e-bannerx.com allows you to convert Banner Impressions of one Size to another Size. Considering the fact that the Banners of different Sizes are likely to be of different value, there are Size Conversion Factors defined by e-bannerx.com.

Example:
If current 81x30 --> 468x60 Conversion Factor is 0.25, then you can convert, say, 1000 81x30 Impressions into 250 468x60 Impressions.