If both I and my husband paid voluntary income tax on social security benefits throughout 2018, but on 1040 3/4 of benefits are taxable income? Why?

UPDATED FOR TAX YEAR 2019

Whether your Social Security benefits are taxable depends on how much other income you have. 

The voluntary income tax payments you had withheld from your social security are included as a credit for taxes paid on your tax return on line 17 of your 1040

Here’s a quick way to find out if you have to pay taxes on your Social Security benefits: Add one-half of the Social Security income to all other income, including tax-exempt interest. Then compare that amount to the base amount for your filing status. If the total is more than the base amount, some of your benefits may be taxable.

Base Amounts

• $25,000 – if taxpayers are single, head of household, qualifying widow or widower with a dependent child or married filing separately and lived apart from their spouse for all of the year
• $32,000 – if they are married filing jointly
• $0 – if they are married filing separately and lived with their spouse at any time during the year

Here is an interactive tax assistant tool from the IRS to help you fully answer the question. Click here: Are my Social Security Benefits Taxable 

[Edited | 4/15/2020 | 8:58am PDT]