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CoastMath charges each expense in every year it’s active, at the amount you entered grown to that year by its growth setting. An expense set to repeat is charged only in its first active year and every interval after it. In the year a start or end milestone is reached, the charge covers only the active months.
Expenses are paid from the year’s cash flow; any shortfall is withdrawn from your accounts in drawdown order. Expenses aren’t tax-deductible unless they are pre-tax payroll deductions.
Each expense has a name, an amount, a category (essential, discretionary or a custom split), a growth setting and an optional schedule. Amount frequency can be weekly, biweekly (every two weeks), semimonthly (twice a month), monthly, quarterly or annual; a new expense starts as monthly. CoastMath multiplies the amount by 52, 26, 24, 12, 4 or 1, and every calculation below works on that annual amount. The frequency is saved with the expense, so the form reopens in it and the expense list shows the amount in it. An expense that repeats every few years is listed by its yearly charge.
Charge in a year=Entered annual amount×growth since plan start×change per occurrence×share of the year active
- Rises with plan inflation multiplies by cumulative plan inflation, which steps up each January. The first plan year uses the amount as entered.
- Custom rate per year multiplies by (1 + rate) for each year since plan start, whether or not the expense was active in those years. It replaces inflation linking. An optional growth cap limits the grown amount to the cap each year; with Cap rises with inflation checked, the cap itself grows with plan inflation. A negative rate shrinks the amount.
- Fixed nominal amount never changes, so its value in today’s dollars falls with inflation.
- Rises with plan inflation
- Custom 5%, capped at $18,000
- Fixed nominal amount
Plan inflation is 3%. The custom rate is 5% with a fixed $18,000 cap, reached in 2035.
Show data
| Year | Rises with plan inflation | Custom 5%, capped at $18,000 | Fixed nominal amount |
|---|---|---|---|
| 2026 | $12,000 | $12,000 | $12,000 |
| 2027 | $12,360 | $12,600 | $12,000 |
| 2028 | $12,731 | $13,230 | $12,000 |
| 2029 | $13,113 | $13,892 | $12,000 |
| 2030 | $13,506 | $14,586 | $12,000 |
| 2031 | $13,911 | $15,315 | $12,000 |
| 2032 | $14,329 | $16,081 | $12,000 |
| 2033 | $14,758 | $16,885 | $12,000 |
| 2034 | $15,201 | $17,729 | $12,000 |
| 2035 | $15,657 | $18,000 | $12,000 |
| 2036 | $16,127 | $18,000 | $12,000 |
| 2037 | $16,611 | $18,000 | $12,000 |
| 2038 | $17,109 | $18,000 | $12,000 |
| 2039 | $17,622 | $18,000 | $12,000 |
| 2040 | $18,151 | $18,000 | $12,000 |
| 2041 | $18,696 | $18,000 | $12,000 |
| 2042 | $19,256 | $18,000 | $12,000 |
| 2043 | $19,834 | $18,000 | $12,000 |
| 2044 | $20,429 | $18,000 | $12,000 |
| 2045 | $21,042 | $18,000 | $12,000 |
| 2046 | $21,673 | $18,000 | $12,000 |
| 2047 | $22,324 | $18,000 | $12,000 |
| 2048 | $22,993 | $18,000 | $12,000 |
| 2049 | $23,683 | $18,000 | $12,000 |
| 2050 | $24,394 | $18,000 | $12,000 |
| 2051 | $25,125 | $18,000 | $12,000 |
| Entered amount, 2026 dollars | $20,000 | |
|---|---|---|
| × | Custom growth at 5% for 10 years1.05 raised to the power 10 | 1.6289 |
| = | Full-year amount in 2036 | $32,578 |
| × | Active months, September–December | 4 ÷ 12 |
| = | Charged in 2036 | $10,859 |
From 2037 the full-year amount applies, growing 5% a year. A graduation milestone in June 2040 limits that year to January–May.
With Repeats every few years on, an expense is charged in its first active year and then every Repeat every (years) after it, with nothing in between. The interval counts from the year the expense first becomes active, not from plan start. The amount still grows from plan start, so a replacement ten years out costs ten years’ more inflation.
Change per occurrence multiplies each later charge by (1 + change) once per earlier occurrence, on top of the growth setting. Use it for costs that step up or down with each replacement.
| Year | Expense | Charged |
|---|---|---|
| 2026 | Car: $35,000 fixed, every 8 years, +10% per occurrence | $35,000 |
| 2031 | Roof: $25,000 inflation-linked, every 10 years from a January 2031 milestone | $28,982 |
| 2034 | Car, second occurrence | $38,500 |
| 2041 | Roof, second occurrence | $38,949 |
| 2042 | Car, third occurrence | $42,350 |
The roof’s first charge is $25,000 grown by five years of inflation, although the expense only starts in 2031.
For a one-time purchase, turn on repeats, give it a start milestone for the purchase date and set an interval longer than the rest of the plan (up to 100 years). It’s charged once.
An expense becomes active in the year its start milestone is reached and stops in the year its end milestone is reached. Months only matter in those boundary years:
- In the start year, charges begin in Start Month. If it’s blank, they begin in the month the milestone is reached: the birthday month for an age milestone, the date’s month for a date milestone, and January for other milestone types.
- In the end year, charges run through End Month. If it’s blank, they stop after the month before the milestone is reached.
- A start or end month without the matching milestone has no effect. Months aren’t a seasonal schedule.
A plan that starts partway through a year also charges only the remaining months of its first year; see Your plan’s start date and first year.
The category matters in one calculation: Flexible spending rules can reduce discretionary spending after weak market years, raise it after new portfolio highs if you ask it to, and never touches essential spending. Cash you choose to spend at year end is added as discretionary “Leftover spending”. Choose Essential (0% can be cut), Discretionary (100%) or Custom, which sets the discretionary share from 0 to 100% with a slider or a number: groceries might be 30% discretionary, travel 90%. A custom split is listed as “30% flexible”, and spending totals that leave out essential or discretionary expenses count each part separately. The categories are otherwise treated the same.
Apps that show only essential or discretionary label a split expense by its larger part: 50% or more is discretionary.
- General cash flow (the default): the year’s income pays expenses first, and any shortfall is withdrawn in the order described in How shortfalls are funded.
- Fund from a specific account: only the selected accounts pay this expense, in the order you list them within each account type; your drawdown order decides which type goes first. If they run short, the remainder is reported as unfunded. These expenses are outside flexible spending rules.
- Payroll Deduction: the amount comes out of the chosen paycheck while that income is active. Pre-tax deductions lower taxable wages; see Payroll deductions and pre-tax benefits.
Each year’s cash flow lists every charged expense by name, in future dollars, alongside health coverage, loan payments and gifts. A repeating expense appears only in the years it’s charged. Switching the display to today’s dollars divides each amount by cumulative inflation; see Today’s dollars and future dollars.