Card and Paper Sizes
Starting from A1 (the biggest), the sizes go down as if you have folded the previous sheet of paper in half.
Card Blank Sizes
With card blanks, the size given refers to its folded size i.e. the front face of the card. So for example, an A5 card blank is a card with an A5 front, even though the sheet of card used to make it is A4.
Square card blanks have simple measurements in cm or inches, and tall DL card blanks are in a category all their own. They are the same size as a sheet of A4 folded to a third of its size, like a business letter. The measurements of a DL card blank are 21 x 9.9 cm (8.3 x 3.7 in).
Insert papers for cards are available either printed or blank. Sold in standard sizes, they are slightly smaller than the card size to give a professional finish.
Envelope sizes are usually given as 'C' plus a number, and this number corresponds to the paper or card blank size that fits inside the envelope. For example, C4 envelopes will fit an A4 card blank or unfolded sheet of A4 inside. The measurements are slightly bigger for the envelope i.e a C4 envelope will be a bit larger than A4, to allow plenty of room.
There are also square envelopes to fit square card blanks, and DL envelopes to fit tall DL card blanks. Again these will be slightly larger than the card blanks they are designed to hold.
Card and paper is available in various "GSM's". Although this is normally taken to be an indication of the thickness, to be precise, it is the weight in grams i.e. Grams pre Square Meter.
A simple way to check the GSM is to weigh an A4 sheet on kitchen scales. Take the weight in grams and multiply by 16. For example a 240gsm A4 card weighs 15 grams. Trust us we've tried it!