" Even a pair of 70 yr olds getting the max $5108/mo CURRENTLY pay ZERO in federal income taxes."
Two things.
One, that's how it already is: if your only income is SS, you pay no federal income tax.
Two, if you have other income, it starts making SS taxable very quickly, where the first level is 35% of SS becomes taxable, then 50% taxable at the second level, then 85%. And if you're in a pocket where that percentage jumps levels, you might go from an expected 12% marginal rate to, say, a 31% rate on that last $1000.
Bottom line: Social Security, by itself, isn't taxable. It's OTHER income which makes it taxable. Generally speaking, the higher the other taxable income, the larger percentage of SS will be taxable.
" run some numbers using that info?"
Okay, let's add stuff:
Still using 70yr olds w/max SS. $122,600.
Add $13,000 in wages, add $2,400 in overtime, add $25,000 in tips.
$163,000 of income. ZERO federal income taxes due.
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That said, the above scenario would pay $3100 in PAYROLL taxes during the year.