Service companies project estimation paradox

Hi guys, My question is that when the client releases the requirements for the project, in our company we have to estimate and then give those estimates to the client, if they are happy, they give us a project else look for a different service provider. But how can I give 100% proper estimation when I have never worked on that project, currently there is a project where we have to upgrade Java version of the app, I don't know how many things will break once I do it and the Project manager wants me to estimate it. But I don't know how can I give an estimate to him, what if I give very few days and it turns out many things are breaking, the PM will blame me then for giving poor estimate. He is already trying to go aggressive on timeline so that our company can get the project. How do you folks in service based companies deal with these estimation related issues when you haven't worked on the project because the client will give us contract based on those estimation only. Does the contract become void if we are not able to deliver on those estimates, who takes the blame for these?

boredcoder

Freelancer

6 months ago

currently we are doing similar kind of migration in our project. Need to check the current version used in project and targetted version. You can take some more information about tech stack currently in use. List down the baward incompatible changes, deprecated things. This two majorly where you will have to work upon. Rest you will have to do some guess work according to tech stack, this will require some experience. Try to fetch more information about things which are in production right now

boredcoder

Freelancer

6 months ago

*backward

Code32

Coforge

6 months ago

It does not matter that much, make a rought estimation , try to add as much hours/day in it. Then add buffer time and then submit to the manager. If he still asks for time reduction, ask him for more PPL , anyways some what here n there is okay and understandable