I am new to making cider and have started a few 5 gallon carboys. They are almost done with their first fermentation and was wondering if I could still add sugar to boost my alcohol levels when I do my first racking.I did not do this step before setting in the original yeast so i’m afraid the alcohol levels will not be where they should be. And no, I did not measure the sg in the beginning either.
You can certainly add more sugar if you want, at any time during the fermentation. Just be sure to give the extra sugar enough time to ferment out – it will most likely take several weeks, not days.
Thanks!!! Is there any way to measure the alcohol level of the cider after all the fermentation is complete and ready to bottle? Like I said earlier, I never measured the sg from the start-
If you never measured SG at the beginning, then… no, you cannot know exactly your alcohol level, at least not without a Masters in Chemistry and fancy analytical equipment.
You can probably get close, though, if you can replicate your source for the apple juice and measure the SG of that. Then use software with the amount of added sugars, and you’ll get in the right ballpark.
If you have a refractomer, or someone that live near by, you can back calculate the OG.
In other words, put your location in your profile. 
Your initial gravity is probably something in the ballpark of 1.045 (assuming you started with store bought juice.) That will go to 1.000 or less, so put those two numbers into the calculator (http://www.brewersfriend.com/abv-calculator/ is one of the many online) and it gives you about 6% ABV.
Adding sugar will raise the starting gravity. A pound of brown sugar will give you 45 gravity points (points per pound per gallon). So one pound in 5 gallons of cider will raise the gravity by 45/5 or approx .009 (to 1.054).
You don’t technically need to add sugar to cider, there is enough natural sugars there to get you to a 6% alcohol by volume level without adding anything. I think the idea of adding sugar to cider is a carryover from home winemaking where the practice is to add sugars to get the OG up to some typical level. I’ve made a few batches, one where I used a couple gallons of store-bought juice and added a couple cans of concentrate; another where I used only the juice, and then the last batch I made was with store-bought juice and then added brown sugar to get to 1.060. They all worked. I would guess that the higher the amount of alcohol the longer it will take to age to something tasty.
