The formula for converting Fahrenheit to Celsius is:

```

°C = (°F - 32) / 1.8

```

where °C is the temperature in degrees Celsius and °F is the temperature in degrees Fahrenheit.

Substituting -10 °F into the formula, we get:

```

°C = (-10 - 32) / 1.8

°C = -42 / 1.8

°C = -23.33

```

Therefore, -10 Fahrenheit is equivalent to -23.33 degrees Celsius.