What do the various paper sizes mean?

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If you are in a hurry and want some images that explain the different sizes quickly, then click on one of the above or below links:

Some information of the "A", "B" and "C" series of paper sizes.

A1, A3, A5, DL — these are paper sizes, and these can be very confusing. This page has some graphics and some words to assist in clarifying what all those letters and numbers mean.

"A" series paper sizes — a more detailed explanation.
"B" series paper sizes — a more detailed explanation.
"C" series paper sizes — a more detailed explanation.

"A" series paper sizes

First, A4 is the standard size that almost everyone knows. A4 is the size of a standard letterhead, and it measures 297mm x 210mm (or 29.7cm x 21cm).
An image of A4 is below.

A4 is the size of a standard business letter.

Second, all "A" series paper is based upon the A0 size. This is one square metre of paper, and it measures 1,189mm x 841mm (or 118.9cm x 84.1cm).
All the other paper sizes are scaled down versions of A0, with:

A1 being half the size of A0;
A2 being half the size of A1, or one-quarter the size of A0;
A3 being half the size of A2, or one-eighth the size of A0;
A4 being half the size of A3, or one-sixteenth the size of A0.

The sizes under A4 are better described in relation to the standard A4 size:

A5 is half the size of the standard A4;
A6 is one-quarter the size of A4 (i.e.: half of A5);
A7 is one-eighth the size of A4 (i.e.: half of A6).

An image is shown below of all the paper sizes in the "A" series. What you will see in this image is that each paper size is either double the size of the category below it, or half the size of the category above it.

A4 is half the size of A3, and twice the size of A5
A5 is half the size of A4, and twice the size of A6
A6 is half the size of A5, and twice the size of A7
DL is an odd size — DL is the same width as A4, but only one-third the height of A4

"B" series paper sizes

All "B" series paper is based upon the B0 size. B-series paper is in the same proportion as the A-series. However, the starting point is NOT one square metre of paper, but the length of the shortest side (i.e.: it is based upon the width of the page). B0 paper has a width of 1 metre, and a length of 1.414 metres.

Below is a comparison of the standard A4 and the non-standard B4. You can see that the non-standard B4 is quite a bit larger than the standard A4.

Each of the B-sized papers are half the size above it, and twice the size below it, with:

B1 being half the size of B0, and twice the size of B2;
B2 being half the size of B1, and twice the size of B3;
B3 being half the size of B2, and twice the size of B4;
B4 being half the size of B3, and twice the size of B5;
B5 being half the size of B4, and twice the size of B6;
and so on and so on..

This is displayed in the below image — the only things that are normally made in the B-sizes are B1 and B0 posters.

"C" series paper sizes

The "C" series of paper sizes is normally used for envelopes. As such, each "C" series size is slightly bigger than the "A" series.

Below is an Image of that shows the size of and A4 sheet to a C4 sheet. As you will see, C4 is about 10% larger than A4, and an envelope that was C4-sized could easily accommodate a number of A4 sheets — C-sized products are almost always envelopes.

As with the A-series and B-series of sizes, each C-size is twice the size of the one below it, and half the size of the one above it. So:

C1 is half the size of C0, and twice the size of C2;
C2 is half the size of C1, and twice the size of C3;
C3 is half the size of C2, and twice the size of C4;
C4 is half the size of C3, and twice the size of C5;
C5 is half the size of C4, and twice the size of C6;
and so on and so on.
DLX is an odd size. It is especially made to accommodate business letters that are folded into thirds — DLX envelopes are often called DL envelopes.