These are two different steps in your wash, not two versions of one product. Olaplex No. 3PLUS is a pre-shampoo treatment you rinse out; K18’s mask is a leave-in you apply after shampoo. Pick Olaplex if you can give the shower an extra few minutes, K18 if your routine cannot absorb another rinse. Per ounce, Olaplex is far cheaper, though how far each bottle goes is disputed, so that gap is not a cost per use.
| Product | Price | Size | Format | Key Ingredient | Type | Rating | Buy |
|---|---|---|---|---|---|---|---|
| Olaplex No. 3PLUS Hair Repair Pre-Shampoo Treatment | $$ Mid | 3.3 fl oz (100 ml) | Pre-shampoo, rinse-out | Patented Olaplex bond-building technology + damage-defense complex (brand's terms) | Bond-repair treatment | 4.5 (1,963 reviews) | Check price (Affiliate link) |
| K18 Leave-In Molecular Repair Hair Mask | $$ Mid | 0.5 fl oz (15 ml) | Leave-in (no rinse) | K18Peptide (bioengineered peptide) | Bond / molecular repair | 4.3 (32,204 reviews) | Check price (Affiliate link) |
Swipe the table to compare specs and ratings. The Buy column stays pinned.
Olaplex vs K18, or K18 vs Olaplex: the short answer
Three inputs decide this. Damage and wash-day tolerance point to the step: rinse-out before shampoo, or leave-in after. Cost per ounce points to Olaplex, about $10.30 against about $58 for the K18 mini. Dose is the unknown, because neither brand states one, so nobody can turn that per-ounce gap into a cost per use.
We went through 38 Reddit threads on these two products, 28 of them pulled for this page, and the same argument runs through most of them: people treat these as two versions of one product and then pick the wrong version for their hair. We researched and compared both; we did not test them. Every mechanism claim below belongs to the brand, to a cited source, or to the community, never to us.
They are not the same product, or the same step in your wash
Olaplex No. 3PLUS goes on wet hair before shampoo, sits three minutes by the label, and gets rinsed. K18 goes on towel-dried hair after shampoo, with conditioner skipped, sits about four minutes, and stays in. Different moment, different mechanics, different failure modes.
The chemistry each brand points at is different too. Bleach and permanent color strip the cuticle lipids and lower the cross-link density that gives hair much of its strength, a mechanism the hair-cosmetics literature describes in its overview of chemical processing. Olaplex says its patented molecule rebuilds all three hair bonds; science communicator Michelle Wong explains how the active is designed to help broken disulfide bonds reconnect. K18 says its bioengineered peptide works on keratin chains instead, and describes the product as reversing damage in four minutes, an asterisked brand claim we quote rather than adopt.
A stylist trained in both brands, writing on r/HaircareScience, put the split as Olaplex on the disulfide bonds and K18 deeper on the polypeptide chains. It is the most-repeated explanation in our threads. The one peer-reviewed side-by-side measured a different thing, which is the next section. If your problem turns out to be cost rather than chemistry, our roundup of Olaplex alternatives prices five Olaplex alternatives per ounce, one of them a pre-shampoo bond-builder at the same step.
Which one fits your routine? The decision table
The axes that decide this are step, dwell time, whether you rinse, whether conditioner is allowed, how often the brand says to use it, and what an ounce costs. The last two rows are evidence quality, not a verdict. This page compares the 100 ml Olaplex bottle with the 15 ml K18 mini, the sizes our links go to.
