor, or walkway. Six inches with rebar or wire mesh for a residential driveway. RV pads, workshops, and anything loadbearing are engineered at 8 inches or more with a stamped drawing. Add control joints every 24 to 36 times the thickness, which is every 8 to 12 feet on a 4 inch slab.

How many 80 lb bags of concrete for a slab?

Divide the slab volume in cubic feet by 0.6, the yield of one 80 pound bag per Quikrete's data sheet. A 12 by 20 foot patio at 4 inches is 88 cubic feet with waste, so 147 bags. That is a tow-behind mixer and a long day; past about a cubic yard (45 bags), a ready-mix truck is cheaper and far less work.

MATERIALFOREMAN · SHEET C-001 · CONCRETE WORKS

CALC-001 · DRAWING NO. C-001 · REV A

Concrete slab calculator.

Drafted to scale · cited sources · honest numbers

Enter the slab dimensions. The calculator works out cubic yards, 80 lb bag equivalents, and a delivered-cost range from national-baseline ready-mix and bagged concrete pricing.

◈ DRAFTING PANEL · CONCRETE TAKEOFF · N.T.S. SHEET C-001 · REV A

Redline · scope notice Materials takeoff only. Does not evaluate subgrade, reinforcement, frost depth, or structural loading. Consult ACI 332 or a licensed engineer for any structural pour.

Note 01 · Field Procedure

How to measure the pour

Square the forms. Lay out the form boards and measure the inside dimension on the long side. Check that the diagonals are equal before recording anything.
Record length and width in feet, inside the forms. Round up to the nearest half foot for irregular edges.
Measure thickness from the top of the compacted gravel base to the top of the form board, not to ground level.
Check for a crown in the subgrade. If it dips in the middle, add 0.5 inch to thickness. The crown is the #1 reason DIY pours run short.
Rectangles only. L-shaped patios, curved pads, and dog-ears: break into rectangles, run the calc once per rectangle, and sum the cubic yards at the end.

Note 02 · Equation

The formula

yd³ = ( L × W × T ) ÷ 324 × ( 1 + waste ÷ 100 )

Llength in feet

Wwidth in feet

Tthickness in inches

324ft³→yd³ factor for T in inches (12 × 27)

Note 03 · Sanity Ranges

Typical slab thicknesses

T	Use case	Notes
4"	Patio / shed floor / walkway	Non-structural, light foot traffic.
5"	Heavy walkway	#3 rebar at 18" o.c. in freeze-thaw zones.
6"	Residential driveway	Passenger cars; rebar or WWM required.
8"+	RV pad / workshop / garage	Engineer-designed; stamped drawing.

Note 04 · References

Sources

Authorities cited on this sheet

ACI 332-20: Residential Code Requirements for Structural Concrete · American Concrete Institute. Defines minimum thicknesses, reinforcement schedules, and aspect-ratio limits for residential concrete slabs. Paid standard; the ACI store page is the canonical link. For free residential slab guidance, see the NRMCA CIP series below.
Quikrete 80 lb Concrete Mix Technical Data Sheet · Yield 0.6 ft³/bag, the basis for the 80 lb bag count line on the spec slip.
NRMCA Concrete in Practice information series · National Ready Mixed Concrete Association reference library on ready-mix delivery, short-load fees, and pour preparation.

Note 05 · Field Manual

What the sheet count does not tell you

A 12 by 20 patio, worked out

Take a 12 by 20 foot patio poured 4 inches thick. Volume is length times width times thickness in feet: 12 times 20 times one-third of a foot is 80 cubic feet, or 2.96 cubic yards. Add the 10 percent waste the trade orders for spillage and uneven subgrade and the number is 3.26 cubic yards. That clears the 3 yard plant minimum, so a ready-mix truck delivers it without a short-load fee. In 80 pound bags the same pour is 147 bags, which means a tow-behind mixer and a long day. Delivered ready-mix runs about 440 to 570 dollars; bags cost less per yard but buy you the labor.

Bags or ready-mix, and where the line sits

The line between bagging it and calling a truck sits around one cubic yard. Below a yard, bagged concrete wins: no short-load fee, no truck to schedule, and you mix what you need. A cubic yard is about 45 bags of 80 pound mix, and mixing 45 bags by hand is a brutal day, so most crews rent a tow-behind mixer past half a yard. Above a yard, ready-mix takes over on both price and effort. Plants set a minimum, usually around 3 yards, and add a short-load fee of 50 to 150 dollars below it. A pour that lands at 2 yards is the awkward middle: price the truck plus the fee against the bag count plus a mixer rental before deciding.

Thickness, joints, and the crown

Thickness is set by use, not preference. Four inches carries a patio, a shed floor, or a walkway. A residential driveway wants 6 inches with rebar or wire mesh. An RV pad or a workshop floor is engineered work with a stamped drawing. Whatever the thickness, control joints have to follow: ACI 332 spaces them at 24 to 36 times the slab thickness, so a 4 inch slab needs a tooled or sawn joint every 8 to 12 feet or it cracks on its own schedule. The other silent failure is the subgrade crown. If the middle of the pad dips, the slab is thicker there than the forms say and the pour runs short. Check for the dip and add half an inch to thickness if you find one; that is the number one reason a DIY slab orders a yard light.

Note 06 · Common Questions

Common questions

How much concrete do I need for a slab?: Multiply length by width by thickness with thickness in feet, then add about 10 percent waste. A 12 by 20 foot patio at 4 inches is 80 cubic feet, or 3.26 cubic yards with waste. The calculator returns yards, 80 pound bag count, and a delivered-cost range. For anything structural, get the dimensions from a stamped drawing first.
Do I need rebar in a concrete slab?: A 4 inch non-structural slab (patio, shed floor, walkway) can use welded wire mesh or fiber for crack control, or careful jointing on a small pad. A driveway at 6 inches wants #3 or #4 rebar on a grid, typically 18 inches on center. Anything carrying real load is engineered. Reinforcement controls cracking and load transfer; it does not change the concrete volume the calculator returns.
How thick should a concrete slab be?: Four inches for a patio, shed floor, or walkway. Six inches with rebar or wire mesh for a residential driveway. RV pads, workshops, and anything loadbearing are engineered at 8 inches or more with a stamped drawing. Add control joints every 24 to 36 times the thickness, which is every 8 to 12 feet on a 4 inch slab.
How many 80 lb bags of concrete for a slab?: Divide the slab volume in cubic feet by 0.6, the yield of one 80 pound bag per Quikrete's data sheet. A 12 by 20 foot patio at 4 inches is 88 cubic feet with waste, so 147 bags. That is a tow-behind mixer and a long day; past about a cubic yard (45 bags), a ready-mix truck is cheaper and far less work.

PROJ MATERIALFOREMAN

SHT C-001 / 014

REV A · 2026-06-07

DRAWN MF