>can’t calculate fahrenheit into celsius on my own

For a rough guess, simply subtract 30 and divide by 2.

This would make 70°F ≈ 20°C (actually 21°C). 100°F ≈ 35°C (actually 37.78°C) and 30°F ≈ 0°C (actually -1°C).

This quick and dirty formula starts to break down at cooking temperatures, but still great for a rough guess at weather.