The gross estate includes the value of all property that the deceased owned at the time of death and will also include the following items, which are generally classified as non-probate property:
Life insurance proceeds payable to the estate or, if the deceased owned the policy, to the beneficiaries or heirs, (When is life insurance subject to the federal estate tax? Read more here.), the value of certain annuities payable to the estate or the deceased's beneficiaries or heirs, the value of certain property the deceased transferred within 3 years of death, and property held by a trust established by the deceased or by someone else in which the deceased has certain powers over the disposition or use of the trust property.
The allowable deductions used in determining the taxable estate include the marital deduction, funeral expenses, charitable deductions, and estate administration expenses. (Read more at the IRS site)
The marital deduction is subtracted by the Federal Estate Tax Calculator below. This amount is generally the value of all the property that passes from the estate to a surviving spouse. If you are uncertain about the amount being given to a surviving spouse and do not have a valid will, open the Intestacy Calculator℠ for your state of interest and see how an estate is divided with just a few clicks.
Of course, this is a simplified presentation of a complex matter and is designed to give only a general idea of the topic. Factors such as the gift tax, generation skipping transfer tax, and, in recent years, portability of the estate tax exemption, will affect the amount of federal estate tax that is ultimately due.
See Also: Table of Federal Estate Tax Rates