Grout has a way of holding onto every year of neglect. Even bathrooms that get scrubbed regularly end up with lines that look gray no matter how much bleach spray gets thrown at them. The problem isn't effort - it's that most store-bought cleaners sit on the surface instead of breaking down what's actually trapped inside the porous grout lines: soap scum, mineral buildup, and mildew that's worked its way below the top layer.
The fix that keeps showing up in cleaning circles, and for good reason, is a lot simpler than anything on a supermarket shelf: baking soda and hydrogen peroxide.
Why this combination works
Baking soda is mildly abrasive and alkaline, which helps lift dirt without scratching the grout. Hydrogen peroxide is a mild oxidizer - it breaks down organic stains and mildew at a chemical level rather than just masking them. Mixed together, they form a paste that gets into the texture of the grout instead of sliding off it, and it keeps working baking soda and hydrogen peroxide grout cleaner for several minutes after it's applied, which is exactly what grout needs since the grime sits below the surface.
Unlike vinegar-based methods, this combination won't damage grout sealant over time, so it's a safer long-term habit for anyone maintaining tiled floors or a shower.
The basic method
Mix baking soda with just enough hydrogen peroxide to form a thick paste - roughly a 2:1 ratio. Apply it directly onto the grout lines with an old toothbrush or grout brush, focusing on lines that look visibly discolored. Let it sit for 10 to 15 minutes so the peroxide has time to break down the stain, then scrub in small circular motions before rinsing with warm water.
For grout that hasn't been cleaned in years, a second application usually finishes the job - this method works gradually rather than instantly, which is part of why it's gentler on the grout itself.
A few things that make a real difference
Warm water in the paste speeds up the reaction slightly. Working in small sections keeps the paste from drying out before scrubbing. And doing a final rinse with a microfiber cloth instead of letting the residue air-dry prevents the powdery baking soda film that sometimes gets left behind.
For anyone who wants the exact ratios, sealant-safe warnings, and a version adapted for colored grout, SimplyGreenHome put together a more detailed breakdown of this method, including what to do when grout is severely stained rather than just dull: this baking soda grout cleaner method.