Can I make dua silently in my head?

Allah hears what's in your heart

Allah knows your thoughts before you put them into words, so a dua that only happens in your mind still reaches Him. The Quran tells us to call on Him quietly:

Prophet Zakariya, praying for a child in old age, "called to his Lord a private supplication" (Quran 19:3), and his dua was answered. When some Companions shouted their dhikr on a journey, the Prophet ﷺ told them, "you are not calling a deaf or an absent one, but One Who is with you" (Sahih al-Bukhari 2992).

When a silent dua is perfect

• In public: on the train, in the office or in a meeting, you can ask Allah for help without anyone noticing.
• Private matters: some things are too personal to say out loud, even alone.
• Moments of panic: before an exam, an interview or a hard phone call, a quick "O Allah, help me" in your heart counts.
• Places where you don't speak dhikr: such as the bathroom. See Can I do dhikr in the bathroom?

In a hadith qudsi, Allah says, "If he remembers Me in himself, I too, remember him in Myself" (Sahih al-Bukhari 7405).

Inside the prayer

Salah is different. For the recitation and the words of the prayer, most scholars say thinking them isn't enough: you need to move your tongue and lips, even if no one hears you. Some, including many Hanafi and Shafi'i scholars, say you should say them loud enough to hear yourself if nothing prevents it. Personal duas in sujood can be whispered the same way. See Can I make dua during sujood?

Adhkar with a promised reward

Many hadith say "whoever says" a certain phrase gets a certain reward, such as "SubhanAllah" 33 times after prayer. Many scholars hold that to earn that specific reward you should actually say it, at least in a whisper. Remembering Allah in your heart is still worship and still rewarded, just not counted as "saying" the dhikr. See What should I say after every prayer?