I was born in XXX. Since I became Baligh till year XXX I used to pray but I was not punctual with my prayers. Some days I prayed all five and some days I prayed two three and some days I prayed none. Problem is that I don’t remembers exactly how many prayers I prayed and how many I missed from baligh age till XXX. How can I calculate how many days or prayers I need to make up?
From XXX when I performed Hajj till now I have been praying and did not miss any prayers
Calculation I need is from baligh age till XXX. Being born in XXX

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Shariyat has give a us the option that whatever calculation you have done even you can accept the minimum number in this case and do it again, I mean perform it’s Qaza. So if you guess that for example all total your 11 or 14 years Namaz has been leftover then you can choose 11 years not the highest number that was 13.
Now you please calculate from XXX till XXX because only you know your way of life and schedule you lived with that so you please do calculations. Of course you can’t get exact nunmber or year but as I said if you reached to 2 figures you may choose the least one.

For example 50%-60% Namaz you have prayed in these years from XXX till XXX. Therefore if you have prayed your 50% salat in these 25 years, it means around 12 years Namaz is remaining on your shoulders. You can calculate in this way I hope you will reach to a satisfactory Numbers.