[Philbrew]

Papazian and Palmer and some folks on this blog have said that 3/4 cup (5.3 oz.) of corn sugar (or 5/8 cup of table sugar) is the correct amount to prime 5 gal. of beer for bottle carbonation.  I assume they mean for most beers.  I'm also assuming that "most beers" are around 2.3 to 2.5 volumes of CO2.
But if you go to the online calculators, you would need to final ferment your beer at 94* F to need that much sugar.  Or you are making a Belgian blond at 2.8-2.9 volumes and fermenting at 68 F.  Try a Brown Ale at 2.1 volumes and you gotta ferment at 143 F.  Good luck with that! 

Is the "3/4 cup" just a holdover from the old days of home brewing?  Or does it not really matter that much and the calculators are putting too fine a point on it?

[klickitat jim]

Something is odd with your calculator.  For 2.1 volumes in 5 gallons that was at 68º I show 3.57 oz corn sugar or 3.25 oz table sugar

[dmtaylor]

Volume matters.  Temperature does not matter much.  I use 5/8 cup table sugar in 5 gallons and always get perfect results.

[Jimmy K]

Different versions of corn sugar have different densities. I've seen some where 3/4 cup weighs ~3oz.

[Philbrew]

Something is odd with your calculator.  For 2.1 volumes in 5 gallons that was at 68º I show 3.57 oz corn sugar or 3.25 oz table sugar

That is what I am saying, I guess.  Either the calculators are off or the old 3/4-5/8 cup is off.  Or does it matter that much?  3.57 oz. is nowhere near 3/4 cup, according to the calculators.

Go tohttp://www.northernbrewer.com/priming-sugar-calculator/ enter 2.1 volumes and 5 gal. and increase the temp until you get 3/4 cup (.75) of corn sugar needed.  It's 143 F!

[Philbrew]

Different versions of corn sugar have different densities. I've seen some where 3/4 cup weighs ~3oz.

But the calculators call for 5.3 oz. of corn sugar equal to 3/4 cup.  And cane (table) sugar is very consistent, 4.8 oz = 2/3 cup.

[pete b]

I have much better results since I have weighed my sugar and used the northern brewer priming calculator. I pay no attention to temp. I just always condition warm (70ish) for a week or so then move to the cellar. Also, I have switched to organic cane sugar. After spending good money on high quality ingredients it seems silly to use crappy gmo corn products.

[Philbrew]

Volume matters.  Temperature does not matter much.  I use 5/8 cup table sugar in 5 gallons and always get perfect results.

Yes, the temp part of the equation that these calculators use goes off into silly land pretty quickly. 
Do you weigh out 4.5 oz. or do you actually measure 5/8 cup?  If so, where did you find a dry measure that does eighths?

[dmtaylor]

I have a 2/3 cup measuring cup and I fill it slightly short.  It works.  I never weigh my priming sugar.  In small batches (currently I usually brew 1.7 gallons), I run some calculations and then measure 1/4 cup plus or minus X number of teaspoons if required.

[reverseapachemaster]

I used the northern brewer calculator and set five gallons at 75F and I only had to get to 2.7 volumes of carbonation to get to 0.74 cup of corn sugar or 5.22 ounces (by weight). That is not far off on the 3/4 cup recommendation but 2.7 volumes is too much for most styles. 3/4 cup is a convenient measurement that was easy to repeat. It is also why lots of homebrew tends to be overcarbonated for its style.

[denny]

Something is odd with your calculator.  For 2.1 volumes in 5 gallons that was at 68º I show 3.57 oz corn sugar or 3.25 oz table sugar

That is what I am saying, I guess.  Either the calculators are off or the old 3/4-5/8 cup is off.  Or does it matter that much?  3.57 oz. is nowhere near 3/4 cup, according to the calculators.

Go tohttp://www.northernbrewer.com/priming-sugar-calculator/ enter 2.1 volumes and 5 gal. and increase the temp until you get 3/4 cup (.75) of corn sugar needed.  It's 143 F!

Which is why it's more accurate and repeatable to weigh it rather than measure volume.

[HoosierBrew]

Which is why it's more accurate and repeatable to weigh it rather than measure volume.

Yep

[Philbrew]

So, looking at all of the above, I get:

1.  Give the measuring cups back to the Missus and weigh your priming sugar.  Exception could be if you are very careful and using table sugar, which is very consistent.

2.  The old 3/4 cup thing was for writer convenience and gives over carbonation for most styles.

3.  Use a priming calculator with the temp set at around 70 F.  Most beers we brew finish fermenting in the 66-75 F range, even lagers with a D rest.

4.  Some of the priming calculators do go to too fine a point.  Do you really need to weigh your priming sugar to a hundredth of an oz.?

5. RHAHB  You probably can't tell the difference between 2.4 volumes of CO2 in a beer and 2.5 volumes.  At 2.7 or more, yeah.