1. How Much Seawater Do You Need to Get One Kilogram of Salt?
Seawater is salty, but not very salty. NOAA, the US ocean agency, gives the average as about 35 parts per thousand, which means that about 3.5 percent of the weight of seawater is dissolved salts [1].
Try some arithmetic. If 1 kilogram of seawater holds about 35 grams of dissolved salts, then 1,000 ÷ 35 is about 28.6. To collect a kilogram of dissolved salts you would need to handle about 29 kilograms of seawater, which is nearly fifteen 2-liter bottles of soft drink. Almost all of that is plain water that must be boiled or dried away. This is a rough guide, not an exact recipe: the 35 grams are all the dissolved salts together, and table salt (sodium chloride) is only the biggest part of them, so the real yield of table salt is a little smaller.
So the hard part of making salt is not finding salt. It is getting rid of the water. Every method in this article is a different answer to that problem.
2. The Main Idea: Make the Water Saltier First, Then Finish the Job
In sunny, dry places, the first step can be done by simply waiting. The Tobacco and Salt Museum in Tokyo describes the usual world pattern: salt comes from rock salt, from salt lakes, or from seawater, and where the climate allows it, sea salt is made by letting seawater evaporate in huge shallow ponds under the sun. Mexico and Australia are named as examples [2]. A researcher at Japan's Salt Industry Center writes that rock salt and this sun-dried salt together make up about 99 percent of the world's salt production, while Japan accounts for less than 0.5 percent [3]. Solar salt is still used in the United States as well: the US Geological Survey estimates that about 10 percent of the salt sold or used there in 2023 was solar salt [7].
Japan is a different story. The islands are surrounded by sea, but they are humid and rainy, and the same museum article says this climate does not suit making salt by sunshine alone [2]. Japanese salt makers therefore worked in two stages. In the first stage, called saikan in Japanese, they turned seawater into a much saltier liquid called kansui ("strong brine"). In the second stage, called sengo, they boiled that brine until salt crystals formed [2][4].
Why does the first stage matter? A rough example with round numbers shows it. A kilogram of dissolved salts needs about 29 kilograms of seawater, so about 28 kilograms of water must be boiled off. In a brine that is 20 percent salt (the figure we meet again in section 6), the same kilogram sits in about 5 kilograms of liquid, so only about 4 kilograms of water are left to boil. This is only an illustration, not a number from the sources, but it shows why the first stage is worth the effort.
Next, how did people do that first stage when all they had was seaweed, sand, and sunshine?
3. How Did Ancient People Make Strong Brine With Seaweed and Clay Pots?
An exhibition document from the Ibaraki Prefectural Museum of History says that salt making in Japan began in earnest about 3,200 to 3,300 years ago, at the end of the Jomon period, and that traces of it have been found at the Hirohata shell mound near Lake Kasumigaura, east of Tokyo [5]. People boiled brine in special thin clay pots called salt-making pots [5]. The Okayama Prefecture ancient-culture center explains that salt crystals soaking into the clay break the pots into small pieces, which is why archaeologists find so many broken fragments in dump heaps [4].
How they made the brine in the first place is much less clear. Old Japanese poems and records use phrases such as "pouring seaweed salt" and "burning seaweed salt." Okayama's archaeologists read the first as pouring seawater over seaweed and letting it dry, and the second as pouring seawater over the salty ash of burned seaweed. They also say that only a few real examples of ancient brine-making have been found, so a lot remains unknown [4]. A salt-industry researcher describes one method from the 6th and 7th centuries: dry seaweed was soaked in seawater, dried in the sun, and soaked again, over and over, to collect strong brine, which was then boiled in clay pots [3].
The sources differ on the details, so it is safest to say that the seaweed methods were not one single recipe. What they share is the two-stage pattern: get the water saltier using sun, wind, and seaweed, then boil. The same researcher notes that these methods made it hard to collect the roughly 3 percent of salt in seawater [3]. They depended on the weather and on how much seaweed was available. Could the beach itself do the work?
