I just need a little math help...
November 6, 2008 2:41 PM Subscribe
[ConstructionFilter] How do I convert the square footage of a house to the necessary amount of concrete, in cubic yards, for flatwork?
By "flatwork" do you mean a basement floor, or perhaps a slab on grade? Are you including footers?
posted by jon1270 at 2:51 PM on November 6, 2008
posted by jon1270 at 2:51 PM on November 6, 2008
Hmm. Is there a direct equation between the footage of your house and the sidewalk? Does it go along the frontage of your lot? If so, LengthxWidthxDepth/27 = yardage.
Widths depend on locality, but most commonly sidewalks are 5 feet wide and 4 inches (.333 feet) thick.
posted by hwyengr at 3:04 PM on November 6, 2008
Widths depend on locality, but most commonly sidewalks are 5 feet wide and 4 inches (.333 feet) thick.
posted by hwyengr at 3:04 PM on November 6, 2008
Response by poster: Thanks everybody. I actually don't know all of the details. A friend was trying to figure something out at her fairly new job. Of course, I have informed her that the largest source of info on teh intarwebs can be found here.
Again, much thanks!
posted by mitzyjalapeno at 3:32 PM on November 6, 2008
Again, much thanks!
posted by mitzyjalapeno at 3:32 PM on November 6, 2008
Wait, you were trying to convert cubic feet into cubic yards? That's what the marked best answer does.
posted by RustyBrooks at 3:38 PM on November 6, 2008
posted by RustyBrooks at 3:38 PM on November 6, 2008
I keep re-reading your question and it just makes no sense, especially when you start talking about sidewalks.
posted by RustyBrooks at 3:40 PM on November 6, 2008
posted by RustyBrooks at 3:40 PM on November 6, 2008
I keep re-reading your question and it just makes no sense, especially when you start talking about sidewalks.
Yeah -- I'm not sure how the square footage of a house has any correlation to how much concrete you need for a sidewalk. The square footage would have something to do with a floor slab, though. The perimeter of a house might have something to do with a sidewalk.
posted by LionIndex at 4:03 PM on November 6, 2008
Yeah -- I'm not sure how the square footage of a house has any correlation to how much concrete you need for a sidewalk. The square footage would have something to do with a floor slab, though. The perimeter of a house might have something to do with a sidewalk.
posted by LionIndex at 4:03 PM on November 6, 2008
Just in case somebody moonlighting as an accountant really, truly doesn't know this already:
To work out how much concrete you need to get to make a sidewalk, multiply the length of the sidewalk by its width, which will give you the area; then multiply that again by the thickness of concrete you want to lay, to give you the volume of concrete required.
Then, the only problem is converting units. If you were working in metric units, all the conversion factors would be nice simple multiples of 10. Since you're using imperial measurements, the least error-prone way is to convert everything to inches before you do the multiplication, which will then give you a result in cubic inches. And since there are 36 inches in a yard, there are 363 = 46656 cubic inches in a cubic yard.
For example: say you wanted to make a sidewalk twenty feet long, four feet wide and four inches thick.
First, convert the length to inches: 20 feet = 240 inches.
Next, convert the width: 4 feet = 48 inches.
Multiply those together to get the area: 240 inches * 48 inches = 11520 square inches.
Multiply that by the thickness in inches: 11520 square inches * 4 inches = 46080 cubic inches.
Convert to cubic yards: 46080 cubic inches / 46656 cubic inches per cubic yard = 0.99 cubic yards; near enough to 1.
posted by flabdablet at 4:45 PM on November 6, 2008
To work out how much concrete you need to get to make a sidewalk, multiply the length of the sidewalk by its width, which will give you the area; then multiply that again by the thickness of concrete you want to lay, to give you the volume of concrete required.
Then, the only problem is converting units. If you were working in metric units, all the conversion factors would be nice simple multiples of 10. Since you're using imperial measurements, the least error-prone way is to convert everything to inches before you do the multiplication, which will then give you a result in cubic inches. And since there are 36 inches in a yard, there are 363 = 46656 cubic inches in a cubic yard.
For example: say you wanted to make a sidewalk twenty feet long, four feet wide and four inches thick.
First, convert the length to inches: 20 feet = 240 inches.
Next, convert the width: 4 feet = 48 inches.
Multiply those together to get the area: 240 inches * 48 inches = 11520 square inches.
Multiply that by the thickness in inches: 11520 square inches * 4 inches = 46080 cubic inches.
