How to Clean Grout and Get It White Again

The short version: most grout goes grey because of trapped soap scum and dirt, not because the grout itself is failing, and a baking soda and hydrogen peroxide paste left on for 15 minutes will fix 80 percent of it. The other 20 percent, the stained, cracked or mouldy stuff, needs resealing or regrouting, not another scrub.

I am not a cleaning expert. I am someone who has spent a stupid number of hours on her knees with a toothbrush because I could not justify paying someone else to do it, and I got curious enough to work out what makes a difference and what is just internet noise. So this is the practical version, not the Pinterest version.

Why grout turns grey or yellow in the first place

Grout is porous. That is the whole problem in one sentence. It is basically sand, cement and water mixed into a paste, and once it dries it still has tiny gaps that soak up whatever gets dropped on it: soap residue, shower gel, cooking oil, dust that sticks to damp floors. Over months that build-up darkens and you end up with grout lines that look grey even though you have mopped the floor a hundred times.

The bit nobody tells you is that mopping often makes it worse, not better, if you are using a dirty mop or too much soap. You are not removing the grime, you are pushing diluted grime into the porous grout and then letting it dry there. So the floor looks clean for an hour and then the grout goes dull again. That is why people end up thinking their grout is just permanently stained when they have been sealing dirt into it for years.

What you need before you start

• Baking soda (about 200g for a standard bathroom floor)
• Hydrogen peroxide, 3 percent, the kind sold in pharmacies for cuts
• A stiff grout brush or an old electric toothbrush
• White vinegar
• A spray bottle
• Rubber gloves
• A microfibre cloth

Skip the bleach pens and the branded grout whitening markers for now. They cover the stain with white paint rather than clean the grout, which looks fine for about three weeks and then flakes off looking patchy and worse than before you started.

The method that works

Here is a worked example so you can picture the timings. Say you run a small bathroom, roughly 2 metres by 2 metres, with white tiles and grout that has gone grey around the shower and near the toilet.

1. Step 1, sweep or hoover first. Loose dirt turns into sludge the second it gets wet, and sludge just gets reground into the grout lines. Thirty seconds with a hoover saves you ten minutes of extra scrubbing later.
2. Step 2, mix your paste. Combine baking soda with just enough hydrogen peroxide to make a thick paste, roughly the texture of toothpaste. For a 2m by 2m floor you will use about 150 to 200g of baking soda.
3. Step 3, apply it directly to the grout lines, not the tiles. Use an old spoon or a piping bag if you want to be tidy about it. Leave it for 15 minutes. Not two minutes, not an hour. Fifteen minutes is long enough for the peroxide to break down the trapped grime without drying rock hard and becoming annoying to remove.
4. Step 4, scrub. Use the stiff brush in small circular motions, then go over each grout line in one straight pass. This two-direction scrub lifts more dirt than scrubbing the same way twice.
5. Step 5, rinse with warm water and a cloth, going over each section twice so you are not just moving paste residue around.
6. Step 6, for anything still dull, spray neat white vinegar directly on the line, leave for 5 minutes, then scrub again. Do not mix vinegar and hydrogen peroxide together at the same time, use them one after the other with a full rinse in between, because combining them can create a corrosive compound called peracetic acid.

For that 2m by 2m bathroom, the whole job takes about 45 minutes including drying time. A full kitchen floor will take closer to 90 minutes because you are usually dealing with more grease.

The stains that this will not fix

This is the part most grout articles avoid saying plainly: if your grout is black with mould rather than grey with dirt, scrubbing harder will not solve it. Mould grows inside the porous grout, not just on top of it, so a surface clean removes the visible colour for a week or two and then it comes straight back, often darker than before. If you are seeing black spotting that returns within days of cleaning, that is mould in the structure of the grout, and the only real fixes are a real mould-killing product left on for the full time stated on the bottle, or digging out and replacing that section of grout entirely. There is no scrub that permanently removes mould that has colonised porous material. Anyone telling you a magic spray will sort it forever is not being straight with you.

Similarly, if your grout is crumbling, cracked, or coming away from the tile edge, cleaning it will not make it structurally sound again. At that point you are looking at raking out the old grout with a grout saw and reapplying, which is a weekend job, not a Tuesday evening one.

Keeping it white once you have done the work

This is where the real payoff is, because nobody wants to repeat a 45 minute scrub every month.

• Seal it. A penetrating grout sealer, applied with a small brush or sponge applicator, soaks into the porous grout and stops liquids and soap scum getting trapped in the first place. It takes about 20 minutes to apply to a bathroom floor and needs to cure for 24 hours before the area gets wet again. Reapply once a year in a bathroom, once every two years on a kitchen floor.
• Squeegee the shower after use. Thirty seconds with a squeegee removes most of the soap scum before it dries onto the grout. This single habit does more than any cleaning product.
• Use less soap, not more. Soap residue is the single biggest cause of grey grout in bathrooms. Switching to a lower-sudsing body wash or rinsing the shower walls after each use cuts build-up dramatically.
• Wipe kitchen floor spills immediately rather than mopping once a week, because oil and grout react fast.

If you want a number to hold onto: a sealed, well maintained floor needs a deep grout clean maybe twice a year. An unsealed floor in a busy family bathroom can need it every six to eight weeks, which is exactly why so many people assume their grout is beyond saving when really it was never protected in the first place.

Frequently asked questions

Does bleach damage grout?

Used occasionally and diluted, bleach will not ruin grout, but repeated undiluted use breaks down the surface over time and makes it more porous, which means it gets dirty faster afterward. Hydrogen peroxide and baking soda do the same whitening job with less long-term damage and no harsh fumes.

Why does my grout go dark again a few days after cleaning?

If it darkens within days, the cause is usually mould rather than dirt, since mould regrows from spores still living inside the porous grout. A surface clean will not remove it permanently, and persistent black regrowth is a sign the grout needs a real mould treatment or replacing in that section.

Can I use a steam cleaner on grout?

Yes, and it is one of the more effective methods because the heat loosens grime without chemicals, but it works best as a first pass before the baking soda and peroxide step, not as a replacement for it, since steam alone often cannot lift years of trapped soap scum.

How often should grout be resealed?

A bathroom floor or shower should be resealed roughly once a year because of constant water exposure, while a kitchen floor can usually go two years between reseals. Resealing takes about 20 minutes per room and is the single biggest factor in whether grout stays white between cleans.