Britax One4Life Slim Multimode Car Seat Review
We remain big fans of the trend toward narrower car seats. Having car seats and booster seats that are narrow enough that several children can fit comfortably in a smaller back seat is a game-changer for a lot of families. We’re thrilled to see that our friends at Britax are introducing narrower versions of seats we already know and love. The latest seat to join this growing group is the One4Life Slim multimode seat. Join us for an in-depth look.
CSFTL Quick Stats
Rear facing weight range: 5-50 poundsRear facing height range: 18-49 inches. The seat is outgrown rear facing when the child’s head is less than 1 inch from the top of the headrestInsert usage:Comfort pads: OptionalHead infant insert: optional but must be used with middle insert, can be used 5-20 pounds in rear facing modeMiddle infant insert: optional but must be used with head infant insert, can be used 5-20 pounds in rear facing modeLower infant insert: optional, can be used 5-20 pounds in rear facing modeForward facing weight range: 30 – 65 poundsForward facing height range: 35 – 49 inches tall and the tops of the child’s ears must be at or below the top of the headrest and the straps are at or above the child’s shouldersForward facing age minimum: None listed – Britax recommends maxing out the seat in rear facing mode before switching to forward facing mode (best practice is to rear face until at least 2 years old), CSFTL also strongly recommends keeping children rear facing until they reach the maximum height or weight limit of the car seat)Booster mode weight range: 40 – 120 poundsBooster mode height range: 44-63 inchesBooster age minimum: CSFTL recommends waiting until a child is at least 5 years old to use a seat in booster modeLower anchor weight limit:Rear facing: 35 pounds *See recall noticeForward facing: 40 poundsBooster: lower anchors be used at any weight when the seat is in booster modeReplacement policy: NHTSA guidelines for replacementRecline positions:Harness mode, rear and forward facing: 1-8Booster mode: BExpiration: 10 yearsFAA certified for use on aircraft in harness mode
Features
ClickTight installationSafewash fabric allowing for machine washing and dryingRemovable and dishwasher safe cup holdersNarrow designNorethread harnessOptional antirebound bar
Measurements
Lowest harness position with insert: 7 inchesLowest harness position without insert: 8 inchesHighest harness position: 18 inchesHighest booster belt guide: 21 inchesCrotch buckle position with inserts: 3 inches from the back of the seatCrotch buckle positions without insert:Under 20 pounds: 2.5, 4.5, 6 inchesOver 20 pounds: 3.5, 5.5, 7 inchesSeat width internally at the child’s shoulders: 14 inchesSeat width externally at the booster lap belt guides: 17 inchesBase width: 14.5 inchesTotal seat weight: 27.5 pounds with all inserts in placeFront-to-back measurement on flat ground (this will vary in/by vehicle:No antirebound bar: 29.5 inchesWith antirebound bar: 30 inches
Assembly
Britax One4Life Slim out of the box
This seat comes fully assembled in the box with the cupholder caps in place and cupholders available in a bag. The seat requires either the cupholders or cupholder caps to be in place at all times. The cupholders are removable and dishwasher safe. The caps and cupholders do require a flathead screwdriver to swap out.
Accessories
The One4Life Slim has a lot of the same types of rear facing inserts we saw with the original One4Life but with some improvements. This seat comes with an optional head, middle, and lower body insert that can be used for a better fit up to 20 pounds. The head and middle inserts must be used together. The lower insert can be used by itself up to 20 pounds. These inserts can only be used when the seat is installed rear facing.
The harness pads on this seat are removable, yay! This is a welcome change for many caregivers.
Anti Rebound Bar
Britax One4Life Slim
This is optional and can be purchased separately for use in RF mode. I found it only took up an inch or so more room when in use.
One4life slim antirebound bar
Installation in Tight Spaces
Britax One4Life Slim next to Britax Poplar
I’ve had the privilege of reviewing the Britax Poplar convertible car seat and now this seat. I put them side by side, literally and figuratively, to help families decide which seat may be best for them. I was pleasantly surprised to see that I could install both seats in my 2023 Crosstrek side by side, independently installed without any bracing or overlap. It wouldn’t have worked next to another One4Life Slim but definitely worked next to an even more narrow seat. As always, we encourage caregivers to check any installations in their own vehicles before heading out for a ride.
Installation
Rear Facing
Recline Angle Settings
Britax One4Life Slim recline button
The seat has 8 recline angle positions, plus position B for booster mode. Any of the numbered positions can be used in harness mode to help install the seat at the correct angle for the child and the vehicle. To adjust the recline, press one of the gray buttons that can be found at the back of the seat on both sides of the base and recline or incline the seat as needed.
