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ISOFIX: how it works and how to check child seat compatibility with a used car

How ISOFIX, Top Tether and support legs work, what R44, R129 and i-Size mean, and how to match a child car seat to a used family car.

October 27, 2026

ISOFIX: how it works and how to check child seat compatibility with a used car

Key takeaways

ISOFIX connects a child car seat to the car's body via two rigid anchor points. Compared with fitting the seat using the seat belt, it reduces the risk of installation errors.
If the seat requires a Top Tether strap or a support leg, using it is mandatory. Both limit the seat's forward rotation in a frontal collision.
R44 classifies seats by the child's weight, R129 by the child's height. i-Size is an approval category within R129. It doesn't simply mean the seat has ISOFIX.
ISOFIX is usually fitted to the outer rear seats and, on some cars, to the front passenger seat too. A rear-facing seat may only go in the front with the passenger front airbag deactivated.
You confirm compatibility in three steps: the child seat table in the car's owner's manual, the seat manufacturer's vehicle fitting list for your car's generation, and a trial fitting of your own seat.

ISOFIX is a child car seat attachment system that makes it easier to fit the seat correctly than with a seat belt. When choosing a used car, finding the word in the equipment list of a listing is not enough. The seat must suit the child, the seating position and the fitting method prescribed by its manufacturer. A family with one or two child seats will also appreciate access through the rear doors and enough space in front of the seat. Fitting three child seats side by side is covered in the article on the family car for 3 kids.

What is ISOFIX and how is it better than a seat belt?

ISOFIX is based on two metal anchor points, usually hidden in the gap between the seat cushion and the backrest. The connectors on the car seat or its base click into them. The seat is then attached directly to the car's structure.

With a seat fitted using the seat belt, the belt must follow the correct route, must not be twisted and must be properly tightened. ISOFIX removes most of these steps. Its main safety benefit is therefore less room for error during installation.

A seat correctly fitted with a seat belt is safe, provided it is approved for that type of installation. ISOFIX, however, makes repeated installation much easier, for example when you move the seat between two family cars.

Fitting method	What holds the car seat	What to focus on in use
Seat belt	The belt routed through the specified guides	Correct routing, tension and an untwisted belt
ISOFIX on a seat with its own harness	Rigid connectors and the specified anti-rotation device	Both connectors clicked in and correct use of the strap or leg
ISOFIX on a high-back booster	The connectors stabilise the seat; the car's belt restrains the child	Correct routing of the belt across the child's pelvis and shoulder

On an infant carrier, ISOFIX is usually part of a separate base that the seat clicks into. The base and the seat must be a combination approved by the manufacturer. The same brand or a similar-looking latch is not enough.

What are Top Tether and a support leg for?

The two lower anchors cannot absorb all the forces acting in a collision. Seats with their own harness therefore have an additional specified device that prevents excessive forward rotation: a top tether strap or a support leg.

Device	How it works	Typical mistake
Top Tether	An upper strap connects the seat to a designated anchor point in the car	Clipping it to a luggage lashing eye instead of the Top Tether anchor
Support leg	Transfers part of the forces from the seat or base to the car's floor	Standing the leg on the lid of an underfloor storage compartment without an approved solution

The Top Tether anchor is usually on the back of the backrest or in the boot behind the seat. You will find its location and the strap routing in the car's owner's manual. A similar-looking hook in the boot is no substitute.

The support leg needs a surface that both the car and seat manufacturers allow. Above an underfloor storage compartment, an approved filler or a different leg position is often required, and some car and seat combinations are ruled out for this reason. A piece of wood as a spacer is no solution.

Top Tether and a support leg are not interchangeable. Always use the device that the seat's instructions specify.

What is the difference between R44, R129 and i-Size?

The approval label on the child car seat shows the standard and the range for which it is approved. In a used car, you will come across seats approved to both R44 and R129. The age of the car therefore doesn't tell you which seat can be fitted.

Label	What it means for your choice	What it doesn't mean
R44	The seat is chosen according to the child's weight group	That every seat to this standard suits every car with ISOFIX
R129	The key figure is the child's height; other limits stated on the seat also apply	That every R129 seat belongs to the i-Size category
i-Size	A category within R129 that simplifies matching the seat to a seating position in the car	That every seat with ISOFIX anchors is an i-Size position

For a seat with its own harness, check the maximum permitted weight of the child as well as the height range. The range on the label takes precedence over the approximate age in the product description.

What does an i-Size seating position in the car mean?

A seating position marked i-Size meets the requirements for seats in this category: an i-Size seat fits it without further checking of lists. The seat's range of use and the fitting instructions still apply.

An older car without i-Size marking may still be compatible with a new seat. In that case, the seat's approval category and the vehicle fitting list issued by its manufacturer decide.

Which seats in the car usually have ISOFIX?

ISOFIX is most often found on the outer rear seats. Only some models and versions have it on the front passenger seat, and it is rare in the centre rear position.

Position in the car	What to expect
Outer rear seats	The most common location for ISOFIX anchors
Front passenger seat	Only on some cars and versions; you can't assume it based on the rear anchors
Centre rear seat	ISOFIX is the exception; a child seat is usually fitted here with the seat belt, if the car's manual allows it

The position of the anchors on a seat is shown by a fabric tag, a plastic cover or a symbol. Which types of child seat may go in each position is set out in the table in the car's owner's manual. "ISOFIX" in a listing doesn't tell you how many such positions the car has.

