You can use your 2017 tax return to estimate.
Here is how to get an extension using TurboTax:
1. Open
your return in TurboTax Online.
(To do this, sign in to TurboTax and select the Take me to my return button.)
2. Select File an extension at
the bottom of the column on the left side of the screen.
3. If
you don't see the File
an extension button in Step 2 above, search for extend in the upper
right and select the Jump
to link at the top of your search results.
4.
Follow the onscreen instructions to
e-file your extension.
For more information and the source, follow this link: How to File an Individual Extension