Most dogs are physically fully grown sometime between 8 and 24 months, with smaller breeds maturing sooner and giant breeds taking the longest. Emotional and behavioral “grown up” maturity often lags behind physical growth and may not appear until around 2–3 years of age.

Key age ranges

  • Toy & small dogs (up to ~24 lb): usually reach adult size around 8–12 months.
  • Medium dogs (~24–59 lb): often fully grown by about 12–15 months.
  • Large dogs (~59–99 lb): typically stop most growth around 15–18 months and keep filling out into year 2.
  • Giant breeds (100+ lb): can keep growing until about 18–24 months, sometimes even closer to 3 years for full muscle “fill out.”

Even after height growth stops, many dogs continue to build muscle and body condition for several more months.

Physical vs. mental adulthood

  • Physical maturity is when the growth plates in the bones close, which is what really defines when dogs stop getting taller.
  • Mental and behavioral maturity (calmer behavior, better impulse control) often arrives later; many owners say their dogs do not truly “grow up” until around age 2 or slightly beyond.

Simple HTML table for sizes

html

<table>
  <thead>
    <tr>
      <th>Dog size</th>
      <th>Approx. adult weight</th>
      <th>Typical age fully grown</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Toy</td>
      <td>5–12 lb</td>
      <td>8–12 months</td>
    </tr>
    <tr>
      <td>Small</td>
      <td>12–24 lb</td>
      <td>8–12 months</td>
    </tr>
    <tr>
      <td>Medium</td>
      <td>24–59 lb</td>
      <td>12–15 months</td>
    </tr>
    <tr>
      <td>Large</td>
      <td>59–99 lb</td>
      <td>15–18 months</td>
    </tr>
    <tr>
      <td>Giant</td>
      <td>100+ lb</td>
      <td>18–24 months</td>
    </tr>
  </tbody>
</table>

What this means for your dog

  • Check your dog’s expected adult size (toy, small, medium, large, giant) to estimate when they’ll be fully grown.
  • Regular vet checkups help confirm whether growth is on track and whether the growth plates have likely closed.
  • Plan to feed puppy food and avoid intense high-impact exercise until your vet is confident growth is nearly complete, especially for large and giant breeds.

TL;DR: Most dogs are fully grown in height by 1–2 years, but big breeds can take up to 2–3 years to finish filling out and acting like true adults.