Civil & Structural Engineering

Concrete Calculator

Concrete calculator for slabs, footings, columns, and circular tubes: calculate cubic yards, bag counts, and cost comparisons.

Formula Volume (rectangular) = L × Wd × DReviewed Sep 8, 2026

Pouring concrete for a slab, footing, wall, column, post hole, or circular tube? This concrete slab calculator and concrete volume calculator turns your pour dimensions into cubic yards - the unit ready-mix concrete is usually sold and quoted in - plus cubic feet, bag count, and separate ready-mix versus bagged cost estimates. Choose a rectangular, round, or tube/ring pour, enter dimensions, quantity, and waste allowance, then review the order-ready volume.

Calculation Bench

Pour Shape

Dimensions

01Length

L · Pour length

02Width

Wd · Pour width

03Thickness

D · Slab, footing, or ring thickness

04Quantity

Q · Identical pours

Mix & Pricing

05Waste Factor

W% · 5-10% is standard

06Bag Weight

wb (lb) · Optional - 40/60/80 lb are standard sizes

07Price per Cubic Yard

$/yd³ · Optional - ready-mix pricing

08Price per Bag

$/bag · Requires Bag Weight to compute cost

Solution

Enter pour dimensions to calculate concrete needed.

Volume (rectangular) = L × Wd × D

Formula Sheet

V = L \times W_d \times D

V = \pi \times \left(\dfrac{D_m}{2}\right)^2 \times D

V = \pi \times \left[\left(\dfrac{D_o}{2}\right)^2 - \left(\dfrac{D_i}{2}\right)^2\right] \times D

V_t = V \times Q

V_{waste} = V \times (1 + W\%)

VConcrete Needed
LLength
WdWidth
DmDiameter
DoOuter Diameter
DiInner Diameter
DThickness / Height
QQuantity
W%Waste Factor
wbBag Weight

Variables & Units

Symbol	Variable	Description	Common Units
V	Concrete Needed	Total concrete volume to purchase, including quantity and waste allowance, in cubic yards.
L	Length	Rectangular pour length.	ft, m
Wd	Width	Rectangular pour width.	ft, m
Dm	Diameter	Round pour diameter.	in, ft
Do	Outer Diameter	Outside diameter for a circular slab, ring, or hollow tube.	in, ft
Di	Inner Diameter	Inside void diameter for a circular slab, ring, or hollow tube.	in, ft
D	Thickness / Height	Slab/footing thickness, column height, or tube/ring thickness.	in, ft
Q	Quantity	Number of identical pours included in the total volume.
W%	Waste Factor	A decimal allowance for spillage and form leakage (e.g. 0.10 = 10%).
wb	Bag Weight	Weight of one bag of pre-mixed concrete, in pounds - used to compute a bag count.

How to Use This Calculator

01Choose Rectangular for a concrete slab, footing, or wall, Round for a column or post hole, or Tube / Ring for a circular slab or hollow tube.
02Rectangular: enter Length and Width. Round: enter Diameter. Tube / Ring: enter Outer Diameter and Inner Diameter.
03Enter Thickness/Height - slab or footing thickness (commonly 4-6 in), column height, or tube/ring thickness.
04Enter Quantity for repeated identical pours, such as multiple post holes or identical footings.
05Set a Waste Factor: 5-10% is standard, to cover spillage, form leakage, and uneven sub-grade.
06Optionally enter a Bag Weight (40, 60, or 80 lb are the standard sizes) for a bag count, plus price per bag or price per cubic yard for cost comparison.
07The calculator shows cubic yards, cubic feet, bag count, and separate ready-mix versus bagged estimates so the two ordering paths are not mixed together. Before ordering, round up according to supplier minimums and your site conditions.

How the Formula Works

For a rectangular pour, volume is simply length × width × thickness - the geometric space the concrete needs to fill. For a round pour, it is the area of a circle (π × radius²) times the height, using half the entered diameter as the radius. For a tube or circular slab with a hole, it subtracts the inner circle area from the outer circle area before multiplying by thickness.

Quantity is applied before waste, so six identical post holes are treated as six full geometric pours and then the waste allowance is added to the total.

The waste factor is applied on top of that raw geometric volume, since a real pour rarely uses exactly the calculated amount - form leakage, an uneven sub-grade, and spillage during placement all add up, and running short mid-pour means calling for another truck or another set of bags.

Ready-mix concrete is quoted and delivered by the cubic yard in the United States, which is why that is the primary unit here even though the underlying math works in whatever length unit you enter.

Bag count uses the industry-standard rule that one bag's yield in cubic feet is its weight divided by about 133 lb per cubic foot - the reason a 40 lb bag yields 0.30 ft³, a 60 lb bag 0.45 ft³, and an 80 lb bag 0.60 ft³, all confirmed on Quikrete's and Sakrete's own technical data sheets. That ratio holds for any bag weight you actually have on hand, not just those three sizes.

Worked Example 01

10x10 ft slab, 4 in thick

Known

Length (L): 10 ft
Width (Wd): 10 ft
Thickness (D): 4 in
Waste Factor (W%): 10%
Bag Weight (wb): 80 lb

Formula

Volume (rectangular) = L × Wd × D

Substitution

Volume = 10×10×(4/12) = 33.33 ft³, with waste = 36.67 ft³ = 1.36 yd³, Bags = ⌈36.67/0.60⌉

Result

≈1.36 yd³ · 62 bags (80 lb)

A 10 by 10 foot slab poured 4 inches thick needs about 1.23 cubic yards of concrete before waste - with a 10% allowance for spillage and form leakage, that's 1.36 cubic yards, or 62 bags at 80 lb each (0.60 ft³ yield per bag).

