Model 0.10 · Updated September 14, 2026 · U.S. dollars
The question we calculate
How much extra spending fits alongside your entered income, contributions, retirement budget, future costs and ending goal? Temporary extra spending stops when you choose. Its effect is still tested until the end of your plan.
When your entered plan passes every historical scenario, the headline shows extra monthly spending to explore, with your total spending below. If the entered plan falls short, we show your current budget and changes to test. A separate comparison can replace your working and retirement budgets with one amount for life.
A worked example
This fictional person is 40, takes home $10,000/month, spends $6,000 and invests $1,000. They have $500,000 in brokerage accounts, $70,000 in savings and $10,000 in checking. Of that cash, $30,000 is protected outside the plan.
They retire at 65 on $6,000/month, receive $2,500/month after-tax income from 67 that rises with inflation, and want $100,000 left at 95. A $25,000 cost at 50 is included. Investments are 60% stocks, 35% Treasury bonds and 5% Treasury bills. The brokerage cost basis is 70% of its starting value; tax on realized gains is estimated at 15%. No fees, payroll contributions or employer contributions are included. Unused income goes to savings.
$7,100 extra, then back to the entered $6,000/month at 50. All 44 complete historical scenarios cover the full plan until the 95th birthday.
Current spending leaves at least $4,047,950 in those scenarios. The changed budget leaves at least $123,797; the middle ending balance is $2,778,495. These are historical outcomes, before remaining withdrawal taxes, not predictions.
A different choice: one constant lifestyle budget of $9,000/month, replacing both entered budgets. That is a lifelong total, not ten years of extra spending.
The account ledger and this example are checked against an independent nominal-dollar calculation using the archived return and inflation data. Automated agreement checks arithmetic; it is not professional financial certification.
What the history test means
We use every complete stretch of 1928–2025 data long enough for the chosen ages. A 55-year plan has 44 starting years. Adjacent stretches share many years, so they are not independent trials or probabilities of future success.
The initial comparison requires every stretch to cover the budget and ending goal. The optional upper limit allows up to 10% to fall short, rounding the required count upward. Neither result guarantees the future. The solver tests above income surplus when assets can fund the gap, and rounds the extra amount down to $50/month. That rounding is display precision, not certainty.
Money, accounts and taxes
Income is after tax. Spending budgets exclude investment withdrawal taxes, which are estimated separately when account details are known. Brokerage sales use your estimated gains rate and remaining cost basis. Traditional retirement withdrawals use your estimated effective tax rate. Roth withdrawals are assumed qualified from the chosen access age, which defaults to 60 in this annual model. Early-access exceptions and Roth contribution access are not modeled.
Withdrawals use available checking, savings and brokerage before accessible traditional and Roth accounts. Protected cash is set aside from checking first, then savings, and excluded from spending and reported ending balances. Contributions from take-home pay enter brokerage; workplace contributions enter traditional retirement accounts. Continuing contributions can coexist with withdrawals.
All invested accounts use the same portfolio mix and annual rebalancing. Different account allocations, exact tax brackets, dividends and rebalancing tax drag, required distributions, Social Security taxation and future tax law are outside this approximation. Unknown account types produce a clearly marked rough comparison.
Timing and purchasing power
Amounts stay in today's buying power. Income does not grow faster than inflation. Fixed pensions lose buying power after their start; inflation-linked income does not. A future $25,000 expense means $25,000 of today's purchasing power at its entered age.
Annual spending deficits and one-time costs are withdrawn before returns. Contributions and unused income are added at year end. This approximates monthly cash flow. Income and contributions stop at retirement. The endpoint is a birthday: planning from 40 until 95 covers 55 years, with no extra year after the 95th birthday. A cost at that birthday is still charged once.
Debt payments belong in current spending; future payoffs, separate household retirement dates, unentered care costs and a complete job-loss/benefits model are not inferred. The optional income-pause test removes take-home pay while maintaining spending and contributions.
Fees and spending ideas
Your annual investment fee estimate reduces the modeled investment return. Zero means no fee drag. It is an approximation, not an exact fund or advisory billing calculation.
A personal one-time idea adds its full cost at the chosen age. A monthly idea increases both base budgets for the whole planning horizon, including retirement. Your other goals and selected extra stay included. Every applied comparison is recalculated and can be undone.
BLS household expenditure totals are reference-only figures in their source-year dollars. They include pension and Social Security contributions, so they are not automatically inserted as retirement living budgets.
The data
Stocks use U.S. large-company total returns, including dividends; bonds use 10-year U.S. Treasury returns; investment cash and the variable savings proxy use Treasury bills. These are long-history proxies, not actual SPY or bank account history. Checking earns zero nominal interest.
From 1928–2025, this stock series averaged about 11.86% arithmetically, compounded at 10.02% before inflation, and 6.78% after inflation. Those are different statistics. The model uses the actual changing annual returns, not a constant average.
- NYU Stern return series, archived September 4, 2026
- BLS CPI-U via FRED, December-to-December inflation
- BLS spending tables, optional budget examples
- IRS retirement distribution guidance
- IRS capital gains guidance
- CFPB emergency reserve guidance
Financial interpretation and suitability require more than a historical test. This tool provides comparisons to investigate, not permission to spend.
Questions or a correction?
Contact Eyal Cohen at hello@spendit.lol. For partnerships, use the sponsorship and contact page.