Each measure has a different job
BMI relates weight to height for population screening. Waist-to-height ratio adds central-fat context. Circumference-based body-fat equations and FFMI attempt to separate fat and fat-free mass, but inherit measurement error. Agreement across several trends is more useful than one isolated score.
Standardisation beats false precision
Use the same scale, tape, anatomical landmarks, time of day and method. Small week-to-week changes can be noise. Evaluate monthly direction alongside strength, energy, blood pressure, laboratory results and professional advice when relevant.
Build a measurement dashboard
Choose one primary outcome and one complementary measure. For example, combine a monthly waist measurement with a weekly-average weight, or pair a body-fat trend with strength performance. Record the method beside the number. This small dashboard makes contradictory day-to-day readings less powerful and keeps the decision focused on health and function.
How to combine the results
You do not need every calculator on the same day. Choose the primary measure that changes your decision, record the conditions and follow it for several weeks. Add a second tool only when it provides useful context. When results conflict, better measurement or professional assessment is more valuable than extra decimal places.