You are nodding your head I know! It is ok even I have to think hard about this stuff annually.
The three different sizes shown A5, B5 and A4 are common sizes. I've added the dimensions to show their actual sizes.
A4 is twice the area of A5, A5 is twice the area of A6 etc. Similarly B4 is twice the area of B5, B5 is twice the area of B6 etc.
These three sizes are all ISO216 paper sizes, ISO216:2007 specifies three different series of paper sizes, A, B and C. One thing in common is that the ratio of height against width is the square root of two. So if you multiple the width by 1.4142 (the square root of 2) you will magically get the height to within 1 mm. Alternatively multiple the height by 0.7071 and you will get the width of the page.
Because they follow this pattern we can easily scale from one size to another when printing. So for instance if you have a Word file that is set to A4 size to print it on A5 paper you scale the page by 0.7071, or more conveniently 70.7% or 71% if you can't input a decimal number.
Note that the reduction between each size is not 50% even though the smaller size has half the area. The scale factor for reduction is calculated by dividing the longer edge of the smaller size by the longer edge of the larger size (or the shorter edge of the smaller size by the shorter edge of the larger size).
It wouldn't be so bad if you used this information every week, it might become second nature, but for most of us it is information we only use once or twice a year.
So let us start with a picture.
The three different sizes shown A5, B5 and A4 are common sizes. I've added the dimensions to show their actual sizes.
A4 is twice the area of A5, A5 is twice the area of A6 etc. Similarly B4 is twice the area of B5, B5 is twice the area of B6 etc.
These three sizes are all ISO216 paper sizes, ISO216:2007 specifies three different series of paper sizes, A, B and C. One thing in common is that the ratio of height against width is the square root of two. So if you multiple the width by 1.4142 (the square root of 2) you will magically get the height to within 1 mm. Alternatively multiple the height by 0.7071 and you will get the width of the page.
Because they follow this pattern we can easily scale from one size to another when printing. So for instance if you have a Word file that is set to A4 size to print it on A5 paper you scale the page by 0.7071, or more conveniently 70.7% or 71% if you can't input a decimal number.
Note that the reduction between each size is not 50% even though the smaller size has half the area. The scale factor for reduction is calculated by dividing the longer edge of the smaller size by the longer edge of the larger size (or the shorter edge of the smaller size by the shorter edge of the larger size).
Using that rule we can devise how to go from A4 to B5, so we take 250mm and divide it by 297mm to get 0.841 or just 84% should be close enough.
What is the significance of B5 size you might ask.... B5 is Deskfax size, whilst Filofax have discontinued this size years ago, it still remains a popular size, although printed inserts for this size are difficult to come across. With the knowledge of the scaling factor of 84% you can take any of the Philofaxy diary inserts intended for A5 size and print them at 84% and you will have Deskfax pages.
All the Philofaxy diary inserts for A5 are in fact sized to A4 this is so if you use the 'Booklet' print facility available on some printers they will be printed the correct size on A4 paper (two pages per side) If you are printing the files directly on to A5 paper you have to scale the print to 71%.
To help you remember this information I have created an 'aide memoire' which you can either put on your notice board or print out and put in your organiser.
