Thanks for your figures. Now I can be more specific to your particular "what if". Since I'm still making estimates, feel free to adjust the calculations as needed.
For the sake of argument, let's assume that your SS benefits are 50% taxable. (It could be as high as 85%.) Let's also assume that you're working part time until age 70 so that we don't have too many different intervals to compute.
Finally, let's assume a fed marginal tax rate of 22% and a state marginal rate of 6% for a combined 28% rate. We have to pick some figure to work with to make this concrete.
I gather from your followup that you need an extra $20K/year (after tax) over and above your part time income for your expenses until you fully retire (assumed age 70). The fact that you'd be drawing from your IRA for this money suggests no money in taxable accounts - since that's what
conventional wisdom says to deplete first.
So if you defer benefits, you'll need to draw $27,778/year from the IRA. (72% of this gives $20K post tax).
OTOH, if you take benefits at age 67, you'll get to "bank" $14,028/year, assuming you bank it in that same deductible T-IRA:
$35K pre-tax SS benefits = $20K for expenses + $14,028 to IRA + $972 taxes
[ net taxes = tax on SS benefits - tax savings on deductible IRA contribution
$972 = (28% x 1/2 x $35K) - (28% x $14,028) = $4,900 - $3,928]
So from age 67 to age 70, you're either reducing your IRA by $27,778/year or growing it by $14,028. That's a difference of $41,806/year for three
years. In pretax dollars. In post-tax dollars (72%), that's $30.1K.
I already explained how to account for the growth of this amount
in an earlier post in this thread. So I'll just give the results here:
Expected value of $30.1K (post-tax) difference/year over three
years: $104K (
portfolio visualizer), $93K in real (inflation adjusted) dollars.
After age 70, if you've deferred SS, you'll be receiving 132%/108% x $35K, roughly $42.8K/year.
Compared with the $35K you'd get by starting at age 67, that's a $7.8K/year difference pre-tax, real dollars. Post tax, the difference is $7.8K - (28% x $7.8K x 1/2) = $6.7K/year.
The extra savings and growth ($93K real dollars to age 70) that you get by taking SS at age 67 can on average be expected to cover this $6.7K shortfall through age 87 (
portfolio visualizer).
That's a tad under what the new RMD table1 (
single life)
gives as the expected lifetime (88.2) for someone now age 67.
The comment "by the way your dead at the end anyway" suggests that you're not giving much weight to the risk of loss once you're dead, since, well, you're dead anyway. OTOH, the risk of having less money while you're alive is going to matter. A risk averse person who values these two risks (dying before "breaking even", and living "too long") differently will tend to make the choice that reduces the more important risk.
In addition, the estimate that by deferring benefits one will begin pulling ahead around age 87 is a result subject to wide variations. Maybe the market will not produce 7%/year (the figure I used as input), maybe it will swoon early in your drawdown period. Maybe you'll do much better, maybe you'll do much worse.
In contrast, SS is a steady (inflation adjusted) income stream. All else being equal, the risk averse person will take the sure thing.
A final note on the numbers. The calculations above incorporate the effect of taxes and account for investment growth. Your potential shortfall by taking benefits at age 67 and living to 100 still comes out to nearly triple magnitude you hypothesized: ""$10k, 20k, 30k maybe?" Or maybe $87K (13 x $6.7K, post tax,
in real dollars).