How would they even know the post-tax amount? That is going to depend on your unique financial situation.

They set the post-tax amount without knowing the exact pre-tax amount, so whatever the pre-tax amount is they will just pay it. Even if tax rate varies person by person there will not be a huge difference.

I checked one of my offer letter (not from google) and the wording is like "an amount of $XXXXX grossed up for U.S taxes".

>Isn't that the default?

I don't know about the US, but in Europe it was always marked as expenses refunded to me on the payslip, which is tax free.

For relocation costs, those are deductible from your (Federal) taxes under certain circumstances (something like, if you move more than X miles to be closer to a job). If it is just a bonus that is just taxable income.

In the US, if they gave you $1000 straight-up for moving costs, you'd probably see something like:

  1.  Company gives you $1000.
  2.  $750 hits your bank account after withholding.
  3.  You spend $1000 to move.
  4.  You file taxes, claim $1000 deduction for moving expenses.
  5.  You receive $250 refund.

I've seen two options: relocation bonus (pre-tax) or relocation reimbursement (post-tax, usually up to a certain amount).

A hiring bonus would presumably always be pre-tax, since there's no expenses to reimburse.

AFAIK Facebook will let you to choose between taking lump sum (a fixed amount of pre-tax money), or getting flight tickets and moving service ordered for you, plus after-tax reimbursement for other related cost.