When the total of all your itemized deductions is greater than the standard deduction for your filing status -
Standard deductions for 2018
Single - $12,000 add $1,600 if age 65 or older
Married Filing Separately - $12,000 add $1,300 if age 65 or older
Married Filing Jointly - $24,000 add $1,300 for each spouse age 65 or older
Head of Household - $18,000 add $1,600 if age 65 or older
