Free Concrete Slab + Bags Needed Calculator

If you are thinking of building a greenhouse, shed, garage, or covering your patio with cement but are not sure if the concrete slab you are considering should be included in your budget take a look at our calculator.

We have worked hard to calculate all costs related to having a concrete slab poured for your next project. Of course, there are many variables that could affect the final price but we feel that this calculator will get relatively close for most situations.

The following instructions will help guide you through the process of using our Free Concrete Slab + Bags Needed Calculator.

This Calculator includes:

• Curb Calculator
• Footings Calculator
• Concrete Bags Calculator
• Concrete Slab Thickness Calculator

Free Concrete Slab + Bags Needed

Concrete calculator formula

Using the measurements provided, our calculator first works out the volume of the required concrete mix using the formulae given below.

Slabs, Footings and Walls

The total volume is calculated using the formula:

Volume=Length×Width×Thickness

Holes & Columns

The total volume is calculated using the formula:

Volume=π×Radius2×DepthVolume=π×Radius2×Depth

Where

Radius=Diameter2Radius=Diameter2

Circular Slabs and Tubes

The total volume is calculated using the formula:

Volume=π×(Outerradius2–Innerradius2)×HeightVolume=π×(Outerradius2–Innerradius2)×Height

Where

Radius=Diameter2Radius=Diameter2

Curb & Gutter Barrier

The total volume is calculated using the formula:

Volume=Curbvolume+GuttervolumeVolume=Curbvolume+Guttervolume

Where

Curbvolume=Curbdepth×Curbheight×LengthCurbvolume=Curbdepth×Curbheight×Length

and

Guttervolume=(Gutterwidth+Curbdepth)×Flagthickness×LengthGuttervolume=(Gutterwidth+Curbdepth)×Flagthickness×Length

Stairs

The total volume is calculated using the formula:

Volume=Volumeunderplatform+VolumeunderstepsVolume=Volumeunderplatform+Volumeundersteps

Volumeunderplatform=Platformdepth×Rise×Width×NumberofstepsVolumeunderplatform=Platformdepth×Rise×Width×Numberofsteps

Volumeunderstep=Run×Rise×Width×NumberofStepVolumeunderstep=Run×Rise×Width×NumberofStep

Note ⚠️

If there are multiple steps, separately calculate the volume under the steps for each and then sum to find the total volume under the steps (more details in the example).

Want to know the best part?

If you need to construct multiple concrete slabs/columns, etc., then our calculator can work out the total volume required:

Totalvolume=Volume×QuantityTotalvolume=Volume×Quantity

Our calculator also works out the total concrete mix weight and required number of bags (using a standard 40lb bag of concrete which weights 145 pounds per cubic foot):

Weight=Totalvolume×145lb/ft3Weight=Totalvolume×145lb/ft3

Bags=Weight40lbBags=Weight40lb

Also, if you know the price per unit volume, then our calculator can work out the total cost:

Cost=Totalvolume×PriceperunitofvolumeCost=Totalvolume×Priceperunitofvolume

But what happens if I’m measuring in different units?

It’s likely that you are wondering what to do if your units aren’t in feet and pounds.

Well, the solution couldn’t be easier!

Our calculator can do all the conversions for you, meaning you don’t have to.

For each measurement of length, you are able to work in metric units (feet, yards, inches) or imperial units (centimeters, meters).

Weight can be given in pounds (lb), US short tons (t), imperial-long tons (long t) or kilograms.

Simply select the desired units in the drop down options and our concrete calculator will do the following conversions:

1ft=12in=0.33yd=30.48cm=0.3048m1ft=12in=0.33yd=30.48cm=0.3048m

1t=2000lb=0.893longton=907kg1t=2000lb=0.893longton=907kg

1ft3=1728in3=0.037yd3=28,317cm3=0.0283m31ft3=1728in3=0.037yd3=28,317cm3=0.0283m3

Please note, other standard unit conversions that might be helpful are:

145lb/ft3=2400kg/m3=3915lb/yd3145lb/ft3=2400kg/m3=3915lb/yd3

Example calculations

Before we begin please note: The number of bags required must be rounded up, otherwise you’ll be left without sufficient concrete.

You’ll notice that we do this with all our examples (but the calculator does this for you!).

Standard wall slab

Let’s say I need to construct a square concrete slab that is 12 feet wide (an area of 144 square feet) and 4 inches deep.

I want to calculate how many 40 lb bags of concrete I need.

The calculator would perform the following calculations to calculate that the slab would need 174 bags:

Volume=Length×Width×Thickness=12ft×12ft×4in=48ft3Volume=Length×Width×Thickness=12ft×12ft×4in=48ft3

Weight=Totalvolume×145lb/ft3=48ft3×145=6960lbWeight=Totalvolume×145lb/ft3=48ft3×145=6960lb

Bags=Weight40lb=6960lb40lb=174bagsBags=Weight40lb=6960lb40lb=174bags

5 columns

Let’s say I need to construct 5 columns measuring 2 feet in diameter and 3 inches high. I want to estimate how many 40lb bags of concrete I need.

The gravel estimator does the following calculations:

Volume=π×Radius2×Depth=π×1ft2×3in=0.7854ft3Volume=π×Radius2×Depth=π×1ft2×3in=0.7854ft3

Totalvolume=Volume×Quantity=0.7854ft3×5=3927ft3Totalvolume=Volume×Quantity=0.7854ft3×5=3927ft3

Weight=Totalvolume×145lb/ft3=3927ft3×145=569lbWeight=Totalvolume×145lb/ft3=3927ft3×145=569lb

Bags=Weight40lb=569lb40lb=14.2→15bagsBags=Weight40lb=569lb40lb=14.2→15bags

Circular slab with price per unit of volume

I want to construct a circular slab of concrete measuring 5 ft high.

