ABV measurements

I am looking to produce relatively large quantities of cider for the local pubs to sell but I am struggling to understand how to keep the ABV as close to exact very time.
I know that I can blend from several carboys and use the hydrometer before I start fermenting but is the any way to test the resulting abv once fermentation is complete?

There are several methods out there to do the calculations, but perhaps the most common one is to use the hydrometer again when fermentation is complete to get the Final Gravity reading (reached when the number doesn't change between separate readings 3 days apart). From there it's a bit of math:

(Original Gravity - Final Gravity) x 131.25 = ABV (%)

For example:

1.053 - 1.008 = 0.045 x 131.25 = 5.9% ABV


Hope this helps.

Cheers.

Agreed. The gravity difference method is the most common. You could also measure it by boiling off the alcohol and replacing it with water and measuring the change in weight and or density.

Also if you use a refractometer in addition to the hydrometer you can calculate ABV two more ways.

Consistent results come from consistent methods. Starting gravity of the cider will be a bit of a wild card. You can compensate there by adding sugar, or water to get your OG.

Best of luck on your business venture.