350°F equals exactly 176.7°C using the linear formula (F − 32) × 5⁄9, but published gas mark tables pair 350°F with gas mark 4 and 180°C instead of 176.7°C — because gas marks are a fixed historical lookup table of rounded, convenient values, not a direct conversion of the Fahrenheit formula. Using the wrong one of the two systems for a recipe’s Celsius figure can leave an oven several degrees off.
What is the worked gap between 350°F and gas mark 4?
Converting 350°F to Celsius with the exact formula: (350 − 32) × 5 ÷ 9 = 318 × 5 ÷ 9 = 176.67°C. But a UK recipe’s printed gas mark 4 conventionally pairs with 180°C, not 176.7°C — a gap of 3.3°C that exists because gas mark tables were set by convenient round numbers for gas-cooker dials, not by converting a Fahrenheit figure with the linear formula. Converting 425°F the same way gives (425 − 32) × 5 ÷ 9 = 218.3°C exactly, while the published gas mark 7 pairing is 220°C — a smaller but still nonzero 1.7°C gap.
How do I convert Fahrenheit, Celsius, and gas mark myself?
For an exact Fahrenheit-to-Celsius conversion, use (F − 32) × 5 ÷ 9, or the reverse C × 9 ÷ 5 + 32 for Celsius to Fahrenheit; this is a true linear formula with no rounding built in. For gas mark, use a published lookup table rather than a formula, because gas marks were fixed independently for each dial position rather than derived from the Fahrenheit or Celsius scale. The oven temperature converter and cooking temperature converter keep the exact linear conversion and the gas mark lookup as separate operations rather than merging them into one formula.
How do I check an oven-temperature conversion?
A Fahrenheit-to-Celsius linear conversion should always be reversible exactly: 176.67°C × 9 ÷ 5 + 32 = 318 + 32 = 350°F, returning to the start with no drift. A gas mark lookup cannot be checked the same way, because it is a table, not a formula — the only check is comparing against the same published source consistently, rather than recomputing it from Fahrenheit or Celsius.
What mistakes produce a wrong oven setting?
The most common mistake is running a gas mark’s conventional Celsius pairing (say, 180°C for gas mark 4) back through the exact linear formula and expecting it to return 350°F; it returns 356°F instead, a difference that can look like a calculation error when it is really the expected gap between an exact formula and a rounded historical table. A second mistake is applying a “reduce by 20°C for a fan oven” adjustment to a Fahrenheit figure without converting the adjustment itself — 20°C is about 36°F, not 20°F, so subtracting 20°F from a Fahrenheit setting under-corrects a fan-oven conversion. A third is treating a fan-oven adjustment as an exact physical constant rather than a common manufacturer guideline that can vary between ovens.
What does an oven-temperature conversion not resolve?
The Fahrenheit-Celsius linear formula is exact physics and always correct for that pair of temperature scales. Gas mark values are a historical British convention with no single authoritative modern formula; different published tables can round gas mark 3 to either 160°C or 170°C, for example, and a fan-oven adjustment is a manufacturer guideline rather than a fixed physical law. This guide converts the numbers that are given; it does not resolve which specific oven, altitude, or recipe timing correction a particular dish needs.
Source for the external fact
The U.S. National Institute of Standards and Technology confirms the exact linear relationship between the Fahrenheit and Celsius scales in its temperature-units guidance; the gas mark scale itself originated as a British gas-appliance dial standard, documented in general reference sources such as the gas mark reference entry.
A full gas mark reference table, and how far each pairing sits from the exact formula
Comparing the commonly published gas mark pairings against what the exact linear formula alone would give for the same Fahrenheit figure shows the gap is not constant.
| Gas mark | Published °F | Published °C | Exact linear °C from °F |
|---|---|---|---|
| 1 | 275 | 135 | 135.0 |
| 2 | 300 | 150 | 148.9 |
| 3 | 325 | 160 | 162.8 |
| 4 | 350 | 180 | 176.7 |
| 5 | 375 | 190 | 190.6 |
| 6 | 400 | 200 | 204.4 |
| 7 | 425 | 220 | 218.3 |
| 8 | 450 | 230 | 232.2 |
| 9 | 475 | 245 | 246.1 |
The gap is not a constant offset and does not even hold one consistent sign: the published Celsius figure sits above the exact linear value at gas marks 2, 4, and 7, but below it at marks 3, 5, 6, 8, and 9 — a pattern with no single correction that would fix every row, which is itself the evidence that the gas mark scale was rounded to convenient numbers independently at each dial position rather than derived from one offset applied to the Fahrenheit column.
A second worked example starting from a US recipe’s printed Fahrenheit figure
A US recipe printed at 425°F converts exactly to (425 − 32) × 5 ÷ 9 = 218.33°C using the linear formula — the number a UK cook should set on a Celsius-dial oven with no gas mark involved. Looking up 425°F on a published gas mark table instead gives gas mark 7, conventionally paired with 220°C rather than the exact 218.33°C. Both 220°C (gas mark table) and 218.3°C (exact formula) are close enough in practice that either will bake the dish successfully; the two numbers are simply answers to two different questions — “what does the gas mark table say?” versus “what is 425°F exactly in Celsius?” — not two competing calculations of the same thing.
Applying a fan-oven adjustment correctly in both temperature scales
A common manufacturer guideline reduces a conventional oven setting by about 20°C for a fan (convection) oven. Applied to the gas mark 4 pairing of 180°C, that gives 180 − 20 = 160°C for the fan setting. Converted to Fahrenheit with the exact linear formula, 160°C is 160 × 9 ÷ 5 + 32 = 320°F — not 330°F, which is what a cook would get by mistakenly subtracting 20°F directly from the original 350°F figure instead of converting the 20°C adjustment first. The adjustment amount itself is a rounded rule of thumb that can vary by oven manufacturer; the Fahrenheit-Celsius conversion of that adjustment, once a figure is chosen, is exact.
Why 0°C, freezing point, and “room temperature” recipes need the same care
A recipe instruction such as “chill to just above freezing, 34°F” converts with the identical exact formula used throughout this guide: (34 − 32) × 5 ÷ 9 = 1.11°C, not the 0°C someone might guess by rounding 34°F down to “near freezing.” At the other end of the scale, “room temperature, 68°F” converts to (68 − 32) × 5 ÷ 9 = 20.0°C exactly — a case where the exact formula happens to land on a clean number, unlike most oven-temperature conversions in this guide, which is a reminder that a clean-looking result at one point on the scale does not mean the formula itself has changed.
Related calculators
Convert an oven setting with the oven temperature converter, apply the same linear formula to any other reading with the cooking temperature converter, and see how a temperature interval differs from a temperature reading in the temperature-difference guide.