Dimensions of the slots are: 1" high x 3 7/8" wide x 4 1/2" deep.
Ink Pad inserts will create a false back to your slot allowing the smaller ink pads to stay to the front of your unit. Dimensions of the slots with the Ink Pad inserts are: 1" high x 3 7/8" wide x 2 7/8" deep.
You only need the Ink Pad inserts if your ink pads are smaller in size.
Below is a list of ink pads that will fit better with the Inserts: Stamp It ©, Archival ©, Brilliance, VersaMark ©, VersaColor ©, Sea Brights, Picture Perfect ©, Adirondack, Crystal, Stazon, Distress Ink by Ranger ©, Ancient Page, Ancient Page mini (2 per slot), Impress, Darcies Ink It Up, Ranger ©, Palette, Fabrico, Dee Gruening Blending Box, Sea Brights, Smaller/older style, inkpads from Stampin' Up ©, smaller/older style Exclusive Inks © from Close to my Heart
Please Note: The ColorBox (thick style) will not fit in the PortaInks. You can always measure the ink pads you would like to store and then check the dimensions above to see if your ink pads need an insert or not.