Diaper size by weight, plus how many you'll go through per day, week, month, and year. Cost too, if you tell us yours.
Diaper count per day is based on AAP voiding norms. By day 4 of life, a healthy term newborn typically produces at least 6 wet diapers in 24 hours; full-term newborns void roughly 20 times per day in small amounts, and that frequency drops gradually with age. By the toddler years, most kids settle at 4–7 voids per day.
At least 6 wet diapers per day after the first week of life is the AAP threshold for "feeding is working." Fewer than that — or no wet diaper for 8+ hours in an older infant — warrants a call to the pediatrician.
Sizing is brand-overlap. Different brands run slightly bigger or smaller; if a diaper leaks, size up. If you see red marks at the leg, size up. Most parents find Pampers and Huggies fit slightly differently — try both.
Sources: HealthyChildren.org (AAP) — "Baby's First Days: Bowel Movements & Urination"; brand size charts (Pampers, Huggies, Honest).