The metric system is disliked by many Americans because it is so different from feet and inches. Yet it is far more convenient and is very simple to use for linear measurements, if a few basic principles are kept in mind.
You only need to understand linear measurements to build a boat from metric plans. There is no need to fill your head with conversion factors for surface areas, volumes etc, which is where the more complicated stuff comes into the picture.
If you can find them, use tapes and rules which show millimetres and metres only. Then you only have to remember that 1000 millimetres (mm) = 1 meter (m). If your tapes or rules also show centimetres (cm) and decimetres (dm) then ignore these other units.
I did my schooling in feet and inches but my country metricated when I was part way through university. I have now worked for over 30 years in the metric system and I continue to switch back and forth freely in my mind and can work in either system. This comfort with the measurements comes from knowing the basic multipliers which can be used as a selfchecking mechanism when working with either system. Keep these in mind when working with whichever of the two systems is not normal for you.
1 inch is very close to 25mm, 25.4mm to be exact. This is a nice round figure to use for your checks, if you think about it a little. Multiples of 25 are very easy to work with mentally, without a calculator. To multiply by 25, simply divide by 4 and add two zeros. To divide by 25, simply multiply by 4 and then divide by 100.
Use the 25 multiplier as a quick method to check that you are not messing up when setting out your measurements. Don't convert all your metric measurements to imperial by this method then set those out, because they will be wrong. Do your setting out in metric but occasionally divide the number by 25 just to get the approximate number of inches as a check on your work.
Out of this basic multiplier flows a string of others which you can use if you want.
100mm is about 4"
1m = 1000mm which is about 40"
250mm is about 10"
300mm is about 12" = 1'
1.5m = 1500mm which is about 60" = 5'
3m = 3000mm which is about 120" = 10'
If you don't feel comfortable doing your measurements in metric, then you have the option of converting all of the measurements to inches before doing the setting out. You would go over each drawing and divide each millimetre dimension by 25.4, to get the eqivalent in inches. The answers will have the partial inches as decimals rather than the fractions which you will be accustomed to working with. Click here to print a table for converting between inch fractions and decimals.
Metric scales are written in the form 1:5 and 1:20. The first number is the size of the article on the drawing and the second number is the size of the article in real life. The following list shows the size of the item on the drawing in relation to the real article, in terms of percentages.
1:1 is full size or 100%
1:2 is half size or 50%
1:5 is 20%
1:10 is 10%
1:15 is 6.67%
1:20 is 5%
1:25 is 4%
1:33.33 is 3%
1:50 is 2%
1:100 is 1%
