I was looking into how much I could contribute to a Roth IRA based on MAGI. In 2018, my MAGI was below the amount where contribution limits start to decrease, so I should be able to contribute the maximum amount.
However, it was unclear to me from the IRS docs whether the contribution limit for a year is calculated using the previous year's (already known) MAGI or the current year's (as yet unknown) MAGI.
In a way, it makes sense that you'd use the previous year's MAGI - you know it, so the contribution limit is clear. On the other hand, maybe it is the case that contributions for 2019 are limited by 2019 MAGI, even if you don't know it yet.
If it is the latter, the obvious problem I see is that you don't know your 2019 MAGI until the year is over. So, you could accidentally over-contribute. In that case, would a good strategy be to wait until 2019 MAGI is calculated and make your 2019 contribution before tax day 2020? Or would the safest thing to do simply be to take advantage of the "backdoor Roth IRA" strategy?
I ask because my MAGI for 2018 was close to the limit, and I suspect my 2019 MAGI may be above the limit somewhat. Just want to make sure I don't over-contribute and have to deal with that problem!