How to Floss Properly: What the Evidence Actually Shows

Most dental advice about how to floss properly gets the order wrong. You’ve been told to brush first, then floss. Three randomized controlled trials found the opposite sequence removes more plaque and delivers more fluoride to the interdental surfaces where cavities form. The standard advice is backwards.

The reason flossing matters at all is simple. A toothbrush can’t reach between teeth. Even advanced electric brushes clean only the front, back, and chewing surfaces. That leaves the two sides of every tooth untouched, roughly 40% of the total surface area where bacteria accumulate and cavities start. Flossing is the only mechanical method that disrupts plaque in those spaces. The evidence shows it works when done correctly, but the technique most people use is inefficient and the sequence most people follow is suboptimal.

This review synthesizes the published trials on flossing sequence, technique instruction, and clinical outcomes. The findings complicate the generic advice you’ll read elsewhere.

Brushing alone misses 40% of tooth surfaces

Your toothbrush can’t physically reach the contact point where two teeth meet. Bristles clean the buccal (cheek-facing), lingual (tongue-facing), and occlusal (chewing) surfaces effectively, but the mesial and distal sides of each tooth (the sides that face adjacent teeth) remain untouched. A 2014 randomized trial measured site-specific plaque removal and found that toothbrushing alone left interdental plaque levels unchanged, regardless of brush type.⁷

Those interdental surfaces are where most cavities form in adults. Plaque bacteria produce acid as they metabolize sugars, and that acid demineralizes enamel. The process starts in areas where plaque sits undisturbed longest, typically between teeth. Brushing twice daily controls plaque on accessible surfaces but does nothing for the spaces a brush can’t reach.

Floss is the standard mechanical tool for interdental cleaning because it’s thin enough to slide between the contact points of adjacent teeth. The clinical question is not whether floss can access those surfaces (it can), but whether people actually use it correctly and whether correct use produces measurable health benefits. The trials show it does, but only when technique and sequence are right.

Floss before you brush, not after

The conventional advice is brush first, then floss. That sequence is intuitive (you clean the big surfaces first, then the tight spaces), but three independent randomized controlled trials found the reverse order works better.

A 2026 trial assigned 60 participants to either floss-then-brush or brush-then-floss for two weeks, then switched groups.¹ The floss-first sequence reduced interdental plaque scores by 25% more than the brush-first sequence and improved gingival health scores significantly. A 2018 trial in 25 adults found the same pattern. Flossing before brushing removed more interdental plaque and left higher fluoride concentrations in those spaces after rinsing.² A 2015 study in 30 adults confirmed the effect: floss-first reduced plaque and gingivitis more than brush-first over four weeks.³

Flossing sequence effects across three RCTs (n=115 total participants)

The mechanism is straightforward. Flossing first loosens and lifts interdental plaque into the mouth, where brushing and rinsing remove it. Brushing first cleans the accessible surfaces but leaves interdental debris in place. Flossing afterward lifts that debris, but you’ve already rinsed away the fluoride from your toothpaste, so the newly exposed interdental surfaces get less fluoride exposure. The floss-first sequence maximizes both mechanical plaque removal and chemical exposure to fluoride.

Most people still brush first because that’s the standard advice. The trials are recent and the reversal hasn’t propagated widely. If you’ve been brushing first your whole life, switching the order is the single highest-yield change you can make to your flossing routine.

How to floss properly: the C-shape method

How you hold and move the floss matters more than most people realize. The correct technique is called the C-shape method. You wrap the floss around one side of a tooth in a C-curve, slide it gently below the gumline, and pull it up against the tooth surface to scrape off plaque. Then you repeat on the adjacent tooth before moving to the next interdental space.

The steps:

1. Use about 18 inches of floss. Wrap most of it around one middle finger, the rest around the opposite middle finger, leaving 2 to 3 inches of working length between your hands.
2. Hold the floss taut between your thumbs and index fingers.
3. Slide the floss gently between two teeth using a back-and-forth motion. Do not snap it down into the gums.
4. When the floss reaches the gumline, curve it into a C-shape against one tooth.
5. Slide the floss gently into the space between the gum and the tooth until you feel resistance.
6. Pull the floss up against the side of the tooth, scraping from below the gumline to the contact point. Repeat 2 to 3 times.
7. Curve the floss around the adjacent tooth and repeat the same scraping motion on that side.
8. Move to the next pair of teeth with a fresh section of floss.

A 2024 trial tested whether video instruction improves flossing performance compared to no instruction.⁴ Participants who watched a video demonstrating the C-shape technique removed significantly more plaque than those who received no instruction, and adding hands-on coaching on top of the video further improved outcomes. The trial confirmed that most people do not instinctively use the correct technique and that showing them how produces measurable improvement.

The common mistakes are using too little floss (so you reuse a plaque-covered section), skipping the C-curve (so you miss the tooth surface and just push floss through the contact point), and failing to go below the gumline (so you leave subgingival plaque in place). A controlled trial in patients with wide gingival embrasures tested a knotted-floss variation of the C-shape method and found it safe and effective when used correctly, with no soft-tissue trauma.⁵

If your gums bleed when you floss, that’s a sign of inflammation, not a sign you’re flossing too hard. Healthy gums do not bleed with proper technique. The bleeding should decrease over a few weeks of consistent flossing as inflammation resolves.

How to floss properly: type matters less than technique

Waxed, unwaxed, tape, ribbon, flavored, unflavored, expanding, gliding: the variety of floss types in the oral-care aisle implies they deliver different outcomes. The trials show they don’t. As long as the floss is thin enough to fit between your teeth and strong enough not to shred, the type makes almost no difference to plaque removal or gingival health. Technique is what matters.

