Why your oven temperature conversion charts could be wrong – and does it matter?

If you’re like me and have an array of cookbooks, some of which have been handed down or been a complete ‘find’ in an antiquarian bookshop or second hand store you’ll find yourself needing to convert weights, measures and, crucially, temperatures quite often.

However, there is a potential problem here: can you trust the values in those old books and does this impact on your modern oven temperature?

If you have a metric oven and are converting from an old Fahrenheit recipe or have an oven in Fahrenheit and a ‘modern’ cookbook with Celsius values (and a Fahrenheit conversion table) you could be setting your oven to a different temperature than the original recipe. Not a potential vast difference but not exact either. Should this be something that you ought to be even bothered about?

Metric temperatures in recipes have been the norm for the last ten to fifteen years or so within the UK. Although Imperial was originally a UK-developed scale (set out in the early 1800s from the previous ‘English scale’ which gave rise to both the British Imperial and the US systems) it seems metric is in the majority use here. Of course, many (mainly older) home cooks still use Fahrenheit and have ovens still with Fahrenheit scales, but they are rapidly thinning out as time goes on and the march of metric continues. We adopted metric with its smaller increments and simpler decimal readings (0 degrees is freezing, 100 degrees boiling water [at sea level]).

For myself, I was taught in metric at school and I measure in metric. I do frequently cook (converting) from old recipe books which only list ingredients in pounds and ounces though. I wondered one day whether I could really rely on the conversion tables and even the accuracy of historical recipes and decided to investigate further. This post has taken about two months of my spare time to research and write up!

The real question I found myself asking, is does it actually matter about the temperature differences?

Very early recipes tended to list oven heat in descriptive words rather than as temperatures, such as ‘low’, ‘hot’, ‘cool’ and ‘medium’. This was because cooking was done on ranges (at best) and over fires (at most basic). When ranges and ovens became more sophisticated and featured gauges to regulate temperature it necessitated a rethink and to list temperature values. Not only were these early gauges not 100% reliable (so that writing down the temperature for a recipe might mean inadvertently writing down the wrong value) there are suggestions that some recipes might have been simply rewritten, changing the words ‘hot’ and ‘cool’ etc to a value, rather than re-making the recipe from scratch in a variable temperature oven and noting down the temperatures. If the writer was a bit relaxed about ‘guessing’ the proper temperature or used someone else’s recipe without checking or testing it to make sure, that mistake could easily be continued on and on as people republished recipes.

Clearly such variations would only have been minor or the recipe wouldn’t have worked and would have fallen out of use. However, some minor discrepancies could have been continued. Ever baked a cake from an old recipe book and thought ‘What on earth did I do wrong?’ It just might be the temperature was inadvertently a little off-kilter for your modern oven.

I can find no solid evidence either way if this theory is true about the guesswork, but it is possible that some recipes were copied and temperatures not fully checked in the past. What we do know is that every oven (even a similar model) seems to run differently and that over time you get to know the quirks of the one you own and adjust the settings or bake time yourself to suit. This doesn’t make converting old recipes any easier though.

Another thing that’s confusing for conversion tables is that Fahrenheit and Celsius have very few comparable whole number values (one of the few easy ones is 210 ° C is equal to 410 ° F). This means recipes either round temperatures up or down from Celsius to the nearest Fahrenheit value, so it’s never exact. If your oven is Fahrenheit then your temperature increments are quite wide; typically 25° or 50°. Metric ovens go up in 10° increments. it’s this difference in increments and the need to round up or down that causes the discrepancies.

While a few degrees hotter or cooler isn’t much of a bother for most cooking, it might make a difference in baking where precise measurements and precise temperatures are stipulated. For instance, cooking a meringue in an oven which is higher by even just a few degrees might result in the dreaded sepia tinge of a too-hot bake.

I’ve written the table below so you can compare the discrepancies between typical cookbook suggested temperatures to match Celsius and what the real equivalent temperatures really are. I looked at 30 plus references and they all gave the same equivalent temperatures (give or take a few errors and a couple of weird ones), so the column with ‘nearest Fahrenheit temperature normally given’ is likely what you’ll encounter in your own cookbooks.

Rounding up or rounding down the numbers

What is especially intriguing in the list of cookbook equivalents temperatures is that 120°C and 130°C both are listed as 250 °F,  and 200°C and 210°C are listed as 400°F. These are examples of the rounding up or rounding down technique, leaving the values to meet somewhere in the middle! Admittedly (even on metric recipes) there can be only a little difference in these temperatures. However I sometimes knock a recipe up or down by 10°C on purpose (such as lowering it to get a slower but more even bake) it clearly does have some small effect on specific recipes.

Is your oven temperature actually correct?

Remember that on top of all this is that your oven might not be at the temperature that matches your dial anyway, especially if it is a few years old!
There is a way to check (roughly) if your oven is running a bit hotter or colder by relying on the melting point of sugar, which is invariable. I will write a post on how to check using this method (watch this space!).

My suggestions/conclusions on whether it matters if conversion temperatures are correct or not are:

• Don’t worry too much about it – it only matters for very delicate baking or if you have baking OCD (of which I might!)
• If you do want to bother about it:
  • Just get to know your oven and remember how it cooks different items. That’s all it really needs. My fan oven runs hot (even for a fan oven) and then cooks slightly hotter on the right side than the left. You’re probably already adjusting without realising it because you know your oven
  • Can’t remember how your oven ‘behaves’ or it’s new to you? Jot down how your oven copes with your most common bakes and adjust next time if it cooks a little too fast (turn down by 10%) or too slowly (turn up by 10%)
  • Buy an oven thermometer and look at that – not the dial on the front if you’re really into precise bakes (although I’ve never bothered to have one). It will also have both metric and Imperial values
  • For delicate or precise bakes you may want to think about knocking your oven down or up a notch or altering the baking time when the Fahrenheit oven setting given is quite a way out from the exact Fahrenheit temperature. For example, A low-temp bake which is given as 140°C in a recipe would tell you to bake it at 275°F in the recipe’s conversion table. However, the real temperature you should be baking it at should be 284°F – 9°F hotter. To compensate you could leave the bake in longer or see if you can set the dial a little higher between the set increments
  • When in doubt in baking (apart from bread which benefits from higher temperatures) always cook slightly lower and for longer. This is an especially good rule of thumb for cakes, as it should give a flatter top and a more consistent crumb throughout

Temperature conversion chart

° Celsius	° Precise Fahrenheit conversion	Nearest Fahrenheit oven temperature typically given
80	176	– not normally given
90	194	– not normally given
100	212	200
110	230	225
120	248	250
130	266	250
140	284	275
150	302	300
160	320	325
170	338	325
180	356	350
190	374	375
200	392	400
210	410	400
220	428	425
230	446	450
240	464	475

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