BudgetPlan your spendHospital bagFree checklist

Cybex Eternis S

The Cybex Eternis S rear-faces up to 50 lb, forward-faces up to 65 lb, and is 19.8 in wide, per Cybex. A non-rotating Cybex all-in-one with a 12-position headrest that goes from rear-facing through high-back booster.

DiscontinuedNo longer made or sold new. Specs are kept for comparing generations.

  • $299.95List price
  • 50 lbRear-facing to
  • 65 lbHarness to
  • 120 lbBooster to
See it on Cybex's site

When does it expire?

All all-in-one car seats

Eternis S: key facts

  • The Cybex Eternis S rear-faces from 4 to 50 lb and 17 to 48 in tall; forward-faces with a harness from 22 to 65 lb and 28 to 49 in tall; works as a booster from 40 to 120 lb and 44 to 57 in tall, per Cybex.
  • Outgrown by height: "CHILD'S HEAD MUST BE 2.5 CM (1 IN) OR MORE BELOW THE TOP OF THE HEADREST OR SHELL, WHICHEVER IS TALLER."
  • It installs with LATCH or seat belt.
  • Cybex says it expires 10 years after the date of manufacture.
  • Cybex lists it as certified for aircraft ("When used WITH the internal harness and properly installed pursuant to these instructions, this restraint is certified for use only in passenger and multi-purpose passenger motor vehicles, buses and trucks equipped with either lap belts only or lap/shoulder belt systems and for use in aircraft. / When used in BOOSTER MODE, this restraint is NOT certified for use in aircraft. (manual Certification section)").
  • It's 19.8 in wide at the widest point and weighs 26 lb.

From Cybex's manual and published specs. Always follow the limits on your seat's labels.

How the Eternis S stacks up

Against the 58 all-in-one car seats we track. The shaded band is where the middle half of them fall; the dot is this one.

Expiration10 years

The same as most all-in-one car seats (10 years).

This one
9 yearsmiddle 10 years12 years
Rear-facing limit50 lbHighest rear-facing limit of 58 all-in-one car seats

Above the 40 lb most all-in-one car seats list.

This one
40 lbmiddle 40 lb50 lb
Width19.8 in28th narrowest of 34

Above the typical range for all-in-one car seats (18.3 in to 19.5 in).

This one
16.7 inmiddle 19 in21.5 in
Weight26 lb21st lightest of 36

Within the typical range for all-in-one car seats (20 lb to 29.2 lb).

This one
12.2 lbmiddle 25.5 lb35 lb
Price$299.9527th lowest price of 55

Within the typical range for all-in-one car seats ($229.99 to $399.99).

This one
$99middle $299.99$799.95

Flying with the Eternis S

Cybex's label: “When used WITH the internal harness and properly installed pursuant to these instructions, this restraint is certified for use only in passenger and multi-purpose passenger motor vehicles, buses and trucks equipped with either lap belts only or lap/shoulder belt systems and for use in aircraft. / When used in BOOSTER MODE, this restraint is NOT certified for use in aircraft. (manual Certification section)” It is 19.8 in wide at its widest point.

What airlines say about car seats:

  • American: “Each ticketed customer is allowed 1 stroller and 1 car seat to be checked free of charge” “If an unoccupied, adjoining seat is not available, the gate agent will check the safety seat to your final destination.” Source
  • Alaska: “We will transport your child's car seat and stroller free of charge as checked baggage.” Source
  • JetBlue: “Checking a child safety seat will not count towards your checked baggage allowance and there is no fee assessed” Source
  • Spirit: “One (1) car seat will be accepted per child as checked baggage at no charge.” Source
  • Frontier: “strollers and car seats can be checked for free.” Source
  • Allegiant: “There is no charge for checking or gate checking one car seat per fare-paying passenger.” Source
  • Breeze: “Child seats may be carried on board the aircraft if a seat has been purchased for the Infant or Child” Source

Quoted from each airline's page as of 2026-11-02.

Listed features

  • Rear-facing to 50 lb
  • Approved for aircraft use per Cybex
  • In-seat recline plus 6 base recline positions

Worth checking

  • 19.8 in wide
  • 26 lb to carry
  • Cybex lists this version as no longer available

Where to buy the Eternis S

Prices and availability change. We may earn a commission from some links.

good to know
  • Discontinued. You may still find it used or on clearance.
  • Check for recalls: Search the agency's recall list with the model number and date code from the label: NHTSA recall search.
  • Car seat use and installation: see NHTSA's car seat page, which also lists free car seat inspection by certified technicians. Check the label and the maker's instructions, and ask your pediatrician about your child.

Full specs

Cybex Eternis S
TypeAll-in-one
List price$299.95
Rear-facing weight4–50 lb
Rear-facing height17–48 in
Forward-facing weight22–65 lb
Forward-facing height28–49 in
Booster weight40–120 lb
Booster height44–57 in
Seat weight26 lb
Width19.8 in
Head ruleCHILD'S HEAD MUST BE 2.5 CM (1 IN) OR MORE BELOW THE TOP OF THE HEADREST OR SHELL, WHICHEVER IS TALLER.
Newborn insertNewborn Insert (REAR-FACING ONLY); no weight limit stated
InstallationLATCH, seat belt
Load legNo
Anti-rebound barYes
RotatesNo
Harness heights12
Recline positions6
Aircraft labelCertified for aircraft, per the maker
Expires after10 years

Compare the Eternis S with

How each closest rival differs from the Eternis S, from the makers' specs.

Ask the meadow

Eternis S: questions answered

From Cybex's published specs, last checked 2026-11-06.

Does the Cybex Eternis S rear-face longer than other all-in-one seats?

Yes. Its 50 lb rear-facing limit is the highest of the 70 all-in-one seats we track, tied with 14.

Will the Cybex Eternis S fit three across or in a small car?

It's wider than 89% of the all-in-one seats we track, at 19.8 in. The narrowest is the Chicco Fit3x ClearTex Ultra-Slim All-in-One at 16.7 in. Fit depends on your vehicle; check each seat's manual and your vehicle manual. Car seat use and installation: see NHTSA's car seat page, which also lists free car seat inspection by certified technicians. Check the label and the maker's instructions, and ask your pediatrician about your child.

Can you bring the Cybex Eternis S on a plane?

Yes, Cybex lists it as certified for aircraft; look for the red-lettered label that says so. The FAA asks that a car seat go in a window seat so it doesn't block the aisle, and the crew will look for that label. It weighs 26 lb to carry through the airport.

Eternis S or Radian 3RXT: what's different?

By the makers' specs, the Eternis S lists for $0.04 less. See every spec side by side.

Is it OK to buy a used Eternis S?

NHTSA publishes guidance on used car seats on its car seat page. Cybex lists a lifespan of 10 years from manufacture for this model. Check the label and the maker's instructions, and ask your pediatrician about your child.

Still wondering? Write to us at hello@cribpicks.com and we'll look it up.

More all-in-one car seats

Where these specs come from

Last checked 2026-11-06. Spot something wrong? Tell us. Read our editorial and safety policy and corrections.