Celsius to Fahrenheit, both ways.

How do I convert Celsius to Fahrenheit?

Multiply by 9/5 (1.8) and add 32: °F = °C × 9/5 + 32. For example, 20°C × 1.8 + 32 = 68°F.

How do I convert Fahrenheit to Celsius?

Subtract 32, then multiply by 5/9: °C = (°F − 32) × 5/9. For example, (68 − 32) × 5/9 = 20°C.

Why do 0°C and 32°F both mean freezing?

The two scales use different zero points and different-sized degrees: Celsius sets 0° at water's freezing point and 100° at boiling (at sea-level pressure), while Fahrenheit sets those same physical points at 32° and 212°, with 180 degrees spanning that same range instead of 100 — which is why the conversion needs both a multiplication (to rescale the degree size) and an offset (to shift the zero point), not just one or the other.

Is there a quick mental-math shortcut?

Doubling the Celsius value and adding 30 gets you within a few degrees of the exact Fahrenheit figure for most everyday temperatures (20°C × 2 + 30 = 70, vs. the exact 68°F) — close enough for a quick weather check, though not for anything requiring precision like a medical or cooking temperature.

Worked example: normal human body temperature, 37°C, converts to 37 × 1.8 + 32 = 98.6°F exactly.