Looking on the internet deeply has found these results:

i will let you know is the most popular phrase on the web.

i shall let you know 15,830 results on the web	i will let you know 128,200,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Should it be incorrect, I shall let you know exactly what is happening, Mr Alavanos. Si no, le dar� la informaci�n real, se�or Alavanos. I shall endeavour to bang�... Apr 21, 2015 ... ... be that this is a contraction of "I will". If you wanted to stress an intention rather than a prediction, you would have to say "I shall let you know". For example: I shall let you know as soon as she arrives. So, when shall is used in the second or third person as in the quiz sentence, it is NOT a synonym for will �... Should it be incorrect, I shall let you know exactly what is happening, Mr Alavanos. Zo niet, zal ik u de correcte informatie doen toekomen, mijnheer Alavanos.	As soon as I have his head in a box, I will let you know. And I will let you know when I'm ready to look at them. And I will let you know if we trip over any treasure chests. I will let you know when it is finished. But whatever I decide... I will let you know. I will let you know if I meet anyone innocent. I will let you know when I'm joking. But I will let you know when you cracked the case.

i shall let you know

15,830 results on the web

i will let you know

128,200,000 results on the web

More popular!

Should it be incorrect, I shall let you know exactly what is happening, Mr Alavanos. Si no, le dar� la informaci�n real, se�or Alavanos. I shall endeavour to bang�...
Apr 21, 2015 ... ... be that this is a contraction of "I will". If you wanted to stress an intention rather than a prediction, you would have to say "I shall let you know".
For example: I shall let you know as soon as she arrives. So, when shall is used in the second or third person as in the quiz sentence, it is NOT a synonym for will �...
Should it be incorrect, I shall let you know exactly what is happening, Mr Alavanos. Zo niet, zal ik u de correcte informatie doen toekomen, mijnheer Alavanos.