Guide: 18650 vs 21700
The 18650 and 21700 families differ first in form. The 18650 model names in the catalog data are NCR18650GA, NCR18650BD, and UR1865ZM2, at a family nominal voltage of 3.6 V. The 21700 and 22700 model names are NCR2170M, NCR2170N, and NCR2270A, at a nominal voltage of 3.6 V with 3.62 V noted for one series variant.
Dimensions are published for the 21700 and 22700 models on their own page, from the legible part of the catalog data. Dimensions for the 18650 models are not republished here, because the archived 18650 columns do not align cleanly and this guide does not resolve that alignment by assumption.
Per-model capacities are held back for the 18650 family on this site, and no capacity or cycle-life figure is published for the 21700 family either. A comparison that needs capacities must come from datasheets for named models, not from this guide.
For a cylindrical choice, name the model, ask for its datasheet, and confirm the exact model, the capacity, the dimensions, and the ordering steps. Do not choose a model from a family summary alone.
What to confirm before ordering
Which family fits the question? Which listed name, if any, is in question? What documentation is available on request? What are the ordering steps for the named item?