Hair Extension Calculator

Packs Needed is calculated from Extension Method, Desired Volume, Natural Hair Density, and a length bracket keyed off Desired Final Length alone (line 25) -- Current Hair Length is never referenced anywhere in that formula. It only feeds Added Length, a separate output that simply subtracts it from Desired Final Length (line 10), so changing it never moves Packs Needed, Total Weight, Total Pieces, or Installation Time.

Neither -- they move it by exactly the same amount, in opposite directions. Added Length is a straight subtraction, Desired Final Length minus Current Hair Length (line 10), so raising Desired Final Length by an inch adds an inch to Added Length and raising Current Hair Length by an inch removes an inch from it -- a one-to-one tie with no other mechanism involved. (Desired Final Length separately drives the length-bracket factor inside Packs Needed, but that's a different output; it doesn't make either input move Added Length more than the other.)

The length factor inside Packs Needed is not continuous -- it steps between four fixed multipliers at 14, 18, and 22 inches (line 23). Moving Desired Final Length from just under one of those breakpoints to just over it changes the multiplier applied to every pack calculation, which after rounding up with Math.ceil can add a whole extra pack even though the length itself barely changed.

Maintenance Interval is read directly from Extension Method with no other input involved: 6 weeks for tape-in, 12 weeks for keratin bond, 8 weeks for sew-in/weft, and 0 weeks for clip-in, since clip-in extensions are removed nightly rather than serviced on a recurring schedule (line 43). A 0-week result is expected behavior for that method, not a missing calculation.