The empty Toxic Shocker marker has about a 3/4 ounce capacity. Although you can use whatever ink you want in this marker, we would highly recommend using an alcohol or dye based ink. Try using Pilot Ink, Marsh, Molotow ONE4ALL Ink, or Do Em Dirty Inside Job. It has a similar nib to the AP Drip Mop Mini. The nib can easily be replaced.