Car Seat Stage Calculator
Whether your child still fits, and what comes next
The principle behind every stage
One rule underlies all car seat guidance: keep your child in each stage until they reach the maximum height or weight for that seat, then move to the next. Moving up is not a milestone to reach early; each transition is a step down in protection.
The specific limits differ between seats and countries, so your seat's own manual and your local regulations are authoritative. What follows is the general framework.
Rear-facing, for as long as possible
Rear-facing is substantially safer for young children, and the reason is anatomical. A young child's head is proportionally large and heavy while their neck is comparatively weak. In a frontal crash — the most common serious type — a forward-facing child is thrown forward while the harness holds the body, placing enormous force on the neck and spine.
Rear-facing spreads that force across the whole back of the seat and the child's back, dramatically reducing head and spinal loading.
Most guidance now recommends rear-facing until at least age two, and many bodies recommend until age four or until the seat's rear-facing limit is reached. Some countries, notably in Scandinavia, routinely rear-face to age four and have markedly lower child injury rates.
Legs touching the seat back or being bent is not a reason to turn a child forward. Children are comfortable in that position, and leg injuries in rear-facing crashes are rare compared with the head and neck injuries prevented.
Forward-facing with a harness
Once a child genuinely exceeds the rear-facing limits, a forward-facing seat with a five-point harness is next. This should be used until they reach the seat's height or weight maximum, commonly around 40–65 lb depending on the model.
Key fitting points: the harness straps should come from at or above the shoulders when forward-facing, the chest clip sits at armpit level, and the harness should be tight enough that you cannot pinch a horizontal fold of webbing at the shoulder.
Bulky coats must not be worn under the harness. They compress in a crash, leaving the harness dangerously loose. Strap the child in first, then put the coat on backwards over the harness or use a blanket.
Boosters and the five-step test
A booster raises the child so the adult seat belt sits correctly. It is not a small seat — it is a belt-positioning device, and it requires the child to sit properly for the whole journey, which is why maturity matters as much as size.
Before moving out of a booster entirely, a child should pass all five parts of the five-step test:
- They can sit all the way back against the seat back.
- Their knees bend comfortably at the seat edge.
- The lap belt sits low across the tops of the thighs, not the stomach.
- The shoulder belt crosses the middle of the shoulder and chest, not the neck or arm.
- They can stay seated this way for the entire journey without slouching.
Most children do not pass all five until somewhere between 10 and 12 years old, and typically around 4 ft 9 in tall — considerably later than most families move them.
Fitting mistakes that matter
Studies of car seats in use consistently find high rates of misuse. The most common problems:
- Harness too loose — the pinch test is the check
- Chest clip too low; it belongs at armpit level
- Seat installed loosely — it should move less than an inch at the belt path
- Wrong recline angle for a newborn, risking airway obstruction
- Bulky coats under the harness
- Aftermarket inserts and strap covers not supplied with the seat
Many areas offer free car seat checks by trained technicians. It is genuinely worth using one — a correctly fitted seat performs very differently from a nearly correct one.
Buying and replacing
Buy car seats new. Second-hand seats carry unknown crash history, and a seat involved in even a moderate collision should be replaced. Seats also have expiry dates, typically six to ten years from manufacture, because plastics degrade.
Always check the seat is approved for use in your country and is compatible with your vehicle — not every seat fits every car, and the manual for both matters.
Common Questions
How long should a child stay rear-facing?
Most guidance recommends at least age two, and many bodies recommend until four or until the seat's rear-facing limit is reached. Rear-facing is substantially safer because it spreads crash forces across the back rather than loading the neck.
Are bent legs a reason to turn a child forward-facing?
No. Children are comfortable with bent or crossed legs, and leg injuries in rear-facing crashes are rare compared with the head and neck injuries that rear-facing prevents.
When can a child stop using a booster?
When they pass all five parts of the five-step test - sitting fully back, knees bending at the seat edge, lap belt low on the thighs, shoulder belt across the mid-shoulder, and staying seated properly for the whole journey. Usually between 10 and 12 years old.
Can my child wear a winter coat in the car seat?
No. Bulky coats compress in a crash, leaving the harness dangerously loose. Strap the child in first, then put the coat on backwards over the harness or use a blanket.