Trace the assumptions
to a balance.

Calculation method

Eligible pay, your deferral percentage, match rate, and salary cap remain constant for the entire scenario. Each month, the model calculates an employee deposit as annual pay × deferral rate ÷ 12. Eligible deferral for matching is the smaller of that monthly deposit and annual pay × match-cap percentage ÷ 12. Employer deposit equals eligible deferral × match rate. A match rate above 100% can produce an employer deposit larger than the eligible employee deferral; the salary cap still limits which employee deferral is matched.

At each month-end, the existing balance first earns the monthly rate derived from the entered effective annual return: (1 + annual return)^1/12 − 1. Employee and employer deposits are then added at month-end and do not earn a return for that month. This convention reflects end-of-period contributions. Zero return produces zero investment change; a −100% annual assumption wipes out the prior balance over the first modeled month, while that month’s deposits are added afterward. The balance is not allowed to go below zero for the allowed input range.

The chart plots year-end balances. Totals show undiscounted employee deposits, modeled employer deposits, and the remaining balance change attributed to investment gains or losses, all rounded to whole dollars for display. Because results are rounded, displayed totals may differ by a dollar from the displayed ending balance.

What the calculator leaves out

This is arithmetic based on assumptions—not a forecast, target, or recommendation. The model does not apply IRS employee or annual-additions limits; it does not model age-based catch-ups, payroll withholding frequency, a plan’s exact match schedule, eligible-compensation definitions, vesting, true-ups, contribution pauses, loans, withdrawals, salary changes, inflation, investment allocation, fees, taxes, or fluctuating returns. Real pay periods do not necessarily divide annual pay evenly by 12, and your employer may calculate matching differently. Compare every assumption with your plan’s current SPD and payroll process.

Employee and employer deposits are hypothetical and shown before tax or fees. Investment values can go down as well as up; a constant return cannot reproduce market behavior. This tool does not tell you how much to contribute or what to invest in. For the official 2026 employee limit and plan-specific cautions, see the 2026 limits page and the IRS contribution-limit guidance.