Convert to cubic yards: 46080 cubic inches / 46656 cubic inches per cubic yard = 0.99 cubic yards; near enough to 1.
posted by flabdablet at 4:45 PM on November 6, 2008
It's easiest to convert the inches to feet (1 inch = .083 feet), multiply by the area, then divide the volume in cubic feet by 27.
posted by electroboy at 6:43 PM on November 6, 2008
posted by electroboy at 6:43 PM on November 6, 2008
Yeah, but it's less error-prone to work with units that (a) make your numbers big enough to demand a calculator instead of tempting you to use your possibly wonky mental arithmetic and (b) stay as whole numbers until the very last stage.
The main thing is to use the same units for length, width and thickness so that you end up with a volume in cubic somethings, and to know how many of those cubic somethings fit in a cubic yard.
When Australia went metric, I thought it was ludicrous that the dimensions of doors and walls and tables and lengths of timber started being routinely specified in piddly little millimetres, rather than in some unit more proportional to their actual dimensions. It seemed just plain wrong to have to ask for a 2700mm length of timber when I wanted nine foot - especially since nine feet is 2743.2mm, not 2700.
The thing is, though, that using the same unit for everything means that by and large you can simply leave the units out when you're drawing plans or ordering materials and everybody will still understand what you mean. That nine foot length of two-be-four becomes twennysevenundreda niney by fordyfive, which is absolutely unambiguous without being too unwieldy.
Standardising on millimetres rather than centimetres or metres also means that you very very rarely need to work in fractional units, which really does simplify things. It also eliminates gross errors due to decimal points falling off in the photocopier, or inch marks being inadvertently substituted for feet.
posted by flabdablet at 3:28 AM on November 7, 2008
The main thing is to use the same units for length, width and thickness so that you end up with a volume in cubic somethings, and to know how many of those cubic somethings fit in a cubic yard.
When Australia went metric, I thought it was ludicrous that the dimensions of doors and walls and tables and lengths of timber started being routinely specified in piddly little millimetres, rather than in some unit more proportional to their actual dimensions. It seemed just plain wrong to have to ask for a 2700mm length of timber when I wanted nine foot - especially since nine feet is 2743.2mm, not 2700.
The thing is, though, that using the same unit for everything means that by and large you can simply leave the units out when you're drawing plans or ordering materials and everybody will still understand what you mean. That nine foot length of two-be-four becomes twennysevenundreda niney by fordyfive, which is absolutely unambiguous without being too unwieldy.
Standardising on millimetres rather than centimetres or metres also means that you very very rarely need to work in fractional units, which really does simplify things. It also eliminates gross errors due to decimal points falling off in the photocopier, or inch marks being inadvertently substituted for feet.
posted by flabdablet at 3:28 AM on November 7, 2008
I recommend Huff's book How To Figure It if things like this come up again. Fun stuff.
posted by ptm at 4:14 AM on November 7, 2008
posted by ptm at 4:14 AM on November 7, 2008
Response by poster: OK, flabdalet, the friend asking the question had already used the same formulas that are refer. Even I, a senior accountant with no construction experience beyond the costs typically used for each house, actually understood the basic premise behind that formula. She came up with a figure that the sub-contractor laughed at. I came up with the same figure. Neither one of us had any goddamned idea why the sub-contractor thought our figure was wrong. He explained that we should have divided our figure by 75. He had no explanation why. This same man also submits handwritten invoices, so his explanation is not a surprise.
This particular question is for sidewalk flatwork, but the way he explained how he figured it out could be used for either a house or a sidewalk, according to him.
Here are the actual numbers: 66 sq ft, with a city mandated 4 in depth. We are continuing to fuck this up, and I don't want our non-profit organization to get screwed by this sub-contractor. I would love to be able to use one of those formulas and know, without a doubt, that we are estimating correctly.
Also, we work with many sub-contractors who use many different methods and as of right now that chances of using the same units and even nomenclature for their work is slim to none.
Regardless of all of that, this actually is not my job and I know almost nothing about construction except costs, but at this point I just want to get it right and know it's right.
posted by mitzyjalapeno at 7:11 AM on November 7, 2008
This particular question is for sidewalk flatwork, but the way he explained how he figured it out could be used for either a house or a sidewalk, according to him.
Here are the actual numbers: 66 sq ft, with a city mandated 4 in depth. We are continuing to fuck this up, and I don't want our non-profit organization to get screwed by this sub-contractor. I would love to be able to use one of those formulas and know, without a doubt, that we are estimating correctly.
Also, we work with many sub-contractors who use many different methods and as of right now that chances of using the same units and even nomenclature for their work is slim to none.
