I just finished bottling a saison and was a little confused when trying to determine how much corn sugar to use. I used an online priming sugar calculator and it said a carbonation level of 3.2 vols required 7 oz (weight) or 1 cup (volume) of corn sugar. I measured out 1 cup of corn sugar and decided to weigh it and it only weighed 4 ounces. The corn sugar that I recently purchased seemed to be in a bigger bag than usual. So I added some more to get it closer to the 7 ounces suggested by the app. I did check several calculators and all gave me the same, around 7 oz. The 2 measurements seem so far off that I am a little worried. Should I always stick to using weight instead of volume? If I just used the volume suggestion of 1 cup, it seems like it would have been under-carbonated. But I also don’t want beer bombs. Am I missing something here? Thanks for your help.
What’s missing here is the batch size and temp that the beer was sitting at when you bottled, and also where you got your bottling sugar. Corn sugar (i.e. dextrose) sold for homebrew bottling often also contains a “heading agent”, usually an algae-derived sugar like gum arabic or whatnot. The ratio of heading agent to dextrose perhaps threw off the measurement. If anything, the beer would probably be under-carbed if you used a product that contained the heading agent.
I measure my corn sugar by weight based on the finished bottling volume of beer and the desired level of carbonation. The temperature RC is referring to is the temperature in which the bottles will condition at after the bottles are capped.
For solids, weight measure is always to be preferred over volume measure.
Always?
I prefer to add water salts by level tsp. I quit breaking out the gram scale trying to hit an exact number because it’s all just an approximation anyway. Accurately measuring brewing salts in grams gives an illusion of precision.
I just weighed a cup of corn sugar in my brewroom and got 4.5 oz. The OP added more to get closer to the 7 oz.
I would recommend caution when carbonating over 3 volumes in bottles. Its better to be a little low than high. If you over-carbonate or have a bottle that’s a little weak, you may experience a bottle bomb as you mentioned.
It happened once to me even though I thought I was being cautious. That’s one reason I prefer to keg these days.
After that, I stored them in an enclosed area, to prevent damage from flying glass chards. From your description, I would be concerned about this.
Obviously not a baker. 
It’s best to trust weight as opposed to volume, as mentioned, simply because of the way solids like sugar, flour, etc, compact. I bake a lot and I hate it when ingredients are given in cups as opposed to oz or grams because it is never accurate.
That said, I agree with Roger. Be absolutely sure your bottles can withstand that pressure. 2.7-2.8 is about as safe as I feel in standard brown bottles.Much higher in thick walled champagne bottles.
Baking is different. I get a bit more precise because there’s no way to adjust a loaf of bread later.
In brewing I can always add salts down the line (more like cooking).
‘Always’ is just a trap.
I can see for salts but for priming sugar I do think weight is “always” better
Tests I did years ago confirm that.
I can appreciate the “illusion of precision” argument for something like brewing salts where I doubt most people could taste a 10% difference in ion concentration. Heck, even a 25% difference would be tough unless you’re talking very high concentrations. A 10% difference in carbonation is a lot more likely to be noticeable, though.
In general, I think for solids you will generally get a more precise result by using weight rather than volume.
I agree, measuring brewing salts by volume is much easier. Never could understand those who insist moisture will affect volume too much for it to be accurate. I’ve always though it to be the opposite. Moisture generally has a much greater affect on weight.
One corn sugar milling machine may produce a different packing density than another. And then there is tapped or vibrated packing density vs. loose packing density. And density at the top of a column of powdered material vs. density at the bottom of the same column. All sorts of problems may potentially rear their ugly head when attempting to equate weight to volume for certain types of solids.
I agree. …but that’s quite a difference (underlined) that your initial always statement.
When it makes sense, weight should be used for solids, when it doesn’t matter, it’s optional. I simply gave an example of when it doesn’t matter.
Thanks for your help everyone. I will stick with weighing, being careful to not over-stress my bottles. I just found it interesting for the two calculations to be so far off.
On a larger scale weight is always better, on a smaller scale measurement is fine.
If off a few degrees with a compass, at a few meters you will still hit your mark, at several thousand meters you will be lost.
If sights are off on a rifle, at a few meters you will hit the target, at a few hundred meters you will miss by far.
I measure 1 tsp table sugar for a 22 oz bottle, slightly rounded, level, tad under, bottle will still be carbed fine.
If I measure for the whole batch (24 bottles), that little over or under each time has a cumulative effect,
and will be way over or under.
If I bake on a very small scale measuring is fine, or even eyeball it, on a larger scale I switch to weighing.
Learned this the hard way.