Iron Powder Toroid Cores part numbers are presented using format Tddd-m where dd is core diameter in tenths of inch and m is material grade. Also, cores are color coded to represent material grade.
Material grade defines frequency range core may be used for. There is no strict standard for coding but for informal purpose this table can be used.
| Material code |
Color | Frequency range |
| 0 | Tan/Tan | 50 MHz – 350 MHz |
| 1 | Blue/Clear | 150 kHz – 3 MHz |
| 2 | Red/Clear | 250 kHz – 10 Mhz |
| 3 | Gray/Clear | 20 kHz – 1 MHz |
| 4 | Blue/White | 3 MHz – 40 MHz |
| 6 | Yellow /Clear | 3 MHz – 40 MHz |
| 7 | White/Clear | 1 MHz – 25 MHz |
| 8 | Orange/Clear | 20 kHz – 1 MHz |
| 10 | Black/Clear | 15 MHz – 100 MHz |
| 12 | Green/White | 30 MHz – 250 MHz |
| 15 | Red/White | 150 kHz – 3 MHz |
| 17 | Blue/Yellow | 20 MHz – 200 MHz |
| 42 | Blue/Red | 30 kHz – 80 kHz |