On the Plans page, click Compare next to the Duplicate Current button. A panel slides in from the right with the active plan's name on one side and a dropdown to pick which other plan to compare against on the other. The panel stays open as you navigate the planner, so you can flip between the Profile, Accounts, Income and other pages and keep an eye on what's different in real time.
The summary at the top shows how many sections have differences, the total field-level change count, and the number of additions and removals. Click any section row to jump to that page in the planner; click the chevron to expand the section in place. Press Esc or click the X in the panel header to close it.
The compare target persists across pages and sessions, so the panel will remember which plan you were comparing the next time you open it.
The panel has two tabs. Results answers "what did the differences cost?" - it runs both plans and breaks the gap in their ending balances down into cash-flow sources. Changes lists which inputs differ, field by field. The section-difference summary above the tabs (how many sections differ, and the change/addition/removal counts) always reflects the field-level Changes view.
The Results tab
The Changes tab tells you which inputs differ; the Results tab tells you what those differences cost. It runs both plans and decomposes the gap in their ending balances into the cash-flow sources that drive it - so you can see, for example, that a later-retirement plan avoided $162K of housing spending but gave up $135K of work income.
Median ending balance (the header) - At the top, each plan's median ending balance is shown for context. These are the same numbers the Plans list shows, so they line up with what you see elsewhere. They are reference only; the cost figure below is calculated a different way (read on).
Measured on one identical market path - To make the comparison fair, both plans are run over a single, identical sequence of market returns (a fixed random seed). That isolates the effect of the plan changes from market luck: any difference you see is because the plans differ, not because one happened to draw better returns. A consequence is that the cost figure will not equal the plain difference between the two median balances in the header - the header reflects each plan's own median across thousands of markets, while the cost reflects one shared market. This is intentional and is noted in the panel. For long-horizon, stock-heavy plans that shared path can sit well above or below the median, so read the header as the typical outcome and the waterfall as the apples-to-apples cost of the change.
The waterfall - The bridge chart adds up the cash-flow sources to the net effect on the ending balance. Green bars are changes that helped the compared plan (spending it avoided, taxes it saved, income it gained); red bars are changes that hurt it. A Growth & second-order effects bar captures compounding on the retained cash flow plus knock-on effects (different RMDs, tax brackets, and IRMAA tiers that follow from a different balance), and the final bar is the net change in ending balance. Hover any bar for its exact value.
The decomposition table - Below the chart, the same figures are listed by source, grouped into Spending, Taxes, and Income, subtotaled as a Net cash flow delta, then combined with the Growth & second-order residual to reconcile exactly to the ending-balance change. Toggle Cumulative (lifetime totals) versus Per year (that lifetime figure averaged over the compared years).
Year-by-year detail - Expand this to see each source's contribution year by year alongside the running balance difference, useful for pinpointing exactly when two plans diverge.
Different horizons - If the two plans run to different ages, the comparison covers the overlapping years only (through the shorter plan's last age), and the panel notes the cutoff.
The Changes tab
How list matching works - Lists like accounts, expenses, debts, pensions, and windfalls don't have IDs that are stable across plans, so the compare engine pairs items by a section-specific composite key built from meaningful fields - typically a normalized name plus type or category. For example accounts pair on name + type + subtype, recurring expenses on description + from age, debts on name + debt type. Owner is intentionally not part of the match key, so changing the owner of an account or pension shows as a single changed row with an Owner field-diff, rather than collapsing to a misleading added + removed pair. Row order doesn't affect matching - reordering accounts in one plan and not the other still produces zero differences. When the engine isn't confident two items are the same, it represents the difference as added/removed rather than forcing a shaky match.
What is compared - Compare looks at plan parameters - the inputs you control. It groups them into these sections: Profile, Assumptions (returns, inflation, volatility, simulation method), Milestones & Events (your Custom and Relocation events), Accounts, Roth Strategy, Income, Social Security, Pensions, Windfalls, Recurring Expenses, One-Time Expenses, Debts, ACA Insurance, Medicare, and Long-Term Care. Sections with no differences are hidden from the panel so you can focus on what actually changed. Every non-equal value is shown - plan inputs are user-entered, so any difference between two plans was typed intentionally and is worth surfacing, including small ones like a $1 nudge to a premium or balance.
Syncing a section - Once you've spotted a difference and want to bring the other plan into line, expand the section in the Compare panel while you're on its matching planner page (the Accounts section while on Accounts, Income on Income, and so on). A Copy section to "<target plan>" button appears at the top of the expanded section. Click it to replace that entire section in the target plan with the active plan's version - every item that's added, removed, or changed is copied across in one shot, so you don't have to re-key the differences by hand. A confirmation prompt ("Replace Accounts in 'Plan B' with Accounts from 'Plan A'?") spells out exactly what's about to happen before any write occurs.
Direction is one-way: from the active plan into the target. Pick the plan you've curated as the active one and the plan you're bringing up to parity as the target. To sync the reverse direction, switch which plan is active using the TopBar plan picker and reopen Compare.