Calculating Cubic Yards of Concrete

cubic yards = square feet/80

therefore

cubic yards = square yards/9 (approx)

so the "sidewalk" will finish 4" deep

Im no expert in this field but this is the only sense I can make out of it. For a 6" deep sidewalk you would divide the square footage by 54 although the concrete company would probably tell you to divide by 50 for ease.

The standard sidewalk is 4" deap. Any thinner and it will have a tendancy to crack

3' X 3' X 3' =27³ feet = 1³ yard of concrete

in a cubic foot you can get 3 of 4'' X 12 X 12

3 X 27 = 81

The standard sidewalk also used to have an expansion joint ~ every 4' the expansion joint was typically filled with a material that looked like 3/4" or 1" thick tar paper

80 was derived at by taking into consideration this joint.

That's silly. If your allowing for joints then you need less concrete not more! Any number divided by 80 is going to be greater than that same number divided by 81.

Were talking about pouring concrete not rocket science. Close to 4" deep is good enough. Ever see a concrete worker with calibers and micrometers to get just prefect. Be an expensive sidewalk

I've calculated several different areas using the "standard" methond [(area in sq.ft x 0.333')/27cu ft per cu yd] and the numbers come out very close.

for a 4 inch thick slab

4" = 0.333' 1/0.333 = 3 3*27 = 81

For a 6 in thick slab

6" = 0.5' 1/0.5 = 2 2*27 = 54

for a 12 inch thick slab

12" = 1' 1/1 = 1 1 * 27 = 27

All the concrete guys did was solved half of an equation with standard values and made a conversion constant. Hope this helps.

Mathematically, if L is in feet, W in feet, and T in inches, compute volume in cubic yards (L x W x T/12)/27= volume in cubic yards.

For practical purposes when you order read-mix-concrete you need to round-off the volume to the nearest high number.

Ok , lets not get so technical with a simple question !

any concrete finisher will tell you that the simple way(maybe not the precise way) they determine how much concrete to pour is take the square footage divide by 80, then times that by the thickness to get the yardage to use. The reason being is because how many footings and thichnesses of concrete are absolute. They all vary, just depends on who's digging and building the form are if you even have a footing. So put your slide rules away you don't need them, just get some old concrete finisher and pay him the 60 cents a square foot (thats what i paid last)to do it all for you

Thanks for all of your help!

Darvin

W X LENGTH X THICKNESS DIV BY 345 GIVE YOU THE EXACT YARDS

Dividing surface area by 80 is clearly a good, time-tested, rule-of-thumb, for estimating the volume of 4" concrete slabs, such as sidewalks. However, there are other factors at work. First of all, a sidewalk is typically "formed" on a levelled layer sand or (semi-?) compacted earth layer that is within a few hundredths of a foot of the desired level. The usually wooden forms are placed along the full length of the sidewalk. Those forms are usually "2x4's". The concrete is subsequently poured and then screeded, floated, finished and sometimes brushed to it's final, pre-curing elevation(s). However, said 2x4 may only provide a guide level of 3'-1/2" above the sand/earth. For those situations, it mght be better to divide the surface area by about 92.6 (= 3.5 x 27). And further, the sand/earth floor may well absorb much of the water in the concrete mix, and a little of the concrete itself, depending on the compaction of the sand/earth and the water content of the concrete. So, it might be better to divide by about 90 instead. Plus, you should always allow a few percent for wastage, etc. And this is a simple(?) application......

.......... and, of course, it's always better to double-check one's calcs before clicking on "submit" (blush, blush...)

Let me formally issue an addendum to read " 92.4 (= 12 X 27 / 3.5") "

And if the sidewalk went across a commercial driveway, it would need (woven wire mesh ?) to strengthen it against loading heavier than merely (domestic) traffic, which would further reduce the quantity of concrete required in the same space by ..........

Hey hey, New guy here, make this real easy for everyone, and any one that is looking for easy answer. http://www.concretenetwork.com/concrete/howmuch/calculator.htm

Gee guy your lucky if you get 70 not 81 Concrete company know how to fill the truck to there likings. Forget math or add in human greed factor

Been doing this for years 80 is your best number spillage plus if you are using more than 5 yards add 5%

Side walk are usally scraped out using 2x4 a 2x4 is 3.5 inches thick

Always watch for that because 3.5 will crack a lot faster than 4

For a 4 inch depth: Divide (Length*Width) by 81 derived from: 3 * 3 * 3 * 3

The first factor of 3 is derived from converting the 4" depth to Square Feet (4 / 12 = .33): Square Feet = (Length * Width) * .33 or Sq Feet = (Length * Width) / 3

The remaining three factors of 3 are derived from converting cubic feet to cubic yards, one factor of 3 for length and one factor of 3 for width and one factor of 3 for depth as follows:

Length (in Cubic Feet) / 3 = Length (in Cubic Yards)

Width (in Cubic Feet) / 3 = Width (in Cubic Yards)

Depth (in Cubic Feet) / 3 = Depth (in Cubic Yards)

Divide 27 by .333. Typically sidewalks are 4" thick (.333). He just figured the SF and divided by 80, rather than multipling by .33, then dividing by 27.