How to Use Muriatic Acid to Clean a Toilet

I describe how to dilute muriatic acid to create a milder acid that will dissolve lime, scale, and so-called “rust” in a toilet bowl. You will need a bucket, a brush, and some patience.

Before getting into the “how” part, let’s look at the broader topic of what’s in the stain, and what alternative solutions exist.

What is “Rust” in the Toilet Bowl

The first fact: it’s not rust. It’s mostly calcium.

People call the calcium deposits that form from water in the toilet (and on the faucet) “lime”. Lime is a calcium containing mineral, so people call the calcium stains “lime”.

They refer to the streaky or splotchy brown or yellow stains in the toilet “rust”, but it’s more likely that is a calcium depots with small bits of urine and feces in them.

Acid Softens Calcium Lime Deposits

Lime is generally alkaline or basic, and adding an acid causes a reaction. The end result of the reaction are salts, and they can be scrubbed away, revealing an un-reacted layer of lime.

The main acids used are: acetic acid, phosphoric acid, and hydrochloric acid.

Their more common names are: vinegar, cola, and muriatic acid for pools.

Vinegar barely does the job. Just barely. The typical acid sold at stores isn’t strong enough.

I would strongly recommend trying out phosphoric acid before going to muriatic acid. Two popular products that are phosophoric acid cleaners are Lime-A-Way and CLR (aka, Calcium Lime Rust).

You can also use cola. Just let it sit overnight. Cola contains phosphoric acid.

You may notice that the cleaner products smell like cola.

If they don’t work, you need to get the “big guns”. In this case, it’s Muriatic Acid, sold in the garden section of big box hardware stores, over near the pool supplies. Muriatic acid is used to maintain pool pH so it’s always slightly acid.

Diluting Muriatic Acid to Clean the Toilet

Be SAFE: wear rubber gloves, and wear safety glasses. This stuff will burn your skin. If you get hit with some acid, use baking soda in water to neutralize it. Also, this stuff will damage and put holes in your clothes, so wear old work clothes.

You want to mix the bottled acid 5 to 1, with 5 parts water, and 1 part muriatic acid.

Muriatic acid is sold in a 10% concentration, so once you’ve diluted it, it’s 1/5 as strong, or 2%.

You further end up diluting it in the bowl, because there’s around a third of a gallon of water in there.

Knowing this, you can adjust your acidity up or down as needed.

To dilute the acid you need a plastic bucket. The 3 gallon size is good.

Pour in around 1/2 inch or 1 cm of acid. Then add water until it gets up to 2 inches. Due to bumps in the bottom of the bucket, there’s less liquid in the bucket than it appears.

Swirl the bucket gently. The bumps on the bottom will help mix the acid. The acid is heavier than water, by the way.

Cleaning With Acid

Pour a little of the diluted acid into the bowl. If you see the lime bubble, you’re doing well!

You should slowly pour the diluted acid into the bowl, allowing it to coat the sides of the bowl.

Let it sit for a half hour, and then go and give it a light scrub with the brush. The water should cloud up a bit. What’s happening? The scale has reacted with the acid and turned soft. You’re scraping off that layer of soft material, exposing the next layer to the acid bath.

The acid bath is getting weaker, as well.

You might want to rinse the brush off in the sink, because it’ll start to rust.

Repeat this process for a few hours. Just check in every half hour or so, and give it a light scrub.

Eventually, you will either have a clean toilet bowl, or the acid will be exhausted and stop working. Either way, flush it down.

If you still have scale deposits, repeat the process.

Getting Under the Rim

The area under the rim also has deposits, but you can’t see them. To clean those off, I like to just dip the brush in the acidified bowl, and lay some acid onto the rim.

Again, you have to wait a bit for the magic to happen. Then flush.

What if it Doesn’t Work?

If the 2% solution above doesn’t work, or takes way too long, you will need to use a stronger concentration of acid.

I’ve used everything from 10% out of the bottle on down. I prefer to dilute to at least 2:1, because it helps the acid mix with the bowl water. Otherwise, the acid from the bottle is so dense that it sinks to the bottom of the bowl.