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Your projection starts on a start date: today for a Now plan, or the date you choose for a Fixed date plan. The balances you enter are the balances on that date. The first calendar year runs only from the start date to December 31, so income, spending and investment growth in that year are scaled to the share of the year that remains.
From the second year on, every year is a full calendar year. Amounts that grow step up a full year’s growth each January, even when the first year was short.
Now (starts today)
The start date is your local calendar date each time the projection runs. Balances are read as today’s balances, and the first year shrinks as the year goes on. On January 1 the start year becomes the new year and the first year is a full year again.
Fixed date
The start date stays where you set it, including a date in the past or the future. Use it to keep a forecast from a known date, such as the day you retire or the date of your last statements. The date can’t be earlier than your birth month.
Your birth month sets the month of age-based milestones and the earliest allowed start date. When it isn’t set, CoastMath assumes January. Set it in Plan detailsBirth month.
CoastMath measures the first year in days, counting the start date itself.
First-year fraction=Days from the start date through December 31Days in that calendar year
| Days from October 1 through December 31 | 92 | |
|---|---|---|
| ÷ | Days in 2026 | 365 |
| = | First-year fraction | 25.2% |
A plan that starts on January 1 has a fraction of 100% and no partial year.
Income and expenses count only for the part of the year after the start date. An item that also has its own start or end month counts only where its months overlap that period. Price growth compounds for the same fraction of a year, and dividend and cash yields are scaled by it.
First-year growth rate=(1 + annual rate)^first-year fraction−1
| Balance on October 1, 2026 | $500,000 | |
|---|---|---|
| × | Growth for 25.2% of a year at 7% | 1.72% |
| = | Growth through December 31 | $8,600 |
The next year compounds a full 7% on the December 31 balance.
- Salary
- Living expenses
Salary of $120,000 and living expenses of $60,000 a year, both entered for 2026 and rising 3% a year. The first bar covers October through December only.
Show data
| Year | Salary | Living expenses |
|---|---|---|
| 2026 | $30,247 | $15,123 |
| 2027 | $123,600 | $61,800 |
| 2028 | $127,308 | $63,654 |
In 2027 the salary is $123,600 and expenses are $61,800: a full 3% step on the entered amounts, not a partial one. Amounts step up once each January regardless of when the plan started.
Some rules need a whole year’s figure, so CoastMath converts the partial year to a full-year pace for them. A milestone set as a multiple of annual expenses uses the full-year spending pace. Roth IRA contribution eligibility uses a full-year estimate of income unless you enter the income you had earlier in the year.
Tax is always figured for the whole calendar year, with the full standard deduction and brackets. Until you enter earlier history, CoastMath estimates income from active plan streams and treats the matching share of tax as already paid, so only the months after the start date settle from the plan. The year panel labels the estimate estimated pro rata. Earlier contributions, withdrawals and deductions stay zero until entered.
| No earlier history (estimated pro rata) | $90,000 earlier wages, $20,000 tax paid | |
|---|---|---|
| Wages taxed for 2026 | $120,000 | $120,247 |
| Federal income tax | $17,570 | $17,624 |
| Social Security and Medicare tax | $9,180 | $9,199 |
| Total 2026 tax | $26,750 | $26,823 |
| Treated as paid before October 1 | $20,008 | $20,000 |
| Paid from the plan in 2026 | $6,742 | $6,823 |
2026 federal figures: $16,100 standard deduction; 10% to $12,400, 12% to $50,400 and 22% to $105,700 of taxable income. Without history, 273/365 of the salary and of the tax fall before October 1.
For how the balance due or refund is paid in the plan, see Withholding, refunds and balances due.
On the Overview, the year panel labels the first year with its period, such as “Oct 1 – Dec 31”. It adds Partial year (the fraction), Full-year income pace and Full-year spending pace, which divide the period’s amounts by the fraction. The cash-flow diagram notes the partial year and the share of the year its amounts cover.