| Axis | Olaplex No. 3PLUS | K18 Leave-In Molecular Repair Hair Mask |
|---|---|---|
| Step in the routine | Pre-shampoo, on wet hair | Post-shampoo, on towel-dried hair |
| Dwell time (brand) | 3 minutes | About 4 minutes |
| Rinse or leave in | Rinse out, then shampoo and condition | Leave in, no rinse |
| Conditioner compatibility | Conditioner comes after, as normal | Brand says skip conditioner first; two community reports say the brand confirms conditioner is fine after the 4 minutes |
| Frequency (brand) | Every 1 to 3 washes, daily if you want | 4 to 6 washes after a salon service, then as needed |
| Protein | None (brand states protein-free) | Peptide plus hydrolyzed wheat protein (ingredient list) |
| Size, band, cost per ounce | 3.3 fl oz, $$, about $10.30 an ounce | 0.5 fl oz mini, $$, about $58 an ounce |
| What the lab study measured | Olaplex N°0 (not No. 3): +47.6% tensile strength vs bleached, and an inner sheet in the fiber | K18: +19.5% tensile strength vs bleached, acting at the surface. Limits: 3 tresses, one application, no placebo arm |
| What community reports circle | Dryness on fine or undamaged hair; “it stopped working”; the volume of product needed | Straw and tangling when conditioner is skipped; breakage from using it every wash; salon mist is not the home mask |
Cost per ounce is arithmetic from the Amazon prices and sizes we verified on 2026-09-19: $34 across 3.3 fl oz for Olaplex, $29 across 0.5 fl oz for the K18 mini. Check current prices before you buy. Cost per use is a different number, and no source supports one: neither brand states a dose volume, and Reddit’s own bottle-life reports for K18 run from eight applications to several months.
What the one peer-reviewed head-to-head found, and its four limits
One published study puts these two in the same experiment. Martins and colleagues, writing in Cosmetics in 2024, bleached hair tresses and treated them with Olaplex N°0 or K18. They reported a 47.6% rise in tensile strength for Olaplex and 19.5% for K18 against bleached samples.
Four limits travel with those figures every time anyone quotes them. The study used Olaplex N°0, the watery pre-treatment, not No. 3 or No. 3PLUS. It ran on three tresses. It applied each product once, when both are sold for repeated use. And it had no plain-conditioner arm, so there is nothing separating the actives from ordinary conditioning. A self-described molecular biology research scientist on r/HaircareScience raised the sample size, the single application and the missing placebo arm in the thread that read the paper, and called the imaging solid and the design weak; another commenter in the same thread flagged that the Olaplex tested was N°0.
The direction is the interesting part, and it needs care. The study reported that K18 formed an external layer and acted at a superficial level without significant effect on the core of the fiber, while Olaplex N°0 produced a visible inner sheet. That is a measurement of how far into the fiber each product was observed, surface layer against core. A stylist trained in both brands wrote about a different question: which bonds or chains each brand is said to target. Those two senses of “deeper” are not the same claim, and we are not resolving the mechanism here. What the study measured and what the community repeats sit side by side unreconciled, and the honest posture is the one the sharpest posters in those threads take: neither brand has independent proof that its product does what the marketing says.
Olaplex No. 3PLUS: how to use it, and who it disappoints
Olaplex No. 3PLUS Hair Repair Pre-Shampoo Treatment
Best for: The reference product: anyone who wants the original patented bond-builder rather than an alternative, and will pay for it
Olaplex's update to No. 3: a 3-minute pre-shampoo bond treatment with the brand's patented bond-building molecule plus a damage-defense complex, in a 100 ml bottle
- The original patented bond-building chemistry, the reference product in this category
- A short 3-minute pre-shampoo step, protein-free and color-safe by the brand's claims
- The brand's own current listing, not one of the look-alike third-party 'Ola'plex' listings that sit beside it in Amazon search
- Around ten dollars an ounce on a bottle Amazon reviewers call smaller than expected, and Olaplex says use it every 1 to 3 washes
- This is the newer 3PLUS formula, so its rating record is still young, under 2,000 ratings; long-term reviews do not exist yet
- It does not work for everyone: Reddit threads are split, and people with fine or undamaged hair often report dryness, weight, or no change
(Affiliate link) · price may vary
Apply generously to wet hair before you shampoo, leave three minutes, rinse, then wash and condition as usual. Olaplex says every one to three washes, and that daily use is fine. Community practice runs looser: weekly for most, twice a week on fresh bleach damage, tapering to monthly as hair improves.
