All STI Magazines are measured by overall length. Below is a table that breaks down each of the available magazine lengths and their capacity by caliber.
| Length/Caliber | 9mm / .38 Super | .40 S&W / 10mm | .45 ACP |
| 120mm (For VIP Length Pistols only) | 16 Rounds | 14 Rounds | 10 Rounds |
| 126mm | 17 Rounds | 15 Rounds | 11 Rounds |
| 140mm | 20 Rounds | 18 Rounds | 13 Rounds |
| 170mm | 27 Rounds | 23 Rounds | N/A |
