# Metric to Imperial conversion help

Discussion in 'Fiberglass and Composite Boat Building' started by crossram, May 13, 2013.

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### crossramJunior Member

I am following a stack up and the builder calls out 1 300 gram layer and 5 600 Gram layers. I assume it's a square meter. What does this convert to in imperial weights ounce per square foot ?

Is there a chart available?

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### Eric SponbergSenior Member

Crossram,
For a more direct answer to your specific question, you can do the conversion yourself with simple math. Yes, the units should have been more clearly stated as "grams per square meter" or gsm, which are standard units for metric fabrics. In the US, the standard units are ounces per square foot for mat, and ounces per square yard for all other fabrics, wovens and stitched included. It's actually pretty easy:

You can look up certain conversions of units:
454 grams (g) = 1 pound (lb)
1 lb = 16 ounces (oz)
1 meter (M) = 3.281 feet (ft)
9 ft^2 = 1 yard^2 (yd^2)

For example, taking the 300 gsm fabric:

300 x (1 lb/454 g) x (16 oz/1 lb) x (1 M^2/3.281^2 ft^2) x (9 ft^2/M^2)

300 x (1 x 16 x 1 x 9) oz/yd^2
(454 x 1 x 3.281^2)

300 x 0.029464 oz/yd^2

Or, since we don't like seeing tiny numbers, we can invert the tiny number in the numerator to be a big number in the denominator:

300/33.9395 oz/yd^2 = 8.84 oz/yd^2

That is, 0.029464 is the same as 1/33.9395

So, that's the conversion. Whenever you have grams per square meter, just divide the number by 33.9395, or 33.94 to round it a little bit, and you get the equivalent weight in oz/yd^2.

The 600 gsm fabric is twice the weight of the 300, so the imperial weight will be twice as much:

600/33.94 = 17.68 oz/yd^2 (which is 2 x 8.84)

To convert any weight to oz/ft^2, either leave the number 9 out of the conversion equation above, in which case the divising denominator is 305.45, or you can just divide the answers above by 9:

300/305.45 = 0.982 oz/ft^2 (about an ounce per ft^2)

or

8.84 oz/yd^2/9 = 0.982 oz/ft^2 (same answer)

I hope that helps. Conversions are not hard when you break all the units down into their separate parts. Then what I do is I write this conversion down in a handbook that I keep at my desk so that when this problem comes up again, I have the conversion right handy.

Eric

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### crossramJunior Member

Thanks, it would still be nice to have a chart, since most materials are made to similar specs.

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### PARYacht Designer/Builder

Crossram, what Eric is saying is you'll use and eventually memorize the common conversions. I have a reference that does this for the most common uses, as well as fractional to decimal and other needs. At first, you'll need the booklet, but you'll find you're using the same conversions over and over, which commits it to memory. "Convert" is also handy.

6. ### tunnelsPrevious Member

In all my time glassing I never heard of measuring per square foot !! how stupid is that !! its always been per sqr mtr !! where ever I worked for the last how ever long !!

Even when they skinned the arch Noah said it was per sqr mtrs !! what rock and cave you guys been living under ??

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### IkeSenior Member

Tunnels; maybe in China you use the metric system, but the USA has never really converted. But most things now have both systems on the label, mainly because now most things come from somewhere else, more often than not, from China.

8. ### tunnelsPrevious Member

So in other words you don't want to keep up with the rest of the world just stay in the dark ages and play ignorant !! Metric is so much easier to use and USA is really dragging the chain !! and could be one of the last countries to convert !!
Was in the 1970 nz converted and the change over was a nightmare for a while because we were trying to use both and convert back and forth and that is the major problem !!! get past that and stop converting !! we got over it and never looked back !!

9. ### tunnelsPrevious Member

If you keep using tables and conversions you will never learn Just push one thing aside and get on completely with the new !! don't even have to think about silly charts !!
so what does this tell you ???
Because now most things come from somewhere else, more often than not, from China.!!!

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### IkeSenior Member

Couldn't agree more. I am an engineer. I have used metric as long as I can remember, but the general public in the US doesn't, so everything has to be done in both. But if you buy a car, it's all metric. If you design almost anything it's all metric. I carry both metric and english tools because anything more than 15 or more years old is in english units.

Engineers, scientists, designers, technicians all use metric, but the consumers still think in English units. Eventually we will be dragged (kicking and screaming) into the 21st century.

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### JSLSenior Member

For conversion, one source I use is a booklet called 'Pocket Ref' by Thomas J. Glover. Useful for a variety of stuff -
I believe Metric is now SI (System International) and some units are re-named. ie: Pressure of KG.sq.cm. is now KiloPascals.

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There is no renaming, it is a different way of presenting the same, but in different units. The base units of measurement, linear, is metre, or m. But when the numbers are small, it is silly use fractions of a m, when cm will do, or, even mm. Same with modulus values of extrusions/shapes. These are best used in cm^3, rather than m^4.

A kg.sq.cm = kg/0.01^2 = kg/0.0001 in units of kg/m^2 …the reason for cm is to make the numbers manageable.

So taking the same kg.sq.cm or kg/0.0001 kg/m^2

To change this into Newtons, becomes N/0.001 N/m^2

But again this is a bit silly, so change the dominator into m^2, so it becomes 1000 x N/1 = 1kN/m^2. Since 1000 = kilo, or k.

And, a pressure has units in Pascal, and 1 Pa = N/m^2, note the 2 base units of force, N, and area, m x m = m^2.

So 1 kg.sq.cm = 1kPa

More on SI Units here:
http://www.bipm.org/en/si/base_units/
or
http://www.chemie.fu-berlin.de/chemistry/general/si_en.html

Last edited: May 14, 2013
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### redreubenredreuben

These are the numbers you need.
¾ oz csm=228g (200)
1 oz csm=284g (300)
11/2 oz csm=457g (450)
2 oz csm=610g (600)

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### hoytedowCarbon Based Life Form

Might want to dial it back a little. This is still a free country where we can use what we choose.

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### PARYacht Designer/Builder

The USA has converted in all put figurative ways. All industries have made the conversion, some decades ago, though accommodation to the system by the baby boomers (and older), is yet still realized. This is common of all countries that have made this transition, such as Australia and Canada, where a mixed bag of metric and imperial are still in place, just like the USA. Children are taught metric (actually SI) and eventually it will become the common usage, but we do need to wait until the vast majority of the SAE folks die off, which will be a few decades yet.

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