Clean Mildew from Shower Caulk

Materials

• 1 cup Household bleach, 5-6% sodium hypochlorite (Regular unscented Clorox or store brand works fine — avoid splash-free formulas which are too thick) — $3 - $5
• 1 bottle (16 oz) Hydrogen peroxide, 3% solution (Brown bottle from pharmacy section — the clear bottles let light degrade the peroxide) — $2 - $3
• 1 cup White distilled vinegar (5% acidity standard grocery store vinegar, used for prevention not primary cleaning) — $2 - $3
• 1 roll Paper towels (For the compress technique on stubborn stains, not just wiping) — $1 - $2

Step by step

1. Prep · 10 minutes

   Dry Before You Treat

   Wipe down all caulk lines with a dry towel and let the shower air out for two hours with the door open and exhaust fan running. Mildew treatment works best on dry surfaces because water dilutes your cleaning solution and prevents it from penetrating the silicone. If you're in a hurry, use a hair dryer on low heat to speed the drying.

   Tip: Morning is best for this job — the shower's been dry all night and you won't need it again until evening.
2. Treatment · 15 minutes

   Spray and Saturate

   Mix one part household bleach with three parts water in a spray bottle. Spray directly onto the mildewed caulk until it's thoroughly wet, then let it sit for fifteen minutes. For stubborn stains, lay bleach-soaked paper towels directly on the caulk line to keep the solution in contact longer. Work with the bathroom door open and exhaust fan on.

   Tip: Don't mix bleach with any other cleaner — especially not ammonia-based products, which create toxic fumes.
3. Scrubbing · 10 minutes

   Brush Out the Stain

   Use an old toothbrush or dedicated grout brush to scrub along the caulk line in short, firm strokes. Focus on the edges where the caulk meets the tile or tub, where mildew digs in deepest. You're not trying to remove the caulk itself, just the top layer where the staining lives. Re-spray with bleach solution as needed to keep the area wet while you work.

   Tip: A toothbrush works better than a bigger brush here because it concentrates pressure on the narrow caulk line.
4. Rinsing · 5 minutes

   Flush All Residue

   Run hot water over all treated areas for at least two minutes, making sure no bleach residue remains. Use your hand to feel along the caulk line — if it's slippery, there's still cleaner present. Residual bleach will continue breaking down the silicone after you're done, which weakens the seal over time.

   Tip: Hot water opens the silicone pores slightly and helps flush out loosened mildew debris.
5. Secondary treatment · 30 minutes

   Go Deeper If Needed

   For stains that survived bleach treatment, spray undiluted 3% hydrogen peroxide directly on the spot and let it sit for thirty minutes. Hydrogen peroxide works more slowly than bleach but penetrates deeper into silicone without the harsh fumes. This is particularly effective on orange or pink mildew that resists chlorine bleach.

   Tip: Hydrogen peroxide loses potency in clear bottles — keep it in the original brown bottle and it stays active longer.
6. Final check · 10 minutes

   Inspect for Damage

   Wipe down the caulk with a clean towel and run the exhaust fan for an hour. Check the caulk lines in bright light — look for any lifting, cracking, or gaps that might have been hidden under the mildew. If the caulk itself is damaged or if black staining remains after two rounds of treatment, the mildew has penetrated too deep and the caulk needs replacement.

   Tip: If you can scrape away the top layer of caulk with your fingernail, it's degraded beyond cleaning.
7. Prevention · 5 minutes

   Lock in Your Win

   Spray the cleaned caulk with a light mist of white vinegar and let it air dry — the acidic environment discourages new mildew growth. Going forward, squeegee the shower walls after every use and run the exhaust fan for twenty minutes post-shower. Monthly quick-cleaning with hydrogen peroxide keeps mildew from re-establishing.

   Tip: A $12 squeegee eliminates ninety percent of the moisture that feeds mildew — it's the single most effective prevention tool.

Field Notes

• Carmen V. — I stopped using bleach entirely after discovering that hydrogen peroxide works just as well without the fumes. I keep a spray bottle of it in the shower and hit the caulk lines every Sunday morning while I'm waiting for my coffee to brew. Five seconds of spraying, walk away, rinse it when I shower that evening. The mildew never gets established enough to turn black anymore. — Oct 14 — Portland, OR
• James K. — The paper towel compress technique is the only thing that worked on the bottom caulk line where the tub meets the tile surround. Regular spraying just ran off too quickly. I soaked paper towels in straight bleach, pressed them into the caulk line, and left them for an hour. When I peeled them off, the mildew came with them. The caulk looked brand new. — Mar 22 — Atlanta, GA
• Rita S. — In humid climates, you're fighting a losing battle if you don't address the root cause. I installed a humidity-sensing exhaust fan that runs automatically whenever the bathroom hits sixty-five percent humidity. Cost me $85 and an hour of installation, but I haven't had to clean mildew in eighteen months. The caulk stays white without any maintenance beyond normal shower cleaning. — Jul 8 — Miami, FL