How to forecast with higher 401k

[hector-pujol]

How do I model a change to my 401k contribution? 

I have my 2018 taxes done and I'm thinking of increasing my 2019 401k contributions but want to see how that will affect my 2019 taxes (assuming 2019 laws are the same as 2018).

I tried increasing the amount of box 12, Code D on the W2 but didn't see a change reflected in the taxes.

[SteamTrain · Accepted solution]

...don't do it on your actual 2018 tax forms..leave that alone..make sure it is back the way you started back the way it was hen you filed...and saved that way

.....and then do a Save-As to some test file name. and only mess with that.

when you increase your box 12 amount, you must also reduce your box 1 value.  the box 1 value has the major effect.....the box 12 value for that particular entry is primarily an explanation for why box 1 is what it is.   Other  box 12 values can also affect box 1.  So if you increase your box 12 code D by $1,000, you then reduce box 1 by $1,000