Strength of phosphoric acid?

The correct way to answer this question is to check the Ka for phosphoric acid and compare these values with other acids that we classify as strong - moderate strong or weak

The Ka values for H3PO4 are( remember that this is a triprotic acid:
Phosphoric acid, H3PO4
1st 7.1 x 10-3 That is H3PO4 ↔ H+ +H2PO4 -
2nd 6.3 x 10-8 that is H2PO4 2- →H+ + HPO4 2-
3rd 4.5 x 10-13 that is HPO4- → H+ + PO4 3-

How does this compare with other acids:
First H2SO4 - a strong diprotic acid

1St H2SO4 ↔ H+ + HSO4- large I have seen 12-15 quoted
2nd 1.2 x 10-2 that is HSO4- ↔ H- + SO4 2-

We can conclude that H3PO4 is a much weaker acid than H2SO4

But is it a weak acid? Check H2SO3 which is a moderate acid:
Sulphurous acid, H2SO3
1st 1.2 x 10-2
2nd 6.6 x 10-8

Phosphoric acid is weaker than H2SO3

What about H2CO3
The 1st Ka = 4.3*10^-7

This makes H3PO4 much stronger than H2CO3

The conclusion must be that H3PO4 is a moderate strong acid.

Phosphoric acid is a non-toxic, inorganic, rather weak triprotic acid. Therefore, it is a mineral and weak acid.

NO man it is weak acid and to determine its strength its formula is
mass of solute(here H3PO4) / volume of solution in which it is present ;)

Phosphoric acid is a strong acid. Because phosphoric acid has a high dissociation constant, meaning that most of it is converted to hydronium ions and phosphate anions.