How long are tax extensions good for?

Filing tax form 4868

When you file the tax form 4868, Application for Automatic Extension of Time to File US Individual Income Tax Return, you are given an automatic 4 months of extension to file your tax return. You must file this tax extension form before the due date of the tax return which is April 15. When you do, you get an automatic (no questions asked) extension of 4 months.

So, the extended tax filing date is August 15.

Tax extensions for filing of tax returns only

Many people think that getting tax extensions means delaying tax payments. This is not true. The tax extensions are extensions of time to file, NOT TIME TO PAY. You can use the simple worksheet to estimate your tax liability and pay it when you file for a tax extension.

Filing tax form 2688

If you need more than the 4 months extension, then you can file the tax form 2688, Application for Additional Extension of Time to File US Individual Income Tax Return. You need to file this tax form before the extended due date of August 15. The new due date is then October 15 and you have to file your tax return then, if you are granted the additional 2 months extension. This tax extension is not automatic, however, and you have to convince the IRS that you need the extension to file your tax return accurately.