Remineralizing means adding calcium and magnesium back to water that has had almost everything stripped out of it, usually by reverse osmosis or a distiller. People do it for taste and for the sake of the plumbing, since very low mineral water tastes flat and is mildly aggressive toward metal fittings. There are three practical ways to do it.
Why RO and distilled water taste flat
The USGS describes hardness as dissolved calcium and magnesium, classified from soft at 0 to 60 mg/L as calcium carbonate up through very hard above 180. What most people call good tasting water sits somewhere in the middle of that range, because those same minerals carry much of the flavor. Reverse osmosis and distillation remove nearly all of them along with the contaminants, which is exactly what they are for, and the flat, slightly empty taste that results is the trade. Remineralizing puts back a modest amount of the mineral content without putting back the things you paid to remove.
An inline remineralizing cartridge
This is the usual answer on a household RO system. A small cartridge filled with calcite or a calcium and magnesium blend goes after the membrane, and often after the storage tank so contact time is consistent. Treated water passes through the media, dissolves a little of it, and comes out with a modest mineral content and a slightly higher pH. It is passive, it needs no measuring, and the only maintenance is replacing the cartridge when the media has dissolved away, which is why the housing is often clear. The downside is a little extra pressure loss and one more thing on the change schedule. Fitted after an RO unit it is the least fussy option by a distance.
Mineral drops and a pinch of salt
Trace mineral concentrates are sold as drops that you add to a pitcher or a bottle, and they work: a few drops per container restores taste noticeably. They suit people who drink RO water from a jug rather than plumbing a cartridge, and they are the only sensible option for distilled water made in batches. The kitchen version is a very small quantity of unrefined sea salt or a magnesium-rich mineral salt, which mostly changes taste rather than restoring calcium meaningfully. Both approaches share the same weakness: they depend on somebody doing it every time, and they treat the water in the jug rather than the water in the ice maker, the coffee machine or anything plumbed in.
Blending, and whether you need to bother
The third method is blending: letting a controlled small fraction of filtered but unstripped water rejoin the RO permeate. It is common in commercial systems and can be done at home where the plumbing allows, but it is fiddly, and it only makes sense if the source water is safe to drink in the first place. That points at the real question. If your reason for RO was a health contaminant, blending raw water back in undoes part of what you bought. If your reason was taste, hardness or a dislike of chlorine, then remineralizing is purely a flavor decision and a cartridge does it cleanly. Note that remineralized water is mildly harder than it was, though at levels well below what would justify a softener.
Questions people ask about how to remineralize water
Is reverse osmosis water unhealthy because it has no minerals?
Diet supplies the great bulk of dietary calcium and magnesium, not drinking water. Remineralizing is mainly a taste and plumbing decision rather than a health one.
What is the easiest way to remineralize water at home?
An inline remineralizing cartridge after the RO membrane. It is passive, it treats everything the system delivers, and the only upkeep is replacing the media.
Will remineralizing make my water hard again?
It raises mineral content slightly, but nowhere near the levels that cause scaling problems, so it does not recreate the hardness a softener was fitted to remove.
Can I remineralize distilled water the same way?
Yes, though distillers usually make water in batches, so mineral drops added to the container are more practical than an inline cartridge.