On this page
Five plan-wide rates drive every projection: inflation, stock price growth, stock dividend yield, bond return and bond allocation. Each is an annual nominal rate. You can keep a rate fixed or give it a curve that changes with your age. An account can override its own stock growth, dividend yield and bond share.
Inflation does the most work. It indexes spending you enter in today’s dollars. It also drives Social Security’s default cost-of-living increase, the default raise on pay, and the federal tax brackets and limits after 2026.
- Inflation
- 3.0%
- Stock price growth
- 7.0%
- Excludes dividends
- Stock dividend yield
- 0.0%
- Bond return
- 4.0%
- Bond allocation
- 0.0%
- All-stock portfolio
With the defaults, every investment account is 100% stocks growing 7% a year with no dividend. The 4% bond return has no effect until you set a bond allocation above zero.
| Assumption | Moves | Doesn’t move |
|---|---|---|
| Inflation | Expenses set to rise with inflation. Income with a blank growth rate: pay and Social Security. Growth caps marked to rise with inflation. Property tax and maintenance entered in dollars. Health costs without their own medical inflation rate. Federal brackets, deductions and contribution limits from 2027. The today’s-dollars display. | Account balances. Expenses and incomes with a fixed rate or a fixed dollar amount. |
| Stock price growth | The stock share of every investment account: retirement accounts, brokerage, HSA, 529 and Other. | Checking and savings, which don’t grow. Accounts with their own stock growth. |
| Stock dividend yield | Dividends on the stock share. They’re reinvested in every account and taxed only in a brokerage account. | The bond share. Accounts with their own dividend yield. |
| Bond return | The bond share of every investment account, including accounts with their own settings. | An all-stock account. |
| Bond allocation | The mix of every investment account between stock and bond rates. | Accounts with their own bond allocation. |
All rates are nominal: CoastMath never subtracts inflation from a return. The today’s-dollars view divides each year’s results by cumulative inflation afterwards. See Today’s dollars and future dollars.
| First plan year | $60,000 | |
|---|---|---|
| × | 1 + inflation | 1.03 |
| = | Second year | $61,800 |
| × | 1 + inflation | 1.03 |
| = | Third year | $63,654 |
Each January the amount steps up by that year’s inflation rate, including after a partial first year.
Wage growth and Social Security COLA
Pay with a blank growth rate rises each January by the wage-growth rate. Social Security with a blank growth rate rises by the cost-of-living rate. Unless those rates are set, both follow that year’s inflation. Other income with a blank rate, such as a pension, stays at the same nominal amount. A growth rate entered on the income, including 0, always takes precedence.
Choose a curve instead of a fixed rate and enter at least two age points. Each year uses the value at your age in that calendar year. Between points the curve bends smoothly. It never goes above or below the two points on either side. Before the first point and after the last one, the rate stays at that point’s value.
The curve flattens at the 5% point rather than overshooting it. Before 50 and after 70 it holds the end values.
Show data
| Age | Inflation |
|---|---|
| 45 | 3.00% |
| 46 | 3.00% |
| 47 | 3.00% |
| 48 | 3.00% |
| 49 | 3.00% |
| 50 | 3.00% |
| 51 | 3.40% |
| 52 | 3.75% |
| 53 | 4.06% |
| 54 | 4.32% |
| 55 | 4.53% |
| 56 | 4.70% |
| 57 | 4.84% |
| 58 | 4.93% |
| 59 | 4.98% |
| 60 | 5.00% |
| 61 | 4.97% |
| 62 | 4.89% |
| 63 | 4.76% |
| 64 | 4.58% |
| 65 | 4.34% |
| 66 | 4.06% |
| 67 | 3.74% |
| 68 | 3.37% |
| 69 | 2.96% |
| 70 | 2.50% |
| 71 | 2.50% |
| 72 | 2.50% |
| 73 | 2.50% |
| 74 | 2.50% |
| 75 | 2.50% |
Curves follow the primary person’s age. The one exception is a partner’s pay or Social Security with a blank growth rate, which uses the partner’s age on a wage-growth or COLA curve. A bond allocation curve is held between 0% and 100%.
Federal tax brackets, the standard deduction, contribution limits and other federal dollar thresholds use the published 2026 amounts in 2026. From 2027 they grow each year by your inflation rate, counted by calendar year. A plan that starts after 2026 first indexes the 2026 amounts through the intervening years with your inflation rate.
Your own amounts work differently: they’re in dollars of the plan’s first year, and inflation starts applying the following January.
An investment account can replace up to three plan rates with fixed values: stock price growth, stock dividend yield and bond allocation. A blank field keeps following the plan, including its curves. The bond return always comes from the plan.
How the rates combine inside an account, including dividends and mid-year contributions, is in How balances grow each year.