One disclosure before the Reddit evidence. Almost none of it is about 3PLUS. Across our 38 threads, No. 3PLUS is named twice, both times favourably and neither time in detail; everything else describes No. 3 or No. 0. Olaplex’s own explainer calls 3PLUS the evolution of No. 3 at the same step, so the routine transfers, but the anecdotes predate the reformulation and we are not passing them off as reviews of the bottle we link.
With that boundary, the split is real and it is close to even. People with bleach damage describe it as the treatment nothing else replaced, including several who switched to K18 and came back. The complaints cluster just as hard: fine, virgin, or merely dry hair reports dryness and rigidity, a recurring thread asks why it “stopped working” (the community’s own answer is that there was nothing left to repair), a couple mention the smell, and the amount of product a long head needs is a running grievance. The honest caveat from our product record is the same one the threads raise: it does not work for everyone, and fine or undamaged hair often reports dryness, weight, or no change at all. Its rating base is also young, under 2,000 ratings, because 3PLUS is the newer listing.
Olaplex · (Affiliate link)
K18 Leave-In Molecular Repair Hair Mask: how to use it, and why people end up with straw
K18 Leave-In Molecular Repair Hair Mask
Best for: Bleach-, color-, chemical-, or heat-damaged hair that needs a repair step it can leave in rather than rinse out
A leave-in peptide treatment (K18Peptide) that works in about four minutes on clean, towel-dried hair, with no rinse and no cap
- The most-reviewed dedicated bond-repair mask in this lineup, with tens of thousands of Amazon ratings behind it
- Leave-in and fast: about four minutes on clean, towel-dried hair, with no rinse and no cap
- Built around a bioengineered peptide, K18Peptide, which the brand says repairs damage from bleach, color and heat
- The priciest bottle per ounce here on the half-ounce mini, and how far it goes is disputed by users, and the bottle-life reports we read concern larger or unnamed sizes, so your cost per use depends on your dose
- Works best as one step in a fuller routine; some users say they cannot credit their results to K18 alone
- It is a leave-in cream, not a traditional rinse-out mask, and the skip-conditioner, consecutive-wash protocol trips people up
(Affiliate link) · price may vary
Shampoo, skip conditioner, towel-dry thoroughly, work one to three pumps through the lengths, wait about four minutes, and do not rinse. K18 says four to six washes in a row after a salon service, then as needed, and that conditioner beforehand leaves a film blocking the peptide.
A trick worth borrowing from the threads: emulsify the product between your palms before it touches hair, because the cream is hard to distribute and harder still to brush through. That last point is where the straw complaint comes from. Applying a leave-in to unconditioned, towel-dried hair and then combing it is a mechanical problem before it is a chemistry one, and users describe tangles bad enough to cost them length. Over-use is the other cause. The clearest caution thread in our set is someone whose stylist told them to use it every wash; the top replies identify the missing conditioner rather than the product, and a stylist of nineteen years in another thread warns against using either product every wash.
The record is strong and loud in both directions. K18 carries by far the deeper review base of the two, more than 32,000 Amazon ratings against Olaplex’s count of under 2,000, and the enthusiasm in the threads is the single loudest signal we found. Against that: the honest caveat is cost and credit. The half-ounce mini is the priciest bottle per ounce here, about $58 an ounce against Olaplex’s $10.30, and how far a bottle goes is disputed in our threads, from eight applications to several months, though those reports are about larger or unnamed sizes and no thread says how far the 15 ml mini goes, so what it costs you per use depends on your dose. One fan concedes they cannot give K18 all the credit, because it was one step in a fuller routine. Two more notes people rarely mention: the salon mist is a different, more potent product than the home mask, and one user in a humid climate found the same bottle behaved completely differently. Our hair mask roundup covers where K18 sits against rinse-out repair masks.
