Can You Use Bleach on Grout?

When bleach is fine on grout, and when it backfires

Diluted bleach can remove mildew from white or gray cement grout next to glazed tile, but it is a poor choice for colored grout, epoxy grout and natural stone. Cement grout is porous and traps moisture and soap scum, which is why it darkens and grows mildew. Bleach lightens the surface stain, but it does not fix the moisture that caused it, so black spots often return within weeks if the grout is left damp.

For a mildew spot, the CDC says to use no more than 1 cup of household bleach per gallon of water on hard surfaces, while Clorox's own mildew directions use its 1/3 cup per gallon disinfecting strength with up to 10 minutes of contact. Mapei says 1 cup per gallon is usually sufficient for disinfecting grout, but only when absolutely necessary. Apply with a brush or spray bottle, keep it damp, scrub and rinse (our suggested double rinse). Wear gloves and eye protection, and open a window or run a fan; the CDC recommends fresh air and goggles.

Tile Council of North America (TCNA) advises alkaline cleaners over acid ones for cement grout, since cement is alkaline and is dissolved by acids, and says using acid to remove stains is not recommended by grout manufacturers. Mapei warns that regular bleach use can discolor and weaken grout, so colored and dark grout deserve a hidden-spot test. TCNA calls epoxy grout virtually as stain proof as the tile, and LATICRETE cleans it with neutral pH detergent, so it seldom needs bleach. Natural stone like marble is on Clorox's do-not-use list, so keep bleach and runoff off it.

Oxygen bleach (sodium percarbonate) is a milder alternative many people use: mix it per its label, let it sit and scrub with a stiff nylon brush; the soak time here is our suggestion, so follow the product. TCNA says sealer is generally a very good idea for cement grout, so after cleaning, let the grout dry completely and seal it.

The biggest danger is mixing. Many tile and grout products are acidic, and bleach mixed with them or with vinegar or ammonia releases toxic gas. Rinse away one product completely before using another. If a mix happens, leave the room, get fresh air and call Poison Help at 1-800-222-1222.

Where these numbers come from

The formulas and default values in this calculator are based on:

• Tile Council of North America (TCNA), Cleaning Grout FAQ
• Mapei, Can I use bleach on my grout
• LATICRETE, Epoxy Grouts FAQ
• CDC, You Can Control Mold and mold cleanup guidance
• Clorox, Bleach Dilution Ratio Chart
• Natural Stone Institute care page
• Transport Canada CANUTEC, improper mixing of household cleaning products
• America's Poison Centers. Note: oxygen-bleach timing and rinse steps are our judgment
• confirm with your grout maker

This is general household-cleaning guidance and planning math, not medical or professional advice. Product formulas change, so the label on your bottle always wins: follow its directions and warnings, test a hidden spot first, and never mix cleaning products. For a poisoning emergency call Poison Help at 1-800-222-1222 (US), or 911 if someone has trouble breathing.