The answer depends on how on much you're planning to leave your heirs.

First off, retirement accounts such as 401(k)s, 403(b)s, traditional and Roth IRAs, and others are indeed a part of your taxable estate.

However, unless the total assets of your estate plus any taxable gifts you've already given exceed the lifetime exclusion amount, the IRS can't touch a penny. In 2019, this is $11,400,000, and in 2020, the exclusion is rising to $11,580,000. If you add up the entire value of your assets, only the amount in excess of the exclusion will be taxable. In other words, if you have a $12,000,000 estate and pass away in 2020, just $420,000 of your assets would be subject to estate taxes.

As another example, if your assets (including your retirement savings) add up to say, $5 million, your heirs won't have to pay any estate tax at all.

However, although they may not have to pay estate taxes, it's important to remember that withdrawals from most retirement accounts -- with the exception of Roth accounts -- will be considered taxable income. So, estate tax or not, if your heirs are in a relatively high tax bracket, inheriting your retirement savings could certainly add to their tax bill.

Lastly, it's also a good idea to check with your state. Some states have their own estate taxes and have lower exclusions than the IRS.

Link:
Ask a Fool: If I Leave My Retirement Savings to My Heirs, Will They Pay Estate Tax? - The Motley Fool