A 2021 split-mouth trial compared conventional floss to floss with ellipsoidal knots (a textured variant marketed as more effective).⁸ Both types removed interdental plaque equally well. The knot design showed no added benefit. A 2022 randomized trial compared daily flossing to no interdental cleaning over six weeks.⁶ The floss group had lower plaque scores and healthier gums, confirming that floss works, but the trial did not test floss type, only the act of flossing itself.

The evidence on water flossers (oral irrigators) is more interesting. A 2026 systematic review and meta-analysis in orthodontic patients found water flossers reduced plaque and gingival inflammation as effectively as string floss.⁹ A 2024 trial in the same population confirmed the equivalence: water flossers and interdental floss both improved oral hygiene significantly, with no meaningful difference between them.¹⁰ For people with braces or other appliances that make string floss difficult to use, a water flosser is a reasonable alternative. For people with normal spacing, string floss and water flossers are clinically equivalent.

Interdental brushes are not interchangeable with floss. They work well in wide embrasures (the triangular spaces below the contact point) but cannot fit through the contact point itself, so they don’t clean the full interdental surface. A 2024 trial in patients with dental implants compared floss to interdental brushes.¹¹ Both reduced peri-implant inflammation, but interdental brushes were slightly more effective in spaces wide enough to accommodate them. A second 2024 trial tested floss, interdental brushes, and water flossers around single-implant crowns.¹² All three tools improved peri-implant health equally. The choice depends on your anatomy. If you have tight contacts, use floss. If you have open embrasures, interdental brushes or water flossers may be more comfortable and just as effective.

The practical takeaway is simple. Buy floss that fits between your teeth without shredding. Waxed floss slides more easily if your contacts are tight. Unwaxed floss may feel cleaner if you prefer it. The brand, flavor, and texture are personal preference. None of the trials found clinically meaningful differences. Focus on using it correctly, not on finding the optimal product.

What flossing prevents (and what it probably doesn’t)

Flossing prevents interdental plaque accumulation, gingivitis, and interproximal cavities when done consistently. The evidence for those outcomes is strong. The evidence that flossing prevents periodontitis or improves systemic health outcomes (cardiovascular disease, diabetes control) is weaker or absent.

Plaque removal is the direct effect. Flossing mechanically disrupts biofilm between teeth. A 2014 trial measured interproximal plaque before and after flossing and found a significant reduction that toothbrushing alone did not achieve.⁷ Gingivitis (gum inflammation) follows from plaque. When you remove interdental plaque consistently, gingival bleeding and swelling decrease. The 2022 trial cited earlier showed that six weeks of daily flossing reduced gingival index scores and bleeding on probing compared to no interdental cleaning.⁶

Cavities form where plaque sits long enough for bacterial acid to demineralize enamel. Interdental surfaces are high-risk sites. Flossing removes the plaque before cavities start. That claim is mechanistically sound and supported by observational data, but no long-term randomized trial has directly tested whether flossing reduces cavity incidence. The logistical and ethical barriers to such a trial are high. You cannot randomize people to never floss for years and measure cavity rates. The indirect evidence (flossing removes plaque, plaque causes cavities, therefore flossing prevents cavities) is strong enough that no professional dental organization questions the recommendation.

Periodontitis (the advanced stage of gum disease where bone loss occurs) is harder to prevent. Flossing alone does not reverse established periodontitis. A 2016 review noted that behavior change interventions, including flossing instruction, improve short-term periodontal outcomes but have limited long-term impact without ongoing professional support.¹⁴ The majority of adults do not floss consistently. A 2025 trial followed periodontally healthy individuals who received interdental cleaning tools and found that most stopped using them within months.¹⁵ Adherence is the limiting factor, not the tool itself.

The claims that flossing reduces heart disease risk or improves diabetes control are not supported by trials. Those claims appear in some dental-marketing materials because observational studies have found correlations between periodontal disease and systemic conditions. Correlation is not causation. No trial has shown that flossing produces systemic health benefits beyond the mouth. The honest answer is that flossing prevents gum inflammation and cavities between teeth. That’s enough reason to do it. The speculative systemic benefits are oversold.

Teaching kids to floss: what works

Children lack the manual dexterity to floss effectively until around age 10 or 11. Before that, parents need to do it for them. A 2025 trial tested three interdental cleaning tools in children (floss, water flosser, and interdental brush) as adjuncts to toothbrushing.¹³ All three reduced plaque and gingival inflammation compared to brushing alone, and the water flosser was the most effective. That finding makes sense for kids: a water flosser requires less fine motor control than threading string floss through tight contacts.

The behavioral challenge is not finding the right tool but building the habit. The 2016 review on behavior change noted that simply providing tools and instruction does not produce lasting adherence.¹⁴ What works better is setting a specific routine (floss at the same time every day, in the same place), reducing barriers (keep floss visible, pre-cut lengths if threading is hard), and providing feedback (praise when the child flosses, dentist reinforcement at checkups). Nagging does not work. External motivation fades. The goal is to make flossing automatic before the child reaches adolescence, when parental influence drops and the child has to sustain the behavior independently.

For younger children, floss picks (plastic handles with a short length of floss stretched between two prongs) are easier to manipulate than traditional floss. The picks don’t allow you to use fresh floss for each tooth, so they’re less hygienic, but they reduce the dexterity barrier enough that some children can use them independently earlier. The tradeoff is reasonable if it means the child flosses at all.

Teaching how to dental floss properly starts with demonstration. Show the C-shape method on your own teeth, then guide the child’s hands through the motion. Expect bleeding the first few times if the gums are inflamed. Explain that the bleeding will stop after a few weeks of consistent flossing. Most children quit when they see blood because they assume they’re doing it wrong. Framing bleeding as a temporary sign of healing rather than harm keeps them going.