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Over the holidays, my mom tasked me to prepare a Crown Roast of Pork for our Media Noche (basically it's the meal you have as you approach the New Year with fireworks) meal. All of the recipes I found on youtube used the Fahrenheit. The procedure would go:

  1. Preheat Oven to 450 F
  2. Roast Crown Roast for 20 minutes
  3. Reduce heat to 350 F and wait for another 130 minutes

Our Convection Type Oven has a Celsius Degree Scale. Converting those temperatures to Celsius, I got:

450 F = 232.2222 C ~ 240 C
350 F = 176.6666 C ~ 180 C

Now, the dials (yes, our oven has an old school rotary dial for time, temp, and function) are only divided into 10s, from 50 C to 250 C, so I didn't have a clear 232 and 176. What I did was I rounded up to the nearest tens instead and then used the same cooking time, thinking that the Fahrenheit equivalent of the rounded up Celsius values wouldn't be too far off.

When I took the Crown Roast out, the outside was charred a bit but the inside was cooked well, although a bit dry. My siblings described the char to still be edible and didn't have that bad and bitter burnt taste.

So, after almost botching our dinner, I rescaled my values back:

230 C = 464 F (originally 450 F), cooked for 20 minutes
180 C = 356 F (originally 350 F), cooked for 130 minutes

Now, we see that there is quite a disparity between the new Fahrenheit values, and being cooked at a higher temperature dried up my pork and burnt the outside.

My question is, is there a "time-scaling" rule to follow when converting Fahrenheit to Celsius? What I mean by this is that for example, in my case, it might be:

450 F for 20 minutes or 230 C for 25 minutes or 220 C for 18 minutes

Wherein after converting the temperature from Fahrenheit to Celsius, the time you cook it in that temperature would also change since the temperatures or not the same. The temperature may have been decreased or increased in order to accommodate the limitations of the cookware.

Sorry if this winds up being an opinion based question or something that can only be discussed but not answered. As much as possible, I'd like to see if anyone tried something with regards to this, may it be from experience or from a book or a class or a show and if others have used the same method.

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    I think your title is a little misleading - obviously if you are able to convert precisely enough you shouldn't have to change the time at all. It seems like what you're actually asking is how to change the time if your temperature is off by 5-10 degrees. – Cascabel Jan 7 '15 at 17:31
  • You have it back to front. 180°C is the medium oven temperature as defined in a classic French (metric) cooking school. 350°F is the rough approximation of that :-) – TFD Jan 7 '15 at 21:14
  • @Jefromi indeed; coming from a physics background, the title of this question struck me as extremely strange. Converting from one unit system to another doesn't change the actual value at all! Your comment makes much more sense. – David Z Jan 8 '15 at 0:44
  • Something seemed out of whack with the calculations. 230° C = 446° F. (Not 464° F). This would explain the disparity. – Cindy Oct 14 '17 at 15:43
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Rounding to the nearest 10C is more accurate than your thermostat probably is anyway (don't round up, round to the nearest).

Conversion isn't your issue, your thermostat is much more likely your culprit.

Use an oven thermometer, not your dial.

And keep in mind that ovens hover above and below their set temperature by switching on and off.

Use Google or whatever to do your conversions, and then your oven thermometer to keep your thermostat honest. The natural variances you'll get from rounding are fine.

You should also be using a meat thermometer; pull a roast to rest when it is 5F below your target temperature.

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    And if you have ten degree increments on your dial you don't have to round to the nearest ten degrees, you round to five degrees and be obsessive and put it halfway between marks. The fact that that's presumably a little tricky goes right along with the point about thermostat accuracy: the oven's not really accurate to that scale anyway, so there wasn't any reason to make it easy to set on that scale. – Cascabel Jan 7 '15 at 17:33
  • You might also round the initial hot bit up and the longer slower bit down (or possibly vice versa) as well. But the temperature will vary between different bits of the roast by more than that even in a fan oven, and the cooking time will be affected by more than that for taking it out for basting a different number of times (or faster/slower) to their assumption. – Chris H Jan 7 '15 at 19:31
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There's nothing wrong with your conversions, they were fine. What you may not have considered is:

  • Convection versus non-convection ovens. When you see a recipe in F it's most likely from the US, and in the US convection ovens are rare. Convection ovens cook with more intensity than non-convection ovens as the fan blows hot air, so when using a recipe for a non-convection oven you should turn down the temperature. How much depends on the oven but I'd say at least 15C or 30F
  • Many ovens are inaccurate, sometimes wildly so. I've seen ovens be as much as 40C off the temperature set on the dial. Without an oven thermometer you can't be sure whether you are actually cooking at the temperature you set

My money is on the convection, you simply had too much heat going into the roast. Just turn it down next time. Also, get an oven thermometer, they are cheap and can save your dinner.

  • The rule of thumb I've seen is 25F for convection, but in any case, yes, using a convection oven without adjustment is likely to cause problems like this! – Cascabel Jan 8 '15 at 3:18
  • European cookbooks and convenience food packages mostly give both values, and the difference varies wildly from one recipe/package to another. My frozen potatoes (fries/croquettes/rösti) all want 180°C with fan, 200°C without. The frozen duck even gives different durations, IIRC it was 40-45min@220°C without, and 30-35min@200°C with fan. – Alexander Jan 8 '15 at 9:24
  • One of the conversions was wrong. See my comment above. :) – Cindy Oct 15 '17 at 14:13

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