How to remove the hard water ring in a toilet bowl?

Try using a pumice stone

There is a product called Iron Out. Sprinkle some of that in your toilet bowl and let it sit and do it's job. Then brush the inside of the bowl with your toilet brush. Flush and you're done. Clean as a whistle. Just be careful not to breathe in the fine powder.

I may have a few thoughts on this, but first, the color of the ring matters..i.e. rust will appear as a rust color...black will designate mold in your home...off white will indicate calcium...the solutions are different...

Pour 1/3 box of baking powder in the toilet. Squeeze some dish washing liquid in the toilet. Pour some vinegar in the toilet. Let it foam and sit for about an hour. Swish around with a toilet brush and flush.

Bar keepers friend works great on my well water stains.

You can find it in the laundry aisle at your friendly neighborhood grocery store😉

If it's from hard water use plain white vinegar. Try not to get it diluted with the water. Let it sit for a while then brush.

Sorry, Baking Soda.

Tturn water off, flush out upper tank to empty, use toilet brush, in a plumger motion, to get out as much water out of bowl. Pour in Cleaning Vinegar into bowl and tank, could take several gallons, it is 20% acidity, verses 7% for food grade. It is available at most major stores now. This is for first cleaning only, maintenance is easier to keep up. The vinegar will disinfect, deodorize, dissolve all the minerals in the bowl and tank. Make a scrubbing paste from Dawn & baking soda for really stubborn areas, using a green Scotch brand scrubby pad, they are green. Maintenance cleaning can be done either with regular vinegar or Scrub Free. I use them both in the bathrooms, vinegar nearly everywhere else.

Does "Iron out" or any of these suggestions do damage to the pipe lines?

DENTURE CLEANING TABLETS. Just drop one or two in the bowl & let'em clean it for ya. Then, just flush. :)

Go to local commercial janitorial supplier & ask for toilet ring remover and purchase the fabric brush. This is an acid so be careful using it! follow directions exactly. Wear eye protection & rubber gloves, force flush toilet by pouring a bucket of water into bowl, add cleaner to brush and wipe under rim & in bowl several times to remove ring and stains. Once stain is gone, flush several times and rinse fabric brush well. BE VERY CAREFUL NOT TO GET ON CERAMIC FLOOR OR MIRRORS.....it will "etch" either. If you leave it to long in toilet, it will wear away at porcelain . But I have used this for over 20 yrs and my toilets look brand new! toilet ring remover may be used on fiberglass or hard plastic tubs, sinks, shower stalls & glass doors----rinse well! Pumice stone will ruin scratch the porcelain off and make cleaning seem useless. between major cleanings, I use housekeeper #1 (also purchased at commercial supplier) weekly and Clorox or Lysol toilet bowl cleaner as needed.

..Easy, CLR...removes Calcium, Lime and Rust..safe...if mold develops this tells me there is a few things going on...mold has microscopic spores..so if it is in the ring around the bowl, and can be seen, it is a colony..it is in other damp places as well..

I will leave that with you..but initially pour 4 cups of industrial vinegar into the water container that deposits the water into the bowl..leave without flushing the toilet for a few hours...then flush twice..then do the same in the actually toilet bowl...again leave for a few hours and flush away...hopefully this will get rid of any bowl mold..if there is mold on any grout in the bathroom, brush with pure vinegar and an old tooth brush...gently...make sure the grout is always wiped dry, open the window after a bath or shower to move the air about...mold has two ways of replicating....airborne spores, and a network of microscopic "roots"...sort of..sorry for the details on mold, but this impacts allergies if you have them, and can create them if you don't..not to mention the off smell hidden mold can give off.....the source of calcium build up is due to your water resource...if there is that much calcium in the water to build a mineral deposit around your toilet bowl..what is it doing to your body??

I actually know..and can share in another comment..then if there is Rust in the bowl you will see rust in the water tank...don't try to chlorine this away, or use abrasive brushes, pastes, etc. as can actually ruin the enamel surface of the bowl allowing microscopic minerals to lodge..build up and then show themselves...I get it that we can't flush all the time, water is precious..but believe it or not, a packet of powdered lemon juice mix added after the last flush of the evening, will have you surprised in the morning..the citric acid in levels that are safe for us to drink are safe in the bowl, the sewage system....rusty water is usually well water, with a high iron content..and actually it is good water..for cooking, and ingesting...have a good day...and I apologize if you have found me too wordy...as some apparently do....

Try a product called 'Scouring Stick'. It's pumice.