Short answer
His hair was wavy, neither tightly curled nor straight. It usually reached between his earlobes and his shoulders, he parted it in the middle, and at his death he had fewer than twenty white hairs in his head and beard.
The Sunni account, from Anas, al-Bara and Ibn Abbas in Bukhari, is that his hair was wavy, of medium length, sometimes touching his shoulders, parted in the middle, with very few white hairs.
The four schools agree on this.
Explanation
Anas ibn Malik described the Prophet's ﷺ hair as "not tightly curled and not lank", meaning a gentle wave. In another report Anas said his hair touched his shoulders. Other reports describe it reaching his earlobes or the middle of his neck. Scholars explain that the length changed at different times: longer when he had not cut it for a while, shorter after Hajj or Umrah when he shaved or trimmed it.
Ibn Abbas reports that at first he let his hair fall forward, as the People of the Book did, and later he parted it in the middle. He also took care of it: Aisha said she used to comb his hair.
Even in old age he had very little grey. Anas said that when he died there were not twenty white hairs in his head and beard. His beard was thick and full.
On the ruling side, scholars say keeping the hair long like the Prophet ﷺ, or shaving it, are both allowed for men, and following his way out of love is rewarded. Questions about cutting hair are in the dress section. For his complexion see Was Prophet Muhammad white or black?
The evidence
Sahih al-Bukhari 3547
Sahih... ليس بجعد قطط ولا سبط رجل ...
"Not tightly curled and not lank."
Anas describes the wave of his hair.
Sahih al-Bukhari 5903
Sahih... يضرب شعره منكبيه ...
"His hair touched his shoulders."
Anas on the length of his hair at times.
Sahih al-Bukhari 3558
Sahih... ثم فرق رسول الله صلى الله عليه وسلم رأسه ...
"Then the Messenger of Allah parted his hair."
He came to part his hair in the middle.
Where the ruling is recorded
- al-Shama'il al-Muhammadiyya (al-Tirmidhi), Chapter on the hair of the Messenger of Allah. Gathers the reports of the length and style of his hair.
- Fath al-Bari (Ibn Hajar), Commentary on Kitab al-Libas, chapter on long hair. Reconciles the different lengths reported.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.