For children who lack neck control, the tip of the arrow should be in the blue range on the recline indicator. Britax does allow the use of a pool noodle or rolled towel in the event recline 1 does not installat at a deep enough recline for a child without head and neck control.
Britax One4Life Slim recline indicator
For a child with head and neck control, the arrow tip can be anywhere in the blue or green.
Installation: Vehicle Seat Belt (preferred installation method)
Britax One4Life Slim ClickTight release
To install rear facing using the vehicle seat belt, adjust the recline angle, place the car seats on the vehicle seat and open the ClickTight panel by squeezing the two buttons under the seat pad. Next, route the vehicle seat belt through the rear facing belt path and buckle the seat belt.
Britax One4Life Slim rear facing with seatbelt
Remove all of the slack in the vehicle seat belt and make sure the base is flat on the vehicle seat before closing the ClickTight panel.
Britax One4Life Slim rear facing with seatbelt
Check for movement at the belt path using the force of a firm handshake with your non-dominant hand. The seat should not move more than one inch side to side and front to back.
Britax One4Life Slim testing for movement at the rear facing belt path
Lower Anchors
Britax One4Life Slim lower anchor install rear facing
To install the seat rear facing with the lower anchors, adjust the seat to the desired recline position, open the ClickTight panel and route the lower anchor strap through the rear facing belt path and attach lower anchor connectors to the vehicle’s lower anchors.
Remove the slack from the lower anchor webbing by pulling on the tightening strap, be sure not to overtighten. Close the ClickTight panel.
Britax One4Life Slim lower anchor install rear facing
Check for movement at the belt path using the force of a firm handshake with your non-dominant hand. The seat should not move more than one inch side to side and front to back.
Installation: Forward Facing
Recline Position
Britax One4Life Slim recline indicator
For forward facing installations, the arrow tip on the recline indicator must be in the green and blue striped section. Use any of the numbered recline positions to install the seat so the the arrow tip in the green and blue striped setting.
V-Shaped Top Tether
Britax One4Life Slim V shaped tether
The top tether is a crucial part of your child’s car seat when installed in forward facing mode. It reduces the risk of head excursion by 4-6 inches in a crash. Britax uses a V-shaped tether that works a little differently than the single strap most other manufacturers use — it usually needs to be routed around the vehicle headrest.
Vehicle Seat Belt (preferred method)
Britax One4Life Slim open ClickTight panel forward facing
To install forward facing using the vehicle seat belt adjust the recline angle, then disconnect and route the top tether (see the manual for instructions), then open the ClickTight panel by squeezing the two buttons under the seat pad.
Next, route the vehicle seat belt through the forward facing belt path and buckle the seat belt.
Britax One4Life Slim forward facing install
Remove all of the slack in the vehicle seat belt and make sure the base is flat on the vehicle seat before closing the ClickTight panel.
Britax One4Life Slim forward facing install with seatbelt
Secure and tighten the top tether after closing the ClickTight panel.
Britax One4Life Slim forward facing install wtih seatbelt
Check for movement at the belt path using the force of a firm handshake with your non-dominant hand. The seat should not move more than one inch side to side and front to back. If there is interference from the vehicle headrest, remove it the vehicle manual allows it.
Lower Anchors
Britax One4Life Slim checking for movement forward facing belt path
To install the seat forward facing with the lower anchors, set the seat to the desired recline, route the top tether per the manual, open the ClickTight panel and route the lower anchor strap through the forward facing belt path and attach the lower anchor connectors to the vehicle’s lower anchors.
Britax One4Life Slim forward facing install with lower anchors
Remove the slack from the lower anchor webbing by pulling on the tightening strap, make sure not to overtighten. Close the ClickTight panel.
Secure and tighten the top tether after closing the ClickTight panel.
Check for movement at the belt path using the force of a firm handshake with your non-dominant hand. The seat should not move more than one inch side to side and front to back. If there is interference from the vehicle headrest, you can remove it if allowed per the vehicle manual.
Converting to Booster Mode
Britax One4Life Slim converting to booster mode
To convert the seat to booster mode, loosen the harness straps, leaving a little bit of slack in them, then raise the headrest up all the way. Unbuckle the chest clip and crotch buckle, then flip up the head restraint cover and flip down the seat pan cover. Position the harness behind the covers and buckle the chest clip, moving it to the middle of the backrest.
Britax One4Life Slim converting to booster mode
Move the comfort pads below the chest clip and tuck in the buckle tongues inside each comfort pad. I found this step to be challenging due to the buckle tongues being larger than the harness pads. I was able to get them in with the smallest amount of Velcro holding the pads together. Per Britax, this is to prevent caregivers from losing the comfort pads. If you are unable to follow this step, you can opt to keep them in a safe place, perhaps with the rear facing inserts.
