Head to Head
Olaplex No.3 vs K18 Leave-In Molecular Repair Mask
The category's defining comparison: Olaplex No.3's patented active in a ten minute pre-shampoo against K18's peptide in a four minute leave in, judged on protocol, brief and bottle economics.
Olaplex No.3 Hair Perfector
The original: the pre-shampoo perfector that started the bond category, built on the brand's patented Bond Building Technology as published.
K18 Leave-In Molecular Repair Hair Mask
The peptide exception in a bond edit: a leave in treatment the brand publishes under molecular repair naming, used instead of conditioner and never rinsed.
Where They Differ
Protocol and time
No.3 asks for ten minutes before the shampoo and a full wash sequence after it. K18 asks for four minutes after the shampoo and forbids the conditioner step on its day. Total added time favours K18 clearly; integration with a normal conditioning routine favours No.3, since K18 days leave the moisture step to be made up elsewhere in the week.
The published systems
Olaplex publishes a single small molecule active and describes it in bond relinking language. K18 publishes a biomimetic peptide and describes it in polypeptide chain language, printed in its ingredient list as sh-Oligopeptide-78. Neither framing is this site's claim; what a buyer can use is that the products feel different in practice, No.3 firmer and more treatment like, K18 lighter and more forgiving on fine hair, and owner commentary for each matches its own framing.
Bottle economics
No.3 is 100 ml dosed by saturation; long, thick, porous hair can spend a quarter of the bottle on one thorough application. K18 is 50 ml dosed at one to three pumps, with the brand printing 16 to 50 treatments per bottle. Judged per treatment day rather than per millilitre, the prestige priced K18 often works out closer to No.3 than the shelf prices suggest, and for short or fine hair it can work out ahead.
The Specification, Side by Side
| Measure | Olaplex No.3 Hair Perfector | K18 Leave-In Molecular Repair Hair Mask |
|---|---|---|
| Size | 100 ml / 3.3 fl oz | 50 ml / 1.7 fl oz |
| Format | Pre-shampoo cream treatment, bottle | Leave in cream, pump bottle |
| Key Ingredients | Bis-Aminopropyl Diglycol Dimaleate (the brand's Bond Building Technology, as published), panthenol, jojoba seed oil, sweet almond oil | K18PEPTIDE (sh-Oligopeptide-78), hydrolyzed wheat protein, tocopherol |
| Use | Apply generously to damp hair before shampooing, leave 10 minutes, rinse, then shampoo and condition, once weekly or up to 2 to 3 times weekly for very damaged hair, as directed by the brand | After shampoo only, towel dry, 1 to 3 pumps, wait 4 minutes, do not rinse, as directed on the listing |
| Hair Type | All hair types, as printed on the bottle | All hair types, as printed on the listing |
Choose Olaplex No.3 Hair Perfector if
- You want the category's reference routine and the deepest owner record behind your purchase
- Your damaged hair is also dry and needs shampoo, conditioner and masks kept in the same wash day
- You prefer a treatment that rinses out over a leave in whose protocol you must follow exactly
Choose K18 Leave-In Molecular Repair Hair Mask if
- Your hair is bleached or chemically serviced and feels seriously fragile
- You will keep a four minute routine but not a twenty minute one
- Your hair is fine and heavy pre-treatments plus conditioner flatten it
Questions, Answered
Which one should a first time bond buyer start with?
No.3, for most people: it is the reference product, its routine tolerates improvisation better, and it teaches what a bond treatment feels like inside a normal wash day. K18 is the step to take when the damage is severe, the hair is fine, or the pre-shampoo format has already failed to survive your schedule.
Keep Researching
Every page in the Bond Repair and Hair Treatments edit.
- 01Bond Repair and Hair Treatments: the hubHub
- 02Best Bond Repair Treatments 2027Best of
- 03Bond Repair Explained: What Bond Builders Claim and How the Technologies DifferGuide
- 04How Often to Use Bond Treatments, and How to Layer Them with Protein and MoistureGuide
- 05Bond Repair for Bleached and Heat Styled Hair: Routines by Damage PatternGuide
- 06Olaplex No.3 Hair Perfector ReviewReview
- 07Olaplex No.0 Intensive Bond Building Hair Treatment ReviewReview
- 08Redken Acidic Bonding Concentrate Intensive Treatment ReviewReview
- 09Redken Acidic Bonding Concentrate Intensive Treatment vs Olaplex No.3Comparison
- 10Bond Repair Treatments Buying GuideBuying guide
- 11Bond Treatment Care and Keeping: Fixing Stiff, Dry and Underwhelming ResultsCare
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