For a linear synthesis, you multiply the yields of every individual step.

For example, for a synthesis like this : A ---> B ---> C ---> D with yield of each step being AB : 80 %, BC : 50%, CD : 50%, the overall yield of A ---> D will be 0.80*0.50*0.50 = 20%

For a convergent synthesis, you multiply the yields of every individual step in the longer linear sequence :

For example you have this synthesis :

A ---> B ---> C

D ---> E

C + E ---> F

In this case you have 2 linear sequences that are A ---> B ---> C ---> F and D ---> E ---> F. The longest linear sequence in this convergent synthesis is A ---> B ---> C ---> F, and to calculate the overall yield of this convergent synthesis, you need to calculate the overall yield the linear sequence A ---> B ---> C ---> F.

For a linear synthesis, you multiply the yields of every individual step.

For example, for a synthesis like this : A ---> B ---> C ---> D with yield of each step being AB : 80 %, BC : 50%, CD : 50%, the overall yield of A ---> D will be 0.80*0.50*0.50 = 20%

For a convergent synthesis, you multiply the yields of every individual step in the longer linear sequence :

For example you have this synthesis :

A ---> B ---> C

D ---> E

C + E ---> F

In this case you have 2 linear sequences that are A ---> B ---> C ---> F and D ---> E ---> F. The longest linear sequence in this convergent synthesis is A ---> B ---> C ---> F, and to calculate the overall yield of this convergent synthesis, you need to calculate the overall yield the linear sequence A ---> B ---> C ---> F.

Marc Vidal rightly said

Find the below link for ur ready reference.

http://chemistry.syr.edu/totah/che276/support/6a1.handouts/5.yield.pdf

Thanks Mark

For calculating over all yield say in 'n' steps, multiply all percentage yields in all steps and divide by 100n-1, thats all.

Selva Chemist, I am afraid you're not quite right. You calculated the AVERAGE yield, not the overall yield. Marc Vidal is right, you should the yield of every step as fractions of 1, multiply them (but only over a linear sequence) and multiply by 100% at the end.

See https://www.researchgate.net/publication/262180165_New_One-Pot_Methods_for_Preparation_of_CyclohexanecarbonitrileGreen_Chemistry_Metrics_Evaluation_for_One-Pot_and_Similar_Multi-Step_Syntheses

Article New One-Pot Methods for Preparation of Cyclohexanecarbonitri...

Please find the following link

http://www.ch.ic.ac.uk/local/organic/tutorial/2.O1%20part%203%20lecture%206%20(DC).PDF.

thanks Naresh Kumar Sangwan 

mulitiply all yields as fractions. i.e 80%, 70% and 90%= 0.8x0.7x0.9= 0.504 which is 50.4%