Two calculators, no numeric chain between them
A baby's weight gain rate and an adult's one-rep max have nothing to do with each other — no real output from one formula can feed into the other. But both real formulas share a genuine methodological parallel worth pointing out directly.
Baby weight gain: an average rate standing in for a trajectory
A baby who gained from 3,200g to 4,600g over 8 real weeks gets one real number — 175g per week — but that average doesn't capture whether growth was steady, or a slow month followed by a growth spurt. The single average is a practical stand-in for a full growth trajectory that would need many more measurements to actually observe.
One-rep max: a sub-maximal lift standing in for a true maximum
Someone who actually lifted 100kg for 5 reps never tested their true one-rep max directly — the real Epley estimate of 116.7kg and the real Brzycki estimate of 112.5kg are both stand-ins for a maximum that was never actually attempted, extrapolated from a rep count that was.
Why both tools are estimates by design, not by limitation
In both cases, directly measuring the true quantity of interest (a full growth curve, an actual max-effort lift) is either impractical or genuinely risky — so both calculators deliberately trade some precision for a number you can get from data you already have. Understanding that trade-off is what separates using either result as a rough planning figure from mistaking it for a precise, directly-measured fact.