The formula to convert Fahrenheit to Celsius is:

```

°C = (°F - 32) × 5/9

```

Therefore, to convert -40°F to Celsius, we would use the following calculation:

```

°C = (-40°F - 32) × 5/9

```

This calculation gives us:

```

°C = (-72) × 5/9

```

Which simplifies to:

```

°C = -40°C

```

So, -40°F is equal to -40°C.