Either the oven is set in the wrong scale or the forecast is. Here is the arithmetic first, then the six searches that usually follow it.
Both directions, and the version you can do without a calculator.
Multiply by 9, divide by 5, add 32 — so 20°C is 68°F, 30°C is 86°F, and 40°C is 104°F. Written the other way it is simply °C × 1.8 + 32.
Double it and add 30 — 20°C gives 70 against a true 68, and 30°C gives 90 against 86. It is wrong by a few degrees, which is fine for weather and useless for an oven.
Where the exact conversion is not actually the answer you want.
The conventional pairs are 160°C/325°F, 180°C/350°F, 200°C/400°F and 220°C/425°F. They are rounded on purpose — nobody sets an oven to 356°F, and ovens swing more than that anyway.
Drop about 20°C or 25°F from the stated temperature, because moving air transfers heat faster than still air. A recipe calling for 180°C conventional wants roughly 160°C in a fan oven.
Gas mark 4 is 180°C or 350°F, and each mark up adds about 25°F. British recipes older than about 1990 use these almost exclusively.
The other half of the searches, from people who are packing.
30°C is 86°F — properly hot, shorts weather. The useful anchors are 10°C/50°F for a jacket, 20°C/68°F for pleasant, and 0°C/32°F for freezing.
37°C, which is 98.6°F — the two figures people quote in each scale are the same measurement. A fever is generally taken as 38°C or 100.4°F.
The formulas here are exact. The oven temperatures further down are the conventional round pairings cooks actually use — a recipe saying 180°C and one saying 350°F are meant to be the same setting, even though the exact conversion is 356°F.
You just walked 7 searches for “celsius to fahrenheit”. The extension maps the trail right on your results page — for anything you search, not just the journeys we’ve written.