Short answer
In the Hanafi school the beard should be at least one fist long, measured below the chin. Whatever grows beyond a fist may be trimmed, as the Companion Ibn Umar did. Trimming it shorter than a fist is not allowed. Shafi'is prefer leaving it untouched, and Malikis allow trimming when it becomes very long.
Leaving the beard to a fist is wajib. Trimming what is beyond a fist is permitted, and many Hanafi texts call it sunnah. The cheek hair above the beard line and the hair on the neck below the jaw may be tidied.
Other madhhabs
- Shafi'i
- Al-Nawawi preferred leaving the beard as it is without trimming, though taking from its length and sides is not forbidden.
- Maliki
- There is no fixed fist measure; trimming is fine when the beard becomes so long that it looks unkempt.
Explanation
The beard is the hair that grows on the jaw, the chin and the cheeks, including the small tuft under the lower lip. The Hanafi measure is the qabdah: grip the beard in your fist below the chin, and whatever is in or below your fist must stay. Ibn Umar, who narrated the command to grow the beard, used to hold his beard after Hajj or Umrah and cut what went beyond his fist. The Hanafi scholars took his practice as the explanation of the command.
So the Sunnah is not an endless beard and not a short one either. Keep it to a fist or more, and shape the sides so they are in proportion. Stray hairs high on the cheeks may be trimmed so the beard line looks clean, and Abu Yusuf held there is no harm in removing the hair on the throat below the jaw.
Looking after the beard is part of the Sunnah too. Keep it clean, comb it and oil it. Use a fragrance if you like. Wash through it in wudu, and run wet fingers through a thick beard (takhlil), which is a Sunnah of wudu; see how to make wudu.
If your beard is naturally thin, patchy or short, there is no sin: you are only asked to leave what Allah grows. You do not need a transplant or a growth product. For the ruling on shaving, see is shaving your beard haram.
The evidence
Quran, Surah Al-Ahzaab 33:21
لَّقَدْ كَانَ لَكُمْ فِى رَسُولِ ٱللَّهِ أُسْوَةٌ حَسَنَةٌۭ لِّمَن كَانَ يَرْجُوا۟ ٱللَّهَ وَٱلْيَوْمَ ٱلْءَاخِرَ وَذَكَرَ ٱللَّهَ كَثِيرًۭا ﴿٢١﴾
There has certainly been for you in the Messenger of Allah an excellent pattern for anyone whose hope is in Allah and the Last Day and [who] remembers Allah often.
The Prophet's appearance, including his full beard, is a model for believers.
Sahih al-Bukhari 5892
Sahih... خالفوا المشركين وفروا اللحى وأحفوا الشوارب ...
"Be different from the idolaters: let the beard grow fully and trim the moustache short."
The command, after which the same report describes Ibn Umar cutting what went beyond his fist.
Sahih Muslim 261
Sahih... عشر من الفطرة قص الشارب وإعفاء اللحية ...
"Ten things are part of the fitrah (natural way), among them trimming the moustache and leaving the beard to grow."
Places the beard among the practices of the natural way of the prophets.
Sunan Abi Dawud 4163
Graded hasan sahih... من كان له شعر فليكرمه ...
"Whoever has hair should look after it well."
Shows that grooming the hair and beard is encouraged.
Where the ruling is recorded
- al-Durr al-Mukhtar with Radd al-Muhtar, Kitab al-Sawm, chapter on what does and does not break the fast. The fist measure and the permission to trim what is beyond it.
- al-Fatawa al-Hindiyya, Kitab al-Karahiyya, chapter on circumcision, hair and nails. Trimming beyond a fist and removing the hair of the throat.
- al-Majmu' (Nawawi), Kitab al-Taharah, chapter on the siwak and the practices of the fitrah. The Shafi'i preference for leaving the beard untouched.
This answer is pending review by the scholars on our team. 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.