Answered by Shaykh Yusuf Weltch
Question
I have about 17 years’ worth of missed prayers. But in that 17 years, I was praying on and off. So I did not forget all of it entirely. How should I calculate how many prayers to make up? And what happens to those extra prayers if the amount estimated and made up is more than what I missed?
Answer
In the Name of Allah, the Most Merciful and Compassionate
Please see the below-given link for information on calculating missed prayers.
Estimating the Number of Makeup Prayers
Extra Prayers
If, in the end, you made up more prayers than were necessary, those extra prayers will be taken into account as extra voluntary prayers, and you will receive their reward on the Day of Judgment. [Maydani, al-Lubab fi Sharh al-Kitab]
This is why it is safer and more cautious to pray more prayers than to pray less. As you will see in the answer linked above, you should add to your final calculation for the sake of caution.
Hope this helps
Allah knows best
[Shaykh] Yusuf Weltch
Checked and Approved by Shaykh Faraz Rabbani