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401k Growth Calculator

Enter your balance, salary, contribution rate, match and return to project your 401k.

How it is calculated

Contributions are a flat share of salary, paid monthly: monthly = salary × (contribution% + employer%) ÷ 100 ÷ 12. With monthly rate i = annual_return ÷ 100 ÷ 12 and n = years × 12 months, the final balance is balance × (1 + i)n + monthly × ((1 + i)n − 1) ÷ i. Paid in is balance + monthly × n; growth is the final balance minus paid in. Deposits land at month end, and salary and percentages stay fixed for the whole period. Fees, inflation and annual contribution limits are not modelled, so the figure is nominal and pre-tax. At a 0% return the deposits are simply added up. Negative balances, salary, years or percentages, a contribution or match above 100%, or a return of −100% or less show no result.

Worked example

Starting balance 35,000, salary 72,000, contribution 6%, employer 3%, return 7% a year, 25 years. Monthly deposit: 72,000 × 9% ÷ 12 = 540. Over n = 300 months at i ≈ 0.5833% per month, the starting balance and the deposits compound together. Final balance: $637,828.35. Paid in: 35,000 + 540 × 300 = $197,000.00. Investment growth: $637,828.35 − $197,000.00 = $440,828.35, more than twice what was paid in.

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Common mistakes

  • Entering the match formula instead of what the employer really adds. If the employer matches 50% of your first 6%, the employer percent is 3, not 50 or 6.
  • Expecting salary raises to be included. Salary stays fixed here, so a long projection understates deposits if your pay will grow.
  • Reading the balance as today's money. The return is nominal, so after decades of inflation $637,828 buys far less than it does now.
  • Typing the horizon in months or the return as a decimal (0.07). The return is a percent per year and the horizon is in years.
  • Picking an optimistic return. Growth reacts strongly to the rate over 25 years, so also run a lower case.

Sources

About this calculator

A 401(k) balance grows from three compounding sources: your existing balance, your ongoing contributions (a percentage of salary deducted each pay period), and the employer match — commonly structured as something like “100% match on the first 3% of salary, 50% on the next 2%” — all invested and compounded at an assumed annual rate of return, typically modeled around 6-8% for a diversified stock/bond portfolio over the long term.

Employees planning retirement savings use this projection to see the real cost of leaving free money on the table — under-contributing below the match threshold effectively forfeits part of your compensation — and to understand how much of their eventual balance comes from what was paid in (their own contributions plus employer match) versus investment growth compounding over decades, since even small early increases in contribution rate compound into large differences by retirement age.

This calculator takes your current balance, salary, contribution rate, employer match and expected return, and projects your 401(k) balance forward to the end of the period, showing the final balance, the total paid in and the investment growth. The match is one flat percentage of a constant salary (for a tiered match, enter the overall percentage), with contributions and growth calculated monthly.

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