The anchors of two adjacent seats must not be combined to fit a child seat in the middle unless the car manufacturer expressly allows it. A spacing that looks right by eye is not an approval.

When can a child seat go on the front passenger seat?

A rear-facing seat may only go in the front with the passenger front airbag deactivated and under the conditions set out in the manuals for the car and the seat. An airbag inflating against the back of a rear-facing seat puts the child in serious danger.

One example is the Škoda Octavia Combi IV (generation from 2019): the tested 1.5 TSI Ambition version allows the passenger airbag to be deactivated precisely for a rear-facing seat. How the seat is attached in the front position, with the seat belt or ISOFIX, is a separate question for the car's manual.

For a forward-facing seat, both the airbag setting and the position of the passenger seat follow the car's owner's manual. Don't apply the rule for rear-facing seats to it. How crash tests rate equipment for child seats is explained in the article on Euro NCAP.

How to check child seat compatibility with a used car

A reliable answer comes from three sources together: the car's manual, the seat's manual and the seat manufacturer's vehicle fitting list, backed up by one trial fitting. The aim is a confirmed combination of car – seating position – child seat – base, if any.

Checklist before buying the car

Find out the exact generation and year of manufacture of the car. In the manual, look up the child seat table and the positions marked ISOFIX or i-Size. The year of first registration may not be enough to distinguish versions.
Write down the exact name of the seat and the base. One product range often includes several versions with different approvals.
Compare the permitted positions. If the seat requires a vehicle fitting list, the generation, production period and chosen seating position must all match.
Inspect the anchors and the floor. A damaged or bent anchor, a missing top tether point or an unsuitable underfloor compartment are reasons to stop the installation.
Fit your own seat, including the base. All latches and indicators must confirm correct installation, the seat must be at a permitted angle and the support leg in the correct position.
Sit an adult in front of the child seat. The front seat must be positioned as the child seat's instructions allow, and the driver or passenger must be able to sit in it safely.

A trial fitting is especially worthwhile for a rear-facing seat. It takes up more space lengthways than the size of the rear seat cushion suggests. Contact with the front seatback is only acceptable if the manufacturers' instructions allow it.

Three common situations when choosing

Situation	What it means
You already have an infant seat with a base	The whole set must be compatible. Approval of a different base from the same brand is not enough.
A younger child travels in a seat with a support leg	Besides the anchors, you need a suitable floor under the leg and enough space for a permitted seat position.
An older child travels on a high-back booster with ISOFIX	The car's belt restrains the child, so its routing is just as important as the clicked-in connectors.

With two child seats, deal with each position separately. A set-up that fits behind the passenger may get in the way behind a driver who needs the seat further from the steering wheel. What else to try during a viewing is summed up in the article on how to choose a family car.

What to watch out for when using ISOFIX?

Rigid anchors won't prevent mistakes when connecting the seat or strapping in the child. Most problems have the same cause: part of the prescribed procedure is left unfinished.

Only one connector clicked in. The seat may stand straight and still not be correctly attached. The indicators on both connectors must confirm they are locked.
Seat belt and ISOFIX together against the instructions. Extra fastening doesn't automatically increase safety. Only approved fitting methods count.
A loose harness. A firmly attached seat is no help if the harness sits loosely around the child. A thick winter coat makes it harder to tighten.
An incorrectly routed belt for an older child. The belt belongs in the seat's guides, never under the arm or behind the back.
A seat outside its range. The child must meet the height and weight limits as well as the rules for head and harness position.

A family car through Carvago

When a family is looking for a particular generation and version, for example with ISOFIX on the front passenger seat too or with a switchable airbag, it needs a wide choice. Carvago has the widest selection of used cars: roughly a million cars from dealers in several European countries. The choice in practically every segment is therefore larger than on a local market, usually by an order of magnitude.

Every car is inspected by a technician before sale as part of CarAudit™, with up to 270 checkpoints. It includes a check of the history and legal status, an inspection of the interior and technical components, diagnostics and a test drive. The customer knows the result before deciding to buy, and Carvago guarantees the condition stated in the CarAudit™ certificate.

Once the purchase is approved, Carvago uses a power of attorney to register the car in the customer's country, including all the paperwork, and delivers it to the customer's door as standard. You can then fit the child seat at home and drive off straight away.

FAQ

Can ISOFIX be retrofitted to an older car?

Only if a solution approved by the car's manufacturer exists for that car. A universal bracket from an online shop is no substitute for an anchor point. Without an approved installation, choose a seat approved for fitting with the seat belt.

Does an empty child seat need to be secured too?

Yes. A loose seat turns into a projectile under hard braking or in a collision and endangers the occupants. For high-back boosters, securing the seat without a child follows its design and the manufacturer's instructions.

Can a rotating child seat stay turned towards the door while driving?

No. Turning it towards the door is only for getting the child in and is not permitted while driving. Before setting off, the seat must be locked in one of the specified travel positions. During a trial fitting, also check whether the seat can rotate in the cabin without hitting the backrest or the door frame.