Britax One4Life Slim converting to booster mode harness pad storage
Push the buckle tongues and chest clip into the gap until the excess webbing is no longer visible.
Britax One4Life Slim converting to booster mode
Open the ClickTight panel and remove the crotch buckle.
Britax One4Life Slim converting to booster mode crotch buckle storage
Lay the crotch buckle and padding horizontally across the raised section of the seat shell in the center.
Britax One4Life Slim booster mode
Here we see the finished product, this seat is ready for use in booster mode.
Recline Position for Booster Mode
Britax One4Life Slim recline “B” for booster mode
The recline angle must be set to position B for use in booster mode.
Lower Anchors in Booster Mode
Britax One4Life Slim booster mode using the lower anchors
Use of the lower anchors and top tether are allowed in booster mode. Britax recommends securing the seat with the lower anchors prior to storing the harness.
Fit to Child
Rear Facing
Newborn
This seat has a minimum height of 18 inches so unfortunately our doll model was an inch too short to be a candidate for this seat. Since the lowest harness height is 7 inches, I have hopes that this seat will fit average sized newborns that are at least 18 inches long.
Infant
Britax One4Life Slim rear facing 6 month old with lower insert
This model is 6 months old, weighs 18 pounds, and 27 inches tall. He is a big fan of this seat and was happy to show us how well he fit with and without the insert. As the person harnessing him, I preferred the fit without the insert. I found that even when I fully loosened the straps, there wasn’t a lot of slack to maneuver around his arms and legs. Using it without the insert gave me just a little bit more strap length to work with.
Britax One4Life Slim rear facing 6 month old no insert
Here we see our same delightful 6 month old model without the insert. He’s equally happy but has a slightly better fit without it. As always, we suggest that caregivers try the fit with and without optional inserts to see which works best for the kiddo at the time.
One4Life Slim 18 month doll
Our 18 month old doll is nearly weightless and 31 inches long and fits very well rear facing in this seat.
4 Years Old
Britax One4Life Slim rear facing 4 year old with ARB
This model is 4 years old, weighs 38 pounds, and is 42 inches tall. She still fits quite comfortably rear facing and is used to finding all sorts of creative places for her long legs since she still rides like this in many vehicles.
Forward Facing
4 Years Old
Britax One4Life Slim 4 year old forward facing
Our same 4 year old model is still 42 inches and 38 pounds. Forward facing is still somewhat of a novelty for her. She has a lot of room to grow in this seat in forward facing mode with a top harness height of 18 inches.
Booster Mode
Britax One4Life Slim 5 year old booster mode
This rider is 5.5 years old. He is 45 inches tall and 46.5 pounds. He needed help buckling but was able to unbuckle by himself. The One4Life Slim provided a great booster fit for this rider. The lap belt portion of the seatbelt sat properly across the lower hips and the shoulder belt portion of the seatbelt sit nicely between the neck and shoulder on the center of the collar bone.
Important Info: Where to Find
Britax One4Life Slim manual storage
Manual storage: The manual stores in the pocket behind the cover on the lefthand side of the car seat.
Britax One4Life Slim FAA label
FAA approval: The seat is approved for use on aircraft in harness mode. The approval label is found on label by the shoulder on the outside of the shell.
Britax One4Life Slim date of manufacture label
Date of manufacture label: The seat’s lifespan is 10 years. The label is found on the seat pan under the fabric on the lefthand side of the ClickTight panel.
Britax One4Life Slim lower anchor storage
Lower anchor storage: The lower anchors are stored under the ClickTight panel in the rear center of the seat, we do suggest leaving them there and using the vehicle seat belt instead. The lower anchors are not the recommended installation method, Britax strongly recommends that caregivers use the vehicle seat belt.
Britax One4Life Slim top tether storage
Top tether storage: the top tether is stored on the back of the car seat, in the center of the seat at the bottom of the shell.
Overall Thoughts
Britax One4Life Slim cuteness overload
When a manufacturer releases a similar seat to one that’s already on the market, we sometimes wonder what need the new seat will meet that the original did not. Well! We found that Britax took the best parts of the One4Life and created an even better version in the One4Life Slim. It lives up to the name Slim. I found the seat worked really well for infants through booster age children. I personally use car seats for multiple age/size children and this seat was a dream to convert from rear facing to forward facing thanks to Britax’s ClickTight installation. Note: our particular seat was involved in the recall. Our seat is registered and we expect to receive the remedy kit soon. When we do, we will add that experience to this review.
