The §415(c) capacity formula

Your remaining Mega Backdoor Roth room is whatever is left under the IRC §415(c) overall annual-additions limit after everything else that counts is subtracted. The formula is deliberately simple so you can reproduce it by hand.

The formula

MBR room = §415(c) limit
           − elective deferral (Roth + pre-tax)
           − employer match + non-elective
           − after-tax already contributed

What counts toward §415(c)

• Your elective deferral — the Roth + pre-tax money you contribute, capped by §402(g).
• Employer additions — match, profit-sharing, and non-elective contributions.
• After-tax contributions — including the Mega Backdoor Roth room you are trying to size.

What does not count

The age-50+ catch-up (§414(v)) sits on top of the §415(c) limit — it is not subtracted from your after-tax room. This is a common point of confusion, and the calculator handles it correctly: it does not reduce your room for catch-up.

The 100%-of-compensation cap

§415(c)(1)(B) also caps annual additions at 100% of your compensation. For the high earners the strategy serves, the dollar limit binds first, so the cap rarely matters. But for lower compensation it can bind before the dollar limit — and the calculator applies whichever is smaller, flagging when the comp cap is the binding constraint.

Worked examples

Each example below reconciles to the cent in the build's unit tests. Plug them into the calculator above to confirm.

• 2026, deferral maxed, $20k match: $72,000 − $24,500 − $20,000 = $27,500.
• 2026, $9k match, $5k after-tax already in: $72,000 − $24,500 − $9,000 − $5,000 = $33,500.
• 2025, $11.5k employer additions: $70,000 − $23,500 − $11,500 = $35,000.

The limits themselves