"12–18 months" describes an average child who may not resemble yours. It is the least reliable sizing information in any category on this site.
Clothing
Measure the child — chest, waist, height, and inside leg — and then ask for the garment measured flat: chest, length, sleeve, inside leg.
Comparing a child to a label is guesswork. Comparing two measurements is not.
Growth also means a garment that is slightly large is useful and one that is slightly small is not, which is the one place where erring has an obvious direction.
Shoes
Inside length, in centimetres, not the printed size. Measure the child's foot standing, add a little for growth, and compare to the inside length of the shoe.
Printed children's shoe sizes differ across makers more than adult sizes do.
Prams, cots and car seats
Here the measurement that matters is not the child but the space:
- Pram: folded dimensions, and whether it fits your boot and your doorway
- Cot: external dimensions, and the mattress size it takes
- Car seat: the fitting system, and whether it suits your vehicle
Ask for folded and unfolded measurements. Questions to ask.
Before any sizing question
Recall status, completeness of safety parts, and dates. Those come first and can stop the sale entirely. Prohibited items.
What protection covers
Measurements materially wrong or a different item than shown: seven days from delivery. A garment that your child has outgrown by the time it arrives is not.
Questions people ask
Should I buy a size up?
Often sensible for clothing. Not for shoes, where fit matters for a growing foot.
Do age labels differ by brand?
Considerably. Treat them as a hint.
How do I measure a child's foot?
Standing, heel against a wall, longest toe to the wall.
More: buying baby items, what they are worth.