Brushing is more important than flossing, but not by as much as you might think. A toothbrush only reaches about 60% of your tooth surfaces, leaving 40% essentially untouched. That uncleaned territory between your teeth is where gum disease and many cavities get started, which makes flossing far more than optional.
The real answer is that brushing and flossing do different jobs. Pitting them against each other is a bit like asking whether it’s more important to wash the front of your body or the back. But if you could only do one, brushing wins because it covers more surface area, delivers fluoride, and disrupts the bulk of plaque buildup.
What Brushing Does That Flossing Can’t
Brushing cleans the outer, inner, and chewing surfaces of every tooth. It also delivers fluoride from toothpaste directly onto enamel, which strengthens teeth and reverses early decay. Fluoride is the single most effective tool against cavities, and there’s no practical way to get it onto your teeth without brushing.
The chewing surfaces of your molars are the most cavity-prone spots in your mouth. Their deep grooves trap bacteria and food in ways that no amount of flossing can address. In studies of cavity distribution, the grooves on first and second molars consistently show the highest decay rates, and these are surfaces only a toothbrush (or dental sealant) can clean.
What Flossing Does That Brushing Can’t
The spaces between your teeth are essentially invisible to your toothbrush. Plaque sits undisturbed in those gaps, hardening into tarite and inflaming the gums. For every tooth except molars, the surfaces wedged between neighboring teeth show the highest cavity rates of any location, ranging from about 59% to 78% in studies of cavity distribution by tooth surface. These are cavities that brushing alone simply cannot prevent.
Flossing also has a measurable effect on gum disease. A large study in the Journal of Clinical Periodontology found that people who floss at least once a week have 23% lower odds of developing periodontitis compared to people who never floss. Interestingly, flossing two to four days a week provided similar protection to flossing five or more days, suggesting you don’t need to be perfect to get a real benefit.
The Long-Term Cost of Skipping Floss
The most striking data comes from older adults. A five-year study published in the Journal of Dental Research tracked tooth loss in people over 65 and found that non-flossers lost an average of about four teeth over that period, while flossers lost roughly one. That’s not a small difference. Flossers also had less gum disease, less attachment loss (the breakdown of tissue holding teeth in place), and fewer cavities on the visible parts of their teeth.
At baseline, non-flossers were already missing more teeth: 11.5 on average compared to 8.6 for flossers. While other factors like income, smoking, and dental visit frequency also play a role, the protective effect of flossing held up even after adjusting for those variables.
The Best Order: Floss First, Then Brush
If you do both, sequence matters. A clinical study found that flossing before brushing led to better fluoride retention on tooth surfaces. The logic is straightforward: flossing clears out the debris packed between teeth, so when you brush afterward, fluoride from your toothpaste can actually reach those freshly cleaned gaps. Brushing first, then flossing, pushes the fluoride away before it has a chance to absorb.
The American Dental Association recommends brushing twice a day and cleaning between your teeth once a day. They note that either order is acceptable as long as you’re thorough, but the evidence slightly favors the floss-first approach.
If You Hate String Floss, Alternatives Work
Traditional floss isn’t the only option. Water flossers, interdental brushes, and floss picks all clean between teeth effectively. A systematic review comparing water flossers to string floss found that water flossers actually performed better in most studies. One trial showed a 74% reduction in whole-mouth plaque with a water flosser compared to 58% with string floss. Four out of the included randomized controlled trials favored water flossers for plaque removal, particularly in hard-to-reach areas.
Interdental brushes (the tiny bottle-brush-shaped picks) are another solid choice, especially if you have gaps between your teeth or bridgework. The National Institute of Dental and Craniofacial Research notes that the goal is simply timely removal of plaque from between teeth, regardless of the specific tool you use.
What This Means in Practice
If you’re only going to do one thing, brush. It covers more ground, delivers fluoride, and handles the highest-risk surfaces on your molars. But treating flossing as optional means leaving 40% of your tooth surfaces to fend for themselves. Over years and decades, that neglect shows up as cavities between teeth, receding gums, and eventually tooth loss.
The practical takeaway is that brushing is the foundation, but flossing is what protects the parts of your mouth that brushing physically cannot reach. You don’t even need to floss every single day to see a benefit. A few times a week meaningfully lowers your risk of gum disease. If string floss feels like a chore, a water flosser or interdental brush gets the job done just as well, and possibly better.