Timing depends on size and growth, not the calendar alone
Puppy food is built for construction, richer in the nutrients a growing body needs, so the switch to adult food should follow her development rather than a date circled months in advance. A small breed who has reached her adult size and stopped gaining height is usually ready far sooner than a large breed youngster whose frame is still developing, even if both dogs are the same age. If her weight has levelled off, her growth has slowed and your vet agrees she is approaching maturity, she is probably near the right window. Switching too early risks short-changing a dog who is still building bone and muscle, while staying on rich puppy food long after growth finishes can quietly push her weight up. If you are unsure, ask your vet to judge from her size, breed and body condition rather than guessing from the food bag.
Switch the mix gradually over days
A young dog's digestion handles change best in steps. Begin by replacing a small portion of her puppy food with the adult food, keeping the total amount the same, and hold that mix for a couple of days while you watch her response. Then increase the adult share in stages, moving through roughly half and half before the adult food takes over completely, across a week or longer for a dog with a sensitive stomach. Resist adding toppers, treats or table extras during the transition week, because if her stomach does wobble you want to know which change caused it. If a stage produces consistently loose stools, step back to the previous mix for a few days and move forward more slowly rather than pushing through.
Watch stool, appetite and weight as you go
Your feedback during the switch comes from three simple signals. Her stool should stay formed and easy to pick up, her appetite should remain keen at her usual meal times, and her weight and body condition should hold steady, with ribs easy to feel under a thin covering rather than disappearing or standing out. A dog who repeatedly refuses the new food, vomits, has ongoing loose stools, seems unusually itchy or becomes lethargic during the transition needs the switch paused and a conversation with your vet, since those signs may point to something beyond a simple food change.
After the switch, portion for the adult in front of you
Completing the transition is only half the job. Adult feeding is about maintaining a finished body rather than fuelling growth, so review how much she actually needs once the switch is done instead of automatically continuing puppy-sized portions. Feed to her body condition over the following weeks, adjusting the amount gradually if she starts to look rounder or leaner, and divide her food into regular meals that suit your household routine.
Keep every feeder on the same plan
A gradual transition only works if it is actually gradual in every bowl she eats from. In many homes the plan unravels because one person serves the new mix, another tops up with the old food, and a third adds extras out of habit, so the dog never experiences the steady steps her stomach needed. Write down the stages, the days you plan to move between them and the signals you are watching, and put that note where everyone who feeds her will see it. With the stages written down, her response can be judged against the real plan rather than a guess. KinsLe Family Sharing also lets her owner share that profile with selected family members.