How Do Life Jackets Work?

Navigating the world of life jackets, with their diverse range and functionalities, can be daunting.

Whether you’re a boat owner, water sports enthusiast, or beach vacationer, understanding these crucial safety devices is vital.

I’ll unpack the science behind life jackets, breaking down their buoyancy mechanisms, categories, and performance ratings. 

Drawing from personal experience, I’ll convey the topic in clear, accessible information, empowering you to make informed safety decisions for your next water adventure.

At a Glance:

1️⃣ Life Jackets work by using buoynacy, discplacement, Archimedes principle, stability and floatation.

2️⃣ Some can turn the wearer over and keep the mouth and nose out of the water, while others only provide essential buoyancy while the swimming survivor waits for rescue.

3️⃣ Two categories are used to score life jackets, Performance TYPE and Performance LEVEL life jackets.

🏊‍♂️ How Do Life Jackets Keep You Afloat?

A life jacket keeps you afloat by providing additional buoyancy. Here’s a concise explanation:

1️⃣ Buoyancy: Life jackets are filled with materials that are lighter than water, such as foam or gas. This makes the jacket less dense than the water.

2️⃣ Displacement: When you wear a life jacket and enter the water, the jacket displaces a volume of water that weighs more than the jacket itself.

3️⃣ Archimedes’ Principle: According to this principle, an object submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object. Since the life jacket displaces water that weighs more than it does, it experiences an upward force.

4️⃣ Stability: The life jacket is designed to distribute this buoyant force evenly across your body, particularly around the chest and back, keeping you stable in the water.

5️⃣ Floatation: The upward force (buoyancy) counteracts the force of gravity pulling you down, allowing you to float.

There are a handful of form factors available for life jackets

Inherent Jackets

As the name implies, inherent Life jackets are “inherently buoyant.” They are filled with foam or similar material and are rated as follows:

Size Life jacket categoryBuoyancy Rating
AdultTYPE I22 lbs.
AdultTYPE II and III15.5 lbs.
AdultTYPE IV15.5 to 22 lbs.
AdultLEVEL 7015.75 lbs.
Youth TYPE II and III11 lbs.
Youth TYPE V11 to 15.5 lb.
ChildTYPE II7 lbs.
InfantTYPE II7 lbs.

Inflatable Jackets

A gas cartridge is activated, which fills the air compartments and makes the life jacket buoyant.

It’s important to remember that an inflatable jacket won’t work if the bladder has a tear in it.

Learning why regular and correct life jacket maintenance to ensure they work when you need them most is highly recommended.

Two kinds of inflatable jackets are available:

  • Those that automatically inflate.
  • Those that require manual inflation.

The capabilities of inflatable life jackets are listed below.

Size Life jacket categoryBuoyancy Rating
AdultTYPE  I and II34 lbs.
AdultTYPE  III22.5 lbs.
AdultTYPE  IV22.5 to 34 lb.

Hybrid Jackets

Hybrid life jackets combine the capabilities of the inherent (foam-filled) and inflatable life jackets.

Size Life jacket categoryBuoyancy RatingWearers Size 
AdultTYPE I Land III 10 lbs.22 lbs.
AdultTYPE IV 7.5 lbs.22 lbs.
AdultLEVEL 70 9 lbs.15.74 lbs.
Youth TYPE II and III 9 lbs.15 lbs.
Youth TYPE IV  7.5 lbs.15 lbs.
ChildTYPE II 7.5 lbs.12 lbs.
InfantNot available for infantsN/AN/A

Special Purpose Jackets

These life jackets are constructed case-by-case, depending on the requirements.

🎽 Types Of Life Jackets

The available life jacket types are rated by the buoyancy specifications.

They are divided into two categories listed below.

These life vests are classified based on their capacity to keep the wearer in an upright position in the water without any external assistance.

They are suitable for use in normal water environments when the swimmer’s proficiency is uncertain, and assistance is not easily accessible.

The life jackets are categorized into I, II, III, and IV, with a higher rating indicating a reduced ability of the life vest to support the survivor’s head above water.

Performance TYPE I Life jackets

Type I Life jackets have the greatest lifesaving ability and the highest buoyancy rating. Type 1 lifejackets will turn an unconscious person’s face vertically aligned slightly to the rear.

This position ensures the person wearing the lifejacket has their mouth and nose clear from the water

Performance TYPE II Life Jackets 

Type II Life Jackets provide a slightly lower level of protection by keeping the wearer’s mouth and nose clear of the water.

The difference is that this level of lifejacket will not turn the person over.

Performance TYPE III Life Jackets

Type III Life Jackets offer a further degradation in ability. While it will lift the person’s head out of the water, the alignment is lower than the Type II lifejacket, and there is also no requirement to turn the person over.

Performance TYPE IV Life Jackets 

Type IV life jackets provide less buoyancy and lifesaving features.

They are only rated for defined activities like water skiing, boardsailing, or white-water rafting.

Different from the TYPE-rated life jackets, the LEVEL life jacket capabilities increases with the rating.

Level 275 is the highest-LEVEL rating, meaning they have the highest buoyancy.