K18 · (Affiliate link)
Yes, you can condition after K18
Wait the four minutes, then condition. That is the single highest-value fix in our research, it comes from the community, and two separate threads report the brand confirming it: the skip-conditioner rule covers the four-minute window, not the rest of your wash.
The top-voted fix in our whole set, from a user on r/HaircareScience, describes the routine plainly: “What helps make k18 more manageable for me is after the 4 minutes, I rewet my hair and condition or deep condition like normal.” Others in the same threads report the identical result, including someone who used to get brittleness every time and no longer does. If you have run K18 for a few washes and your hair has gone rough and tangly, this is the first thing to change, before you conclude the product failed. What you cleanse with matters as well; our guide to shampoo for damaged hair covers the wash step around a repair treatment.
Can you use both? Order, spacing, and when stacking backfires
Not on the same wash day. Alternate them: K18 after one wash, the Olaplex step on the next. One bond treatment per wash is where the experienced voices in our threads land, and there is no benefit evidence for running both at once.
The stacking question comes up constantly, and the answer rarely varies. The top reply to the classic version of it (Olaplex No. 0 and No. 3 before the wash, then K18 after) is that Olaplex is too heavy to pair with K18 on one day, and to split them across wash days instead. Someone with relaxed hair described a workable schedule: K18 for the first four washes after the chemical service, then mostly Olaplex for maintenance. The counter-risk is well documented by the community too. One of the highest-scoring warnings in our banked bond-builder research is that most people who buy into the category do not need it, and a stylist calls the failure mode over-keratinizing. Crispy, stiff, straw-textured hair after weeks of enthusiasm is the signature of too much repair, not too little.
What neither one does
Neither adds moisture, neither repairs a split end, and neither does anything permanent. If your hair is rough and thirsty rather than snapping, a bond treatment is the wrong purchase and you will feel worse after it, not better.
Split ends are the clearest line. The American Academy of Dermatology’s guidance on stopping hair damage is prevention rather than repair: damaged hair is fragile and tends to break, and its ten recommendations are all about not doing more damage. The stylists in our threads go one step further and say the same thing before recommending any bottle: get the damaged length cut first. Permanence is the second line. The claim that K18’s repair is permanent circulates widely on Reddit, is uncredentialed, and is contradicted inside the same threads by people pointing out that hair keeps getting damaged and every bond product is sold for repeat use. Both brands tell you to reapply, which settles it.
The third line is diagnosis. If you have not bleached, colored, relaxed, or heat-styled hard, the community’s own verdict is that there is little for either product to work on. The recurring correction in those threads is that the asker needs moisture, and a deep conditioner is the cheaper and more likely answer.
Who should pick which
Fresh from a bleach or color service, with no patience for a longer wash: K18, four to six washes in a row, then space it out. Budget-first and fine with the extra rinse: Olaplex No. 3PLUS, roughly a sixth of K18’s price per ounce, though bottle life is disputed, so that is not a per-use figure.
Already gone crunchy on one of them: stop both, add moisture, and let your hair reset before you spend again. If your hair goes stiff or brittle on protein products, that difference is the axis to decide on, because Olaplex states No. 3PLUS is protein-free while K18’s ingredient list carries a peptide and hydrolyzed wheat protein, and our threads frame this as a protein-versus-moisture balance rather than a verdict on either bottle. Hair type does not give a clean rule here and we will not invent one, because the same “fine hair” is blamed in our threads for Olaplex failing and for K18 failing. What does hold is that the wrong pick on low-porosity, coily, or undamaged hair is where our threads report the roughest outcomes.
Not chemically processed and not heat-damaged: buy neither. There is also a third answer for anyone who wants the Olaplex step at a lower price, because the bond-builders we priced per ounce include a pre-shampoo option at a fraction of Olaplex’s price per ounce, while the repair masks we rank cover the rinse-out route.
Want to know how we choose? See about The Hair Roundup, or browse the rest of our hair-care roundups.