How to Get Mildew Out of Shower Grout: Step-by-Step

To remove mildew from shower grout, apply a paste of baking soda and water directly to the affected grout lines, let it sit for 10 to 15 minutes, then scrub with a stiff grout brush and rinse thoroughly. For stubborn mildew that doesn’t respond to baking soda, apply 3 percent hydrogen peroxide, let it dwell for 15 minutes, scrub, and rinse.

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This works for surface mildew — the dark spots and discoloration that appear on grout in bathrooms with regular moisture exposure. If mildew has penetrated deep into grout or spread to walls and ceiling, professional treatment is more appropriate than DIY.

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Why Shower Grout Gets Mildew

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Grout is porous. Unlike the glazed surface of your shower tiles, grout absorbs moisture and organic material — soap residue, body oils, shampoo — creating the ideal environment for mildew to grow. Add warmth and poor ventilation, and you have the three conditions mildew needs: moisture, organic food, and warmth.

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In humid climates like Miami, where ambient humidity averages around 75 percent, bathroom grout faces mildew pressure year-round. There’s no dry season where bathrooms get a break. This is why mildew returns faster in coastal subtropical cities than in drier regions — even with regular cleaning.

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Step-by-Step Mildew Removal

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For light to moderate mildew (surface discoloration):

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- Mix baking soda with water to form a thick paste — roughly the consistency of toothpaste. Apply the paste directly to mildewed grout lines. You want it thick enough to adhere vertically. Let the paste sit for 10 to 15 minutes. This gives the baking soda time to penetrate the surface layer and loosen the mildew’s grip on the grout.

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- Scrub each grout line with a stiff brush. An old toothbrush works for small areas. For larger sections, a dedicated grout brush with stiffer bristles saves time and effort. Scrub along the grout line, not across it, to concentrate force where the mildew lives.

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- Rinse thoroughly with water. Check your results. For light mildew, one round of this treatment typically removes 80 to 90 percent of visible discoloration.

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For stubborn mildew (deep discoloration that survives baking soda):

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- Apply 3 percent hydrogen peroxide directly to the affected grout. You can spray it from a bottle or apply with a cloth. Let it dwell for 15 minutes — hydrogen peroxide needs contact time to work. Scrub with your grout brush and rinse.

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Hydrogen peroxide is an oxidizing agent that kills mildew at a deeper level than baking soda alone. It’s safe for most grout colors, though it can lighten colored grout unevenly — test a small inconspicuous area first if your grout isn’t white.

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What about bleach? Bleach kills surface mildew effectively but doesn’t penetrate porous grout deeply. It also lightens colored grout, damages surrounding surfaces over time, and produces fumes in enclosed bathrooms. Baking soda and hydrogen peroxide achieve similar results with fewer downsides.

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When DIY Won’t Work

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If mildew has penetrated the caulking around your tub or shower, cleaning may improve the appearance but won’t fully resolve the discoloration. Caulking is more porous than grout, and once mildew grows inside the material, the only real fix is removing the old caulking and applying new. Professional recaulking costs $100 to $200 and provides a fresh, mildew-free starting point.

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If mildew covers large areas, returns within days of cleaning, or appears fuzzy or black (rather than the typical gray or pink surface mildew), you may be dealing with a more serious mold issue that benefits from professional assessment.

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Preventing Mildew From Coming Back

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Removal without prevention guarantees you’ll repeat the process monthly. In humid climates, prevention requires active management because the environment works against you.

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- Run your exhaust fan during every shower and for 20 to 30 minutes after. This is the single most impactful prevention measure. The fan removes moisture from the air before it settles on grout. If your fan seems weak, the cover probably needs cleaning — dust buildup reduces airflow significantly.

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- Squeegee shower walls after each use. Thirty seconds of squeegeeing removes the standing water that mildew needs. This habit alone cuts mildew recurrence dramatically.

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- Leave the bathroom door open when not in use. A closed, dark, humid bathroom is a mildew incubator. Open doors allow air circulation that keeps surfaces drier.

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- Address standing water. Anywhere water pools — shower floor, soap dish, around the faucet base — is a mildew invitation. Wipe these dry when you notice them.

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- Keep grout sealed. Sealed grout resists moisture penetration, reducing mildew’s ability to take hold. In humid environments, grout should be resealed annually, or as recommended after professional grout cleaning.

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The Professional Option

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If you’re cleaning mildew monthly and it returns at the same pace, DIY alone isn’t solving the underlying problem. Professional tile and grout cleaning uses specialized equipment that reaches deeper than consumer products — rotary scrubbers, high-pressure extraction, and commercial-grade cleaning solutions that break bonds between mildew and grout at a level baking soda can’t match.

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Professional grout cleaning followed by sealing provides 12 to 18 months of protection in humid climates, compared to the two to four weeks that typical DIY cleaning lasts before mildew returns.

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Regular professional bathroom cleaning — weekly or biweekly — as part of recurring house cleaning keeps mildew from establishing in the first place by catching early growth before it becomes visible.