Tools/Tax & estate/Mega Backdoor Roth Calculator✓ CHECKED AGAINST WORKED EXAMPLES · SEP 29, 2026

How much can I put into a mega backdoor Roth?

See the room your 401(k) could have for after-tax contributions this year, after your own contributions and the employer’s, and what moving that money into a Roth is worth by retirement.

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Your pay from this employer (before tax)$Per year, from the employer that offers the plan. The total limit cannot be more than your pay.Your own pre-tax and Roth contributions$Dollars a year. The 2026 limit is $24,500 before catch-up. Using the full amount is the usual starting point for a mega backdoor.Employer match rate (% of what you put in)%A sample plan, not yours: 100% up to 6% of pay. Your plan’s summary plan description has the real rule.Match cap% of payOther employer contributions$Profit sharing or other non-match employer money, dollars a year. It uses up room too.After-tax amount you plan to put in each year$An example: what you could afford out of take-home pay. It is cut to the room if higher.Tax a taxable account would pay on the growth%Long-term capital gains are 15% for most people, 20% at high income, before any state tax. Applied to the growth at the end.

Your age at the end of 202634Catch-up contributions from 50 do not count toward the total limit.Retirement age65Expected return (before inflation)7%Inflation is taken out at 3%, so the projection is in today’s dollars.

ROOM FOR AFTER-TAX 401(K) MONEY

$38,800/yr

Your $24,500 of contributions and $8,700 of match use $33,200 of the $72,000 2026 limit, which leaves $38,800 a year you could add as after-tax contributions and move to Roth. This is a ceiling: it only works if your plan accepts after-tax contributions and lets you move them to Roth, either with an in-plan conversion or an in-service withdrawal to a Roth IRA. Putting in $10,000 a year for 31 years grows to about $591,697 in today’s dollars at 3.9% a year after inflation; $42,255 of the growth would be taxed at 15% in a taxable account and is not in a Roth.

Plan-wide limit

$72,000

Already counted

$33,200

A month at full room

$3,233

Tax avoided by 65

$42,255

What uses up the 2026 limit

The plan-wide limit is $72,000. Your $24,500 and the $8,700 match come off it, leaving $38,800.

Roth balance against a taxable account, in today’s dollars

RothTaxable after taxPut in

Putting in $10,000 a year, the Roth is worth $591,697 after 31 years and a taxable account $549,443 once tax on the growth is paid: $42,255 apart, in today’s dollars.

How your own contributions change the room

You contribute	Employer match	Counts toward limit	Room for after-tax
$0	$0	$0	$72,000
$10,000	$8,700	$18,700	$53,300
$20,000	$8,700	$28,700	$43,300
$24,500 (yours)	$8,700	$33,200	$38,800

Every dollar you put in yourself uses a dollar of the limit, and the match on it too, so contributing less leaves more room for after-tax money. At $0 the room is $72,000; at $24,500 it is $38,800.

What moves the needle

Each row re-runs the calculation with one change. Click to apply.

How it's computed

FORMULA

Limit = the lesser of $72,000 and your pay (IRC §415(c), 2026)

Counted = your regular contributions (not catch-up) + employer match + other employer money

Room for after-tax contributions = limit − counted

Roth balance = yearly amount × ((1 + monthly real rate)^months − 1) ÷ monthly real rate ÷ 12 per month, in today’s dollars

Tax avoided = (balance − what you put in) × the rate a taxable account would pay on growth

The plan must accept after-tax (non-Roth) contributions and let you move them into Roth, either by an in-plan Roth conversion or an in-service withdrawal to a Roth IRA. Many plans do not, and some cap after-tax contributions at a share of pay; ask your plan administrator.
The 2026 limits are $72,000 in total (or your pay if less) and $24,500 of your own regular contributions (IRS Notice 2025-67). Catch-up contributions from age 50 are on top and do not use up room.
The match is worked out from the rate and cap you enter on pay up to the plan’s $360,000 limit. The default 100% up to 6% is a sample plan, not yours. Nondiscrimination tests can limit what highly paid employees may put in.
After-tax money that earns anything before it is converted has that growth taxed when converted, so the assumption here is that you convert right away. Converted amounts are not taxed again.
The projection is in today’s dollars at the 7% you enter less 3% inflation. A taxable account is modelled as a flat 15% on the growth at the end; dividends and turnover along the way would make its tax bigger, so the benefit shown is on the low side.
You pay for the after-tax contributions out of take-home pay. Whether that is affordable is your call: the page shows the ceiling and the value of what you actually plan to put in.

WORKED EXAMPLE · SAMPLE NUMBERS

The limit is $72,000. Your $24,500 + $8,700 match + $0 of other employer money = $33,200. $72,000 − $33,200 = $38,800 of room a year. You plan $10,000, so $10,000 is used; $10,000 a year for 31 years grows to $591,697 ($310,000 put in), and 15% of the $281,697 growth, $42,255, is tax a Roth avoids.

SOURCES

[1]Notice 2025-67: 2026 amounts relating to retirement plans and IRAsInternal Revenue Service [2]Retirement topics: 401(k) and profit-sharing plan contribution limitsInternal Revenue Service [3]Notice 2014-54: Guidance on allocation of pre-tax and after-tax amounts in rolloversInternal Revenue Service

HSBuilt by Hussain Sehorewala · checked against worked examples · Sep 29, 2026

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Questions about this result

A way to put much more into a Roth than the regular limits allow. If your 401(k) accepts after-tax contributions, you can fill the space between what you and your employer already contribute and the plan-wide limit of $72,000 for 2026, then move that money to Roth right away with an in-plan conversion or an in-service withdrawal to a Roth IRA.

The plan-wide limit ($72,000 in 2026, or your pay if less) minus your own regular contributions (not counting catch-up), the employer match and any other employer money. On the example on this page, with $24,500 of your own money and a 100% match up to 6% of a $145,000 salary, that leaves $38,800 a year.

Two things: after-tax (not Roth) contributions, and a way to move them to Roth, either an in-plan Roth conversion or an in-service distribution. Many plans allow neither. Some also cap after-tax contributions as a share of pay, and nondiscrimination testing can limit what highly paid employees may contribute.

Growth on the after-tax money before it is converted is taxable when converted, so the usual practice is to convert as soon as possible. Money converted to a Roth is then tax-free to grow. The page assumes you convert right away.

No. The catch-up contribution for people 50 and older, $8,000 in 2026 or $11,250 at ages 60 to 63, is on top of the $72,000 limit.

Probably not on a typical salary, and the page does not assume you can. It asks how much after-tax money you plan to put in, with $10,000 a year as an example, and shows what that is worth by retirement compared with a taxable account.

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