You must start take the minimum distribution each year beginning with the year you turn 70 1/2. This minimum amount is calculated by dividing the IRA account balance as of December 31 of the prior year by the applicable distribution period or life expectancy.
The IRS does the math for you; just check out the tables in Appendix B of Publication 590 on their site.
- Table I is used by beneficiaries.
- Table II is for use by owners who have spouses who are both the IRA's sole beneficiary and who are more than 10 years younger than the owner.
- Table III is for use by all other owners.