An object is more buoyant in salt water than it is in fresh water, but why? An object's buoyancy is determined by two forces:

The downward force: equal to weight of the object

equal to weight of the object The upward force: equal to the weight of the water that the object displaces (this is known as Archimedes' Principle)

The upward and downward forces work in opposition to each other. As a result of these forces, the object will either float, sink, or remain suspended in the water. The object's buoyancy may be described in one of three ways:

Negatively Buoyant: The weight of the object is greater than the weight of the water it displaces. The object will sink.

The weight of the object is greater than the weight of the water it displaces. The object will sink. Positively Buoyant: The weight of the object is less than the weight of the water it displaces. The object will float.

The weight of the object is less than the weight of the water it displaces. The object will float. Neutrally Buoyant: The weight of the object is exactly equal to the weight of the water it displaces. The object will remain suspended mid-water and will neither float nor sink.

Salt Water Weighs More Than Fresh Water

A cubic foot of salt water weighs (on average) 64.1 lbs, while a cubic foot of fresh water weighs only 62.4 lbs. The reason for the difference in weight is that salt water has salt dissolved in it.

Dissolving salt in water increases the water's density, or mass per a unit of volume. When salt is added to water, it reacts with the water molecules, forming a polar bond with the water that rearranges the salt and water molecules with an unusual effect:

A cubic inch of salt added to a volume of water will not increase the volume of water by a cubic inch. A simplistic explanation is that the water molecules pack themselves tightly around the salt molecules—squeezing closer together than they do when the salt is not present. When a cubic inch of salt is added to a volume of water, the volume of water increases by less than a cubic inch.

A cubic foot of salt water has more molecules in it than a cubic foot of fresh water and, therefore, weighs more.

Recall that Archimedes' Principle states that the upward force on a submerged object is equal to the weight of the water that it displaces. Salt water weighs more than fresh water, so it exerts a greater upward force on a submerged object. An object that displaces a cubic foot of fresh water will experience an upward force of 62.4 lbs, whereas the same object in salt water will experience an upward force of 64.1 lbs.

Changing Between Fresh Water and Salt Water

At this point, it is possible to make some general predictions about an object's (or a diver's) buoyancy when moved from fresh to salt water and vice-versa. Consider the following cases:

An object that is neutrally buoyant in fresh water will float when placed in salt water. In fresh water, the weight of the object is exactly equal to the weight of the water it displaces, and the downward and upward forces on the object are equal. When the object is moved to salt water, the weight of the water it displaces will increase and the upward force will be greater than the downward force. The object will be positively buoyant in salt water.

In fresh water, the weight of the object is exactly equal to the weight of the water it displaces, and the downward and upward forces on the object are equal. When the object is moved to salt water, the weight of the water it displaces will increase and the upward force will be greater than the downward force. The object will be positively buoyant in salt water. An object that is neutrally buoyant in salt water will sink when placed in fresh water. In salt water, the weight of the object is exactly equal to the weight of the water that it displaces, and the upward and downward forces on the object are equal. When the object is moved to fresh water, the weight of the water it displaces will decrease, and the downward force on the object will be greater than upward force. The object will be negatively buoyant in fresh water.

In salt water, the weight of the object is exactly equal to the weight of the water that it displaces, and the upward and downward forces on the object are equal. When the object is moved to fresh water, the weight of the water it displaces will decrease, and the downward force on the object will be greater than upward force. The object will be negatively buoyant in fresh water. An object that is negatively or positively buoyant in salt water will become more negatively buoyant when placed in fresh water—but we cannot predict whether it will sink or float without more information. An object will experience a weaker upward force in fresh water than in salt water and will be less buoyant in fresh water. However, to determine whether the object will sink or float, it is necessary to know the exact weight of the object and the exact weight of the water it displaces.

An object will experience a weaker upward force in fresh water than in salt water and will be less buoyant in fresh water. However, to determine whether the object will sink or float, it is necessary to know the exact weight of the object and the exact weight of the water it displaces. An object that is negatively or positively buoyant in fresh water will become more positively buoyant when placed in salt water—but we cannot predict whether it will sink or float without more information. An object will experience a stronger upward force in salt water than in fresh water, and will be more buoyant in salt water. However, to determine whether the object will sink or float, it is necessary to know the exact weight of the object and the exact weight of the water it displaces.

Weighting a Scuba Diver

It is clear that a diver will be more positively buoyant in salt water than he will be in fresh water, and will need to adjust his weights accordingly. The diver will need to carry more weight in salt water than he will need to carry in fresh water. The amount of weight the diver must carry will depend upon a variety of factors, including his body mass, his exposure protection, the type of tank he carries, and his dive equipment.

A diver's weight belt is only a small percentage of his total weight; his body weight, tank and dive gear also contribute to his weight and the downward force on his body. Divers often switch wetsuits (or drysuits) and other gear when changing dive locations, and the upward force on the diver may vary according to these factors, as well as according to the type of water.

It is impossible to predict the necessary weight change for an individual diver without knowing his water displacement, total weight, and the salinity of the water he will dive in. The easiest way for a diver to determine proper weighting is to perform a buoyancy test whenever switching between fresh and salt water, and whenever he changes a piece of his dive gear. However, given that all factors remain the same except for the water type, a diver may have to nearly double his weight when moving from fresh to salt water or halve it when changing from salt to fresh water.

Additional Considerations

To make matters more complicated, the salinity of salt water varies around the world. Some bodies of water may be saltier than others. Of course, a diver will be more positively buoyant in saltier water. The average weight of a cubic foot of salt water is 64.1 lbs, but in the Dead Sea, a cubic foot of water weighs about 77.3 lbs! A diver would be significantly more buoyant in the Dead Sea.

Temperature also affects the density of water. Cold water is denser than warm water. Water reaches its maximum density at about 39.2° F, and a diver who ventures into very cold water may notice that he is a bit more negatively buoyant than in warmer water.

Many dive sites require a diver to move through layers of different water temperatures (thermoclines) or layers of different salinities (haloclines). A diver moving between these layers will notice changes in his buoyancy.

Objects (such as divers) will be more buoyant in salt water than in fresh water. Predicting a diver's buoyancy requires knowing his total weight, including gear, as well as the weight of the water he displaces. It is much easier to perform a buoyancy check before a dive than to attempt to mathematically determine the quantity of weight that a diver should carry. Additionally, divers who use aluminum tanks will need to weight themselves to offset the buoyancy change of the tank during a dive; an aluminum tank will become more positively buoyant as it is emptied.