Celsius to Fahrenheit

Multiply by 9/5, then add 32: °F = °C × 9/5 + 32. The 9/5 corrects for the Fahrenheit degree being smaller, and the +32 corrects for the scales starting at different points — 0 °C is 32 °F, not 0 °F. Both steps are needed: 100 °C × 9/5 = 180, and only adding 32 gives the correct boiling point, 212 °F.

At −40, exactly. Solving x = x × 9/5 + 32 gives x = −40, and the table shows it: −40 °C = −40 °F. It is the one conversion nobody can get wrong, and a useful sanity check — below −40, the Fahrenheit number is lower than the Celsius number; above it, higher.

Exactly 356 °F by the formula (180 × 9/5 + 32). Oven dials round it to 350 °F, and the same setting is Gas Mark 4. The 6-degree gap is a dial convention, not a conversion error, and it is well inside the swing of a domestic oven's thermostat.

Double it and add 30 for an estimate: 25 °C gives 80 against the exact 77 °F. Or use the exact mental route — multiply by 9, divide by 5, add 32 — for round numbers: 30 °C is 270 ÷ 5 = 54, plus 32 = 86 °F. The shortcut is fine for weather; use the exact formula for cooking and body temperature.

As a conversion, yes: 37 × 9/5 + 32 = 98.6 exactly. The 98.6 °F figure became famous as 'normal body temperature' precisely because an early measurement of 37 °C was converted. Individual healthy temperatures vary around it by roughly half a degree Celsius either way, which is why the Fahrenheit to Celsius page treats 100.4 °F (38 °C) as the fever threshold rather than any reading above 98.6.