The rating is the buoyancy of each device as measured in Newtons.

Performance LEVEL 50 Life Jackets 

This LEVEL life jacket is intended to be used purely as an aid, assisting competent swimmers close to safety (boat or river bank.)

These life jackets are only suitable for inland, calm waters.

These life jackets can provide a buoyancy of 50 newtons (11.24 pounds).

Performance LEVEL 100 Life Jackets  (ISO12402-4)

This life jacket provides a slightly higher level of buoyancy.

They are intended to be used by competent swimmers while traveling through sheltered waters.

These life jackets can provide a buoyancy of 100 newtons (22.48 pounds).

Performance LEVEL 150 Life Jackets  (ISO12402-3)

The second highest rate LEVEL lifejackets have a rating of 150.

This level of lifejacket provides a flotation capability that will keep a wet person with sodden clothes afloat.

These are primarily used in open water conditions and worn by “conscious” people who can work at keeping their mouths and noses out of the water.

These life jackets can provide a buoyancy of 150 newtons (33.72 pounds).

Performance LEVEL 275 Life Jackets (ISO12402-2)

The most extreme LEVEL life jacket is designed for use in high-risk conditions (such as oil rigs), where the person in the water carries additional weight, possibly in the form of heavy tools.

These life jackets can provide a buoyancy of 275 newtons (61,82 pounds of buoyancy)

⚖️ The Correct Weight Category Life Jacket Must Be Selected

The most appropriate life jacket must be chosen. 

The weight categories of lifejackets are listed below. Don’t be confused by a lifejacket’s buoyancy capability being compared to the wearer’s weight.

The user’s weight that is indicated is on land and not in the water, where a human body has some natural buoyancy.

Adult Life Jacket

These life jackets are designed to keep the wearer who weighs more than 88 lbs. (40 kg) afloat.

Teenager Life Jacket

Youth Life Jackets are designed to keep the wearer who weighs between 55 lbs. and 88 lbs. (25 – 40kg) afloat.

Young Person Life Jacket

Young Person Life Jackets are designed to keep the wearer between 33 and 55 lbs. (15  – 25kg) afloat.

Infant Life Jacket

These life jackets are Child Life and are designed to keep the wearer who weighs less than 33 lbs. (15 kg) afloat.

The required performance of the lifejacket will also be determined by analyzing all potential risk factors, including the following:

  • The potential wearer’s stamina.
  • The time the life jacket needs to work (how close are the potential rescue services.)
  • The activities the vessel undertakes (deep sea or inland waters)

Properly Fitting/Donning A Life Jacket

This section will teach you how to choose a life jacket correctly to ensure it works effectively in an emergency.

A life jacket that does not fit perfectly is better than nothing. But, the optimum solution is to have a life jacket that fits well and the user is happy to wear.

Anyone who regularly travels on the boat should have a well-fitting unique life jacket that is theirs.

It is also worthwhile keeping several general-use life jackets of different sizes for visitors.

A life jacket should be comfortable and worn as a choice rather than by instruction. 

Always let the regular user choose a color and style of lifejacket that suits them. If they are happy with it, they will be more willing to wear it constantly. 

The following is the list of factors to consider when fitting a life jacket:

✔️ Only select life jackets warranted by the United States Coast Guard (USCG) or SOLAS.

✔️ Select the most appropriately designated life jacket (adult, Teenager, Young Person, or infant).

✔️ Choose the most appropriately rated life jacket according to its Performance LEVEL and TYPE.

✔️ Select the size life jacket that matches the measurement of the wearer’s torso (circumference).

✔️ Test the life jacket by putting it on, zipping, or buckling it up and have someone check it for fit.

✔️ Hold the handle at the back and see if you can pick it up without slipping it off over the wearer’s head to check that it fits.

🦺 Additional Safety Features:

Some life jackets have additional safety features incorporated that may include the following:

  • Lights
  • Whistle
  • Neck pillows
  • Pouches or pockets.
  • Rings to thread a rope through and keep all of the survivors together.

🔑 Key takeaways

Carrying and storing store life jackets on inland and offshore waters is required by law.

There are four categories of flotation provided by life jackets (Inherent, inflatable, hybrid, and special purpose).

Two types of rating are used to score life jackets Performance TYPE and Performance LEVEL life jackets.

There are four sizes of life jackets (adult, teenager, young person, and infant).


Do Life Jackets Work If You Can’t Swim? 

Performance TYPE I Life jackets will rotate a person to ensure the wearer faces up in the water.

Once rotated, the life jacket will keep a non-swimmers head and nose above the water. This system works even if the wearer is unconscious. 

How Do Life Jackets Inflate Underwater? 

Life jackets that can inflate under water are called inflatable life jackets.

These devices have a small tablet that dissolves in water that works with a gas cartridge.

As the unit comes into contact with the water, the tablet activates the gas canister causing gas to be released and the life jacket to inflate.

I’m the founder and chief editor here at Sailing Savvy. I spent a decade working as a professional mariner and currently, I mix those experiences with digital publishing. Welcome, and I hope that we can be the hub you need for safe passage.