Is there any formula to calculate fillet length?

The fillet lenght is calculated based on the weld possible resistant section, which in its turn depends on the weld geometry.

For the calculations, you need the nominal base material resistance, the weld metal nominal resistance, and check with ASME or any other applicable code in your area which are the allowable loadings in welds for your case.

Then you are able to calculate the required weld lenght and decide about installation of more reinforcements or not. Add coeficients for dynamic loading and service conditions, and voilà!

Yeah, its not so simple as it seems. Do you have the nominal loads to consider? Maybe you could make a try and post your data here so we can check it out.

Remember: if its a critical application, regarding pressure, temperature, in a commercial installation, please refer to the applicable codes and have someone to design and undersign it for you. Its not just a matter of calculating, but assuming legal responsability about it, ok?

Regards.

Mr,bhrescobar is correct post correct figures alongwith stress involved n will let u know the throat thickness , fillet length , ......

Cheers

Mahi