Worked Example 02

Round column, 12 in diameter, 8 ft tall

Known

Diameter (Dm): 12 in
Height (D): 8 ft
Waste Factor (W%): 10%
Bag Weight (wb): 60 lb

Formula

Volume (round) = π × (Dm / 2)² × D

Substitution

Volume = π×(0.5)²×8 = 6.28 ft³, with waste = 6.91 ft³ = 0.256 yd³, Bags = ⌈6.91/0.45⌉

Result

≈0.26 yd³ · 16 bags (60 lb)

A round column form 12 inches across and 8 feet tall needs about 0.23 cubic yards of concrete before waste - with a 10% allowance, that's 0.26 cubic yards, or 16 bags at 60 lb each. At this size, bagged concrete is typically far more practical than ordering a ready-mix truck, which often carries a minimum order well above what this pour needs.

Worked Example 03

Circular slab or tube, 5 ft outer diameter, 4 ft inner diameter, 6 in thick

Known

Outer Diameter (Do): 5 ft
Inner Diameter (Di): 4 ft
Thickness (D): 6 in
Waste Factor (W%): 10%
Bag Weight (wb): 80 lb

Formula

Volume (tube/ring) = π × [(Do / 2)² - (Di / 2)²] × D

Substitution

Volume = π×[(2.5)²-(2)²]×0.5 = 3.53 ft³, with waste = 3.89 ft³ = 0.144 yd³, Bags = ⌈3.89/0.60⌉

Result

≈0.14 yd³ · 7 bags (80 lb)

A circular pour with a center opening uses the annulus area, not the full outside circle. Subtracting the 4 ft inner opening from the 5 ft outside diameter keeps the estimate from over-ordering concrete.

Applications

01Estimating ready-mix cubic yards for a concrete slab, driveway, patio, walkway, garage floor, or foundation footing pour
02Sizing a bagged-concrete order and bag count for small DIY pours like post footings or fence post bases
03Comparing ready-mix versus bagged-concrete cost for a given pour size before ordering
04Estimating circular slab, ring, or tube pours where an inner void should be subtracted
05Creating an order-ready starting quantity before calling a ready-mix supplier or buying bagged concrete

Standard Concrete Bag Yields (Quikrete / Sakrete)

Bag Weight	Yield (Cubic Feet)	Bags per Cubic Yard
40 lb	0.30 ft³	90
60 lb	0.45 ft³	60
80 lb	0.60 ft³	45

Assumptions

01The pour is a uniform rectangular slab/footing/wall, round column/post hole, or circular tube/ring. Irregular or multi-section pours should be broken into simple shapes and summed separately.
02Bag yield uses the industry-standard ~133 lb per cubic foot density for pre-mixed bagged concrete (Quikrete/Sakrete data sheets) - a different bagged product's yield may vary slightly from this figure.
03Ready-mix pricing is entered per cubic yard, matching how ready-mix concrete is actually quoted and delivered.

Where This Model Stops

01Does not calculate rebar, wire mesh, or other reinforcement - those are sized separately from the concrete volume itself, based on the structural design.
02Does not account for ready-mix minimum order quantities - many suppliers have a minimum (often around 1 cubic yard) or a short-load surcharge below it, which can make bagged concrete more practical and cheaper for small pours.
03Does not model curb-and-gutter or stair geometry directly - use the dedicated stair calculator for stairs, or break curbs into rectangular sections and sum them.
04Does not include delivery fees, short-load fees, reinforcement, excavation, finishing, curing compound, forms, labor, or pump/truck access constraints.

References

[1]
Concrete Calculator - How Much Do I Need? (Bags & Yards)
Concrete Network
Reference for the rectangular/round volume formulas and the standard practice of quoting ready-mix concrete by the cubic yard.
[2]
Concrete Bag Yield / Bags Per Cubic Yard (Quikrete Data Sheet #1101)
Quikrete / Sakrete (via multiple bag-yield reference calculators)
Reference for the 40/60/80 lb bag yields (0.30/0.45/0.60 ft³) and the underlying ~133 lb/ft³ density rule, cross-checked across independent bag-calculator sources.

Frequently Asked Questions

Should I order ready-mix or use bags?

It comes down to volume: most ready-mix suppliers have a minimum order or a short-load surcharge below it, so small pours like a single post footing or a short walkway section are usually cheaper and more practical with bagged mix. Larger pours like a driveway or garage slab are usually cheaper per cubic yard as ready-mix.

Why does bag yield depend on bag weight?

Because a bag's finished (mixed and placed) volume is its dry weight divided by pre-mixed concrete's density, about 133 lb per cubic foot per Quikrete's and Sakrete's own data sheets - heavier bags simply contain more dry material, so they yield proportionally more finished concrete. That's why an 80 lb bag (0.60 ft³) yields exactly twice what a 40 lb bag (0.30 ft³) does.

How much waste factor should I use?

5% is reasonable for a well-prepared, level sub-grade with tight forms. 10% is a safer default for typical DIY conditions, where form leakage, an uneven base, or minor spillage during placement are more likely to eat into the calculated volume.

What thickness should I use for a concrete slab?

4 inches is standard for a typical patio, walkway, or shed slab. 5-6 inches is more common for a driveway that needs to carry vehicle loads, and structural slabs (like a garage floor with heavier point loads) may call for more - check your local code or an engineer's spec for load-bearing applications.

Why does the calculator show both cubic yards and bags?

Ready-mix concrete is usually ordered in cubic yards, while small DIY pours are often bought as 40, 60, or 80 lb bags. Showing both keeps the estimate useful whether the next step is calling a supplier or loading bags at a store.

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