I want to figure out how many bags of 40 lb concrete I need to do this.

The outer diameter is 10 ft and the inner diameter is 9 ft. The price of the concrete is $108 per cubic yard.

The ready mix concrete calculator does the following calculations:

Volume=π×(Outerradius2–Innerradius2)×Height=π×(5ft2–4.5ft2)×5ft=74.6ft3Volume=π×(Outerradius2–Innerradius2)×Height=π×(5ft2–4.5ft2)×5ft=74.6ft3

Weight=Volume×145lb/ft3=74.6ft3×145=10819lbWeight=Volume×145lb/ft3=74.6ft3×145=10819lb

Bags=Weight40lb=10819lb40lb=270.4→271bagsBags=Weight40lb=10819lb40lb=270.4→271bags

Cost=Volume×Priceperunitofvolume=74.6ft3×108$/yd3=$298.45Cost=Volume×Priceperunitofvolume=74.6ft3×108$/yd3=$298.45

But wait, there’s more!

We imagine you are wondering what happens if you want to use concrete bags weigh different amounts. 🤔

Well, our calculator can figure this out for you as well.

Simply select from the multiple options in the drop-down menu to select a different weight.

4 curbs with price per unit volume and 50 lb bags of concrete

For our next example, let’s say we need to construct 4 curbs with the following measurements:

CurbDepth=3inCurbDepth=3in

CurbHeight=4inCurbHeight=4in

GutterWidth=10inGutterWidth=10in

FlagThickness=3inFlagThickness=3in

Length=10ftLength=10ft

The calculator would first work out the total volume of the 4 curbs:

Curbvolume=Curbdepth×Curbheight×Length=3in×4in×10ft=0.8333ft3Curbvolume=Curbdepth×Curbheight×Length=3in×4in×10ft=0.8333ft3

Guttervolume=(Gutterwidth+Curbdepth)×Flagthickness×Length=(10in+3in)×3in×10ft=2.7083ft3Guttervolume=(Gutterwidth+Curbdepth)×Flagthickness×Length=(10in+3in)×3in×10ft=2.7083ft3

Volume=Curbvolume+Guttervolume=0.8333ft3+2.7083ft3=3.54ft3Volume=Curbvolume+Guttervolume=0.8333ft3+2.7083ft3=3.54ft3

Totalvolume=Volume×Quantity=3.54ft3×4=14.167ft3Totalvolume=Volume×Quantity=3.54ft3×4=14.167ft3

Then, just as with the previous examples, the calculator would perform similar calculations to work out total weight, number of 50 lb bags and total cost.

Weight=Totalvolume×145lb/ft3=14.167ft3×145=2054lbWeight=Totalvolume×145lb/ft3=14.167ft3×145=2054lb

Bags=Weight50lb=2054lb50lb=41.08→42bagsBags=Weight50lb=2054lb50lb=41.08→42bags

Cost=Totalvolume×Priceperunitofvolume=14.167ft3×108$/yd3=$56.67Cost=Totalvolume×Priceperunitofvolume=14.167ft3×108$/yd3=$56.67

The volume of 4 steps

In our final example, let’s say we have 3 steps leading up to a platform (for a total of 4 steps), with the following measurements:

Rise=7yardsRise=7yards

Run=5yardsRun=5yards

Platformdepth=8yardsPlatformdepth=8yards

Width=9yardsWidth=9yards

Numberofsteps=4Numberofsteps=4

The calculator first works out the volume under the platform:

Volumeunderplatform=Platformdepth×Rise×Width×Numberofsteps=8×7×9×4=2016yd3Volumeunderplatform=Platformdepth×Rise×Width×Numberofsteps=8×7×9×4=2016yd3

Then, it uses the following formula to calculate the volume (in cubic yards) under each step:

Volumeunderstep=Run×Rise×Width×NumberofStepVolumeunderstep=Run×Rise×Width×NumberofStep

Therefore:

Volumeunderstep1=Run×Rise×Width×NumberofStep=5×7×9×1=315yd3Volumeunderstep1=Run×Rise×Width×NumberofStep=5×7×9×1=315yd3

Volumeunderstep2=Run×Rise×Width×NumberofStep=5×7×9×2=630yd3Volumeunderstep2=Run×Rise×Width×NumberofStep=5×7×9×2=630yd3

Volumeunderstep3=Run×Rise×Width×NumberofStep=5×7×9×3=945yd3Volumeunderstep3=Run×Rise×Width×NumberofStep=5×7×9×3=945yd3

Finally, the calculator sums up the volume under the platform and each step to calculate the total volume in cubic yards:

Totalvolume=Volumeunderplatform+Volumeundersteps=2016yd3+315yd3+630yd3+945yd3=3906yd3Totalvolume=Volumeunderplatform+Volumeundersteps=2016yd3+315yd3+630yd3+945yd3=3906yd3

Our online concrete calculator is a powerful tool that you can use to help ensure your next landscaping project goes smoothly.

Don’t forget, it can be helpful to purchase slightly more concrete than the volume the estimator calculates to allow for spillage and alterations in slab depth.

This dramatically reduces the probability of having an insufficient concrete mix.