Retinol Concentration Calculator

Recommended Current % and Application Frequency are calculated from two entirely separate pathways that never interact. Recommended Current % comes from a fixed six-rung concentration ladder (0.025%, 0.05%, 0.1%, 0.25%, 0.5%, 1.0%) -- Skin Tolerance sets which rung you start on and how many weeks must pass between each step up (8 weeks for sensitive, 6 for normal, 4 for resilient, lines 14-17), and Weeks Using Retinol divided by that per-step interval, rounded down, determines how many rungs you've climbed (lines 20-24). Application Frequency, by contrast, is read from Current Retinol % directly through its own separate bracket (2 nights/week at 0.025% or below, up to 7 nights/week above 0.1%, lines 28-31), and then reduced by 2 more nights, floored at 2, if Skin Tolerance is sensitive (line 33) -- what you actually type as your Current Retinol % never affects Recommended Current % or Weeks to Goal at all; those two pathways are entirely independent.

Weeks to Goal counts whole ladder rungs remaining, not literal percentage distance: it finds the first rung at or above Goal Retinol % (line 36), so a goal that falls between two rungs still targets the rung above it, and multiplies the number of rungs still needed by the same per-step interval used for Recommended Current % (line 38). Buffer Product Recommended triggers on either condition alone, not both together -- Dry skin type or Sensitive tolerance is enough on its own to recommend a buffer product (line 41).