Car Seat Guide Calculator

Car seat sizing matches a child’s weight, height, and age to the correct car seat category and installation method. Using the wrong seat, or installing the right one badly, is behind a large share of child injuries in vehicle crashes.

Seat Categories by Size:

Weight and Height Check Formula:

Child is ready to advance when EITHER:

• Weight exceeds the seat’s rated maximum, OR
• Head is within 2.5 cm (1 inch) of the top of the seat shell

The one exception to that, and it matters:

Leaving the booster is the only step decided by height and age alone. A heavy child is not a big child. The lap belt has to sit low on the upper thighs and the shoulder belt has to cross the collarbone, and whether it does depends on the length of the child’s legs and the shape of their hip bones, not on what they weigh.

The number to reach is 145 cm (4 ft 9 in), usually somewhere between 8 and 12 years old. A short, heavy seven-year-old still needs a booster, and there are high-weight boosters rated past 45 kg for exactly that child. Put an adult belt on them early and it rides across the abdomen, which is the injury mechanism the booster exists to prevent.

Worked Example:

Child: 3 years old, 16 kg, 95 cm tall.

• Not yet at convertible rear-facing weight limit (18–23 kg)
• Head below top of shell
• Keep rear-facing, which is the safest position even at age 3

At 18 kg or when head reaches limit: switch to forward-facing with 5-point harness.

LATCH Weight Limit:

The LATCH system (Lower Anchors and Tethers for Children) is rated to child + seat weight of 29 kg (65 lbs). Above this, use the seatbelt to install even if LATCH hardware is present.

Practical Tips:

• Rear-facing is always safer until the child outgrows the rear-facing weight/height limit
• Harness straps should be snug: you should not be able to pinch the webbing at the shoulder
• Get a certified car seat technician to check your installation. Errors are extremely common, and free check stations exist in most areas

Infant seat or convertible? That one is about hardware, not safety

Worth separating the two decisions this page makes, because they are not the same kind of decision.

Rear-facing versus forward-facing is a safety call, and the answer is to stay rear-facing as long as the seat allows. Infant carrier versus rear-facing convertible is a hardware call: both are rear-facing, and moving between them is about which seat still fits the child, not about how protected they are. The calculator splits them at 13 kg (30 lbs) or 83 cm (32.5 in), which are the figures in the table above.

Those are typical, not universal. Infant carriers on sale run anywhere from 22 to 35 lbs and 30 to 32 inches, and the sticker on the side of your own seat is what actually governs. If the child’s head is within 2.5 cm of the top of the shell, the seat is outgrown whatever the weight says.