Frozen concentrate has 42 kcal per 100 g — dilute before drinking. The concentrate is meant to be mixed with water, so a reconstituted glass sits around 45–55 kcal per 100 ml. Log the prepared juice, not the concentrate weight, or you'll triple-count the calories.
How should I track Grapefruit juice, red, not fortified, not from concentrate, refrigerated?
Liquid calories from Grapefruit juice, red, not fortified, not from concentrate, refrigerated are easy to overlook because drinks do not trigger the same fullness signals as solid food. A meta-analysis (Wehling & Lusher, Am J Clin Nutr 2021) confirmed that people underreport beverage intake more than solid food. Most people pour 30–50% more than the label serving size without realizing it. Use a kitchen scale to weigh your glass or bottle once — you will know your exact pour going forward. This one calibration step can save you from hundreds of untracked calories per week.