Roth Conversions: How Much Can You Convert?

By Alamzeb Khan · Updated August 2026 · 6 min read · Roth Conversion Calculator →

The idea in one line

Move money from a Traditional account into a Roth, pay the tax now while your rate is low, and never pay tax on it again.

Simple enough. The difficulty is that the converted amount counts as income this year - so converting too much pushes part of it into a higher band, taxed at exactly the rate you were trying to avoid.

The room you actually have

On a $40,000 taxable income filing single, the 12% band runs up to $50,400. So there is $10,400 of room before anything gets taxed higher.

Convert within that room, from the Roth conversion calculator:

Every dollar taxed at 12%, and never taxed again. That is what a conversion is for.

What spilling over costs

Now convert $30,000 on the same income:

The average rate on the whole conversion is 18.5% - half again as much as the 12% you were aiming for.

That is not necessarily wrong. If you genuinely expect 24% in retirement, paying 18.5% now still wins. But it should be a decision rather than an accident, and most people converting a round number have never checked where their band line sits.

The mistake that halves the benefit

Where you pay the tax from matters enormously, and it is the easiest thing to get right.

Paying the $1,200 tax from the conversion itself means only $8,800 crosses over. That missing $1,200 would have compounded for twenty years - and its absence halves the benefit.

If you cannot pay the tax from other money, convert a smaller amount instead. Converting $8,800 and paying the tax separately beats converting $10,000 and taking the tax out of it.

When to do it

The whole strategy depends on your rate being lower now than later. Good years to look at:

• Between jobs, or after a redundancy
• Early retirement, before pensions and Social Security start
• A year with a business loss or unusually low income
• Any year your income dips for any reason

That gap between retiring and claiming benefits is the classic window. Income is low, the tax bands are wide open, and there may be years of room before anything forces itself out.

The problem it solves later

There is a second reason, and it grows over time.

From 73 you must take money out of Traditional accounts whether you need it or not, taxed as income. A large balance means large forced withdrawals - see the RMD guide for how big they get.

Those withdrawals can push you into higher bands and across the Medicare income lines, where crossing by one dollar costs over a thousand.

Converting earlier shrinks the balance that will later be forced out. It is genuinely a way of making a future problem smaller.

One thing to be careful about

A conversion cannot be undone. Reversing them used to be allowed and is not any more. Once you convert, the tax is owed for that year.

Also worth knowing: a large conversion this year can raise your Medicare premiums in two years’ time, because that surcharge looks back at your tax return from two years earlier. If you are near 63 or older, check that before converting a large amount.

Before converting

1. Find the top of your current band and work out the room.
2. Convert within it, unless you have a specific reason not to.
3. Pay the tax from outside money. If you cannot, convert less.
4. Check the Medicare lines if you are within two years of 65.
5. Do it in a low year, deliberately.

Work out your own room in the Roth conversion calculator - it shows how much fits before spilling, what the spill costs, and what paying the tax from the pot does.

Common questions about roth conversion