4. How Did Sand-Covered Salt Fields Concentrate Seawater by Sunshine and Tides?
Later Japanese salt makers did use the beach. The Tobacco and Salt Museum describes two kinds of salt field. In the ageham type, people carried seawater up the beach by hand and spread it over the sand. In the iriham type, the rise and fall of the tide brought seawater in. Sunshine and wind then evaporated the water at the sand's surface, leaving salt behind. Workers scraped up the salty sand and poured seawater through it to get strong brine, which was then boiled [3]. The museum article says the tide-fed type appeared in the early Edo period and, until about 1955, supported Japanese salt making for roughly 300 years, mostly around the Seto Inland Sea [2]. The hand-carried type is still used today on the Noto Peninsula [2].
The Okayama center adds a reason the Seto Inland Sea suited the tide-fed type: in some places its tides differ by about 2 meters between low and high water, which let seawater flow into the fields by itself [4]. The rising and falling tide let seawater flow into the fields by itself, and the sun and wind did the drying. But sun and wind still cannot beat a week of rain.
5. How Did Salt Makers Get Around the Rain, and Then the Weather Altogether?
In the 1950s Japan introduced a new kind of salt field. The Salt Industry Center's researcher describes it as a sloped, clay-and-gravel bed plus a tall frame covered with hanging bamboo twigs. Pumped seawater trickles over the bed and down the twigs, where sun and wind take the water away [3]. Spreading seawater over a huge number of twigs exposes a large surface to the air, so it dries faster. That explanation is a plain-language picture, not a quote from the source.
Still, a salt field depends on the weather and needs a lot of land and workers. So in 1955 Japan's state salt monopoly started developing an electrical method using ion-exchange membranes, and in 1972 seven factories, each able to produce 200,000 tons of salt a year, were running at full scale [3]. This method uses sheets called ion-exchange membranes. Seawater contains dissolved salt as electrically charged particles called ions. When ion membranes of two types are stacked alternately and a voltage is applied, positive ions (such as sodium) and negative ions (such as chloride) move in opposite directions and collect in alternate chambers. The result is a strong brine, without waiting for the sun [3].
The idea has changed from evaporate the water away to move the salt together. The brine is still boiled afterward, now in large vacuum evaporators [2]. The researcher reports that between 1945 (the tide-fed field era) and 1985, the amount of salt produced per worker rose by more than 100 times [3]. One thing stayed the same through every era: first make the water saltier, then crystallize it.
6. How Salty Can a Modern Concentrator Make the Water?
The Okinawa Prefectural Industrial Technology Center built a small seawater concentrator that uses ion-exchange membranes. It says that traditional salt fields and boiling are affected by the weather and use a lot of energy. Its document lists the machine's abilities as brine of about 20 percent salt (plus or minus 3 percent) and about 15 liters of brine per hour [6]. Compare 20 percent to seawater's 3.5 percent: 20 ÷ 3.5 is about 5.7, so the brine is roughly six times as concentrated. This is the number for one test machine, not for salt factories in general. The same document says that many Okinawan makers boil seawater directly and make a salt that keeps other minerals such as magnesium, so this machine was designed to give a brine with a similar mix, unlike the purer brine of large Japanese plants [6].
That is the answer to the question in the title. You do not boil away the whole ocean. Whether a salt maker uses sun, seaweed, sand, or electricity, the water is mostly removed before the final step.
7. How Can You Watch the Water Disappear and the Salt Appear at Home?
You can see the idea on a small scale, with no stove. Ask an adult to help. Mix about 3 to 4 grams of table salt into 100 milliliters of water, roughly seawater strength. Pour a little into a shallow saucer and put it on a sunny windowsill, away from rain and pets. Over several days, white crystals may start to appear around the edge as the water disappears. Notice that the weather and the temperature change how fast it dries. This is not a copy of ancient salt making, only a way to see that the water leaves and the salt stays. Do not heat it on a stove, and do not taste the crystals.
If you want to dig deeper, the sources below are good places to start: the Ocean Newsletter articles for Japan's two-stage history, and the Okayama center's page for what archaeologists have actually found. Next time you shake salt on a meal, you could check the package, if it says, to see whether the salt came from a mine, a salt pond, or a factory.