A recipe chart often groups 180 °C, 350 °F, Gas Mark 4, and about 160 °C fan on one line. That line is convenient cooking shorthand, but its values come from three different rules. Exact Celsius–Fahrenheit arithmetic makes 180 °C equal 356 °F. The published Gas Mark 4 row is 177 °C / 350 °F. The 160 °C fan figure follows the approximate conventional-minus-20 guide. Keeping the labels prevents a practical grouping from looking exact.
Exact conversion: 180 °C is 356 °F
Apply F = C × 9/5 + 32. Multiplying 180 by 9/5 gives 324, then adding 32 gives 356. Conversely, 350 °F converts to about 176.7 °C. Therefore 180 and 350 are close settings used together by recipe writers, not exact translations. Use 356 °F when you specifically need mathematical conversion; use the recipe’s stated 350 °F when it deliberately supplies that practical setting.
Published Gas Mark 4 is 177 °C / 350 °F
OvenTemp looks up Gas Mark rather than calculating or interpolating it. The immutable row for mark 4 contains 177 °C and 350 °F. A recipe chart may round that Celsius value to 180 for readability, but the converter keeps the published row visible. It does not create quarter marks, half marks, or a new row to force every Celsius number into a Gas Mark label.
Fan guide: about 160 °C, then check the recipe
Subtracting 20 from the conventional 180 °C setting gives the common starting guide of 160 °C fan. Use it only when the recipe gives no fan value and the appliance manual does not recommend another adjustment. If the recipe explicitly says 180 °C fan, do not subtract again. Begin checking doneness at the recommended time or slightly earlier, because fan strength, cookware, and oven calibration affect the result.