Regardless of all of that, this actually is not my job and I know almost nothing about construction except costs, but at this point I just want to get it right and know it's right.
posted by mitzyjalapeno at 7:11 AM on November 7, 2008
Response by poster: Goddamn, that are referenced, not refer. I hate the preview button.
posted by mitzyjalapeno at 7:18 AM on November 7, 2008
posted by mitzyjalapeno at 7:18 AM on November 7, 2008
Best answer: 66 square feet, at 1/3 of a foot thick, is 22 cubic feet.
That's 22/27, or .81 cubic yards of concrete.
posted by the Real Dan at 9:29 AM on November 7, 2008
That's 22/27, or .81 cubic yards of concrete.
posted by the Real Dan at 9:29 AM on November 7, 2008
I had no intention of suggesting you couldn't worth this out. Quite the opposite: I really couldn't fathom the idea that an accountant could fail to work this out properly - hence the somewhat incredulous tone of my first answer.
In any case, I am now consumed with curiosity about how your friend and her contractor ended up disagreeing by a factor of 75. For starters, 75 is not a conversion factor between any pair of imperial units that I can think of. For seconds, if his figure was 75 times too small, 0.81 cubic yards / 75 is about 0.3 cubic feet, which is only about enough to make a couple of biggish garden gnomes; but if her figure was 75 times too big, her own basic sanity check would have rejected the idea of trying to fit 61 cubic yards of concrete four inches deep into 66 square feet without the contractor having to laugh at it first. So what were the actual numbers involved? Inquiring minds must know!
posted by flabdablet at 2:40 AM on November 8, 2008
In any case, I am now consumed with curiosity about how your friend and her contractor ended up disagreeing by a factor of 75. For starters, 75 is not a conversion factor between any pair of imperial units that I can think of. For seconds, if his figure was 75 times too small, 0.81 cubic yards / 75 is about 0.3 cubic feet, which is only about enough to make a couple of biggish garden gnomes; but if her figure was 75 times too big, her own basic sanity check would have rejected the idea of trying to fit 61 cubic yards of concrete four inches deep into 66 square feet without the contractor having to laugh at it first. So what were the actual numbers involved? Inquiring minds must know!
posted by flabdablet at 2:40 AM on November 8, 2008
Yeah, but it's less error-prone to work with units that ...
Seriously dude, it's concrete work, not the space shuttle. Your excavated area won't be a uniform depth and trucks typically come in 7 yard batches.
Also mitzyjalapeno, don't let your subs dictate how they're going to price your work. You have to tell them how you want it bid. Per square foot of sidewalk, or even per linear foot of sidewalk, since it's a standard width. Makes it easier to compare prices that way.
posted by electroboy at 11:13 PM on November 8, 2008 [1 favorite]
Seriously dude, it's concrete work, not the space shuttle. Your excavated area won't be a uniform depth and trucks typically come in 7 yard batches.
Also mitzyjalapeno, don't let your subs dictate how they're going to price your work. You have to tell them how you want it bid. Per square foot of sidewalk, or even per linear foot of sidewalk, since it's a standard width. Makes it easier to compare prices that way.
posted by electroboy at 11:13 PM on November 8, 2008 [1 favorite]
Response by poster: The actual numbers that I was given are thus: 66 sq feet with a depth of 4 inches. The higher-ups decided to trust the sub (BAD IDEA IME) and the amount of material he said we needed was spot-on. I have no god-damned idea why. The friend asked a couple of other subs for their formulas, and they both divide by 80, but always end up with a little extra.
And electroboy, I'm not even getting into that with the subs. We just hired a new purchasing manager, and it looks like he knows what to do. So I'll trust him to do that and I'll get back to accounting.
Thanks everyone!
posted by mitzyjalapeno at 10:36 AM on November 11, 2008
And electroboy, I'm not even getting into that with the subs. We just hired a new purchasing manager, and it looks like he knows what to do. So I'll trust him to do that and I'll get back to accounting.
Thanks everyone!
posted by mitzyjalapeno at 10:36 AM on November 11, 2008
4 inches is 1/3 foot, and the conversion to cubic yards is 1/27, so (1/3)*(1/27)=1/81, so if you use 1/80 it allows for a little extra. Dividing by 75 gives you even more leeway.
Btw, it's very common for contractors to use this kind of calculation, even if it's not particularly accurate. Obviously it doesn't scale well, but for small jobs, a little extra material isn't a big problem. Running short on concrete is much more problematic than having a little extra.
posted by electroboy at 12:46 PM on November 11, 2008
Btw, it's very common for contractors to use this kind of calculation, even if it's not particularly accurate. Obviously it doesn't scale well, but for small jobs, a little extra material isn't a big problem. Running short on concrete is much more problematic than having a little extra.
posted by electroboy at 12:46 PM on November 11, 2008
This thread is closed to new comments.
posted by curie at 2:50 PM on November 6, 2008