On this page
A Social Security stream is sized one of two ways. With Benefit amount, CoastMath pays the annual amount you enter, starting in the year its owner reaches the claiming age you choose; the claiming-age adjustment is applied only when you use the built-in estimate. With PIA, you enter your monthly benefit at full retirement age and CoastMath applies the reduction or delayed credits for the claim date itself, adds spousal benefits when both partners use PIA, and can compute survivor benefits. Either way the benefit is in today’s dollars and rises every January with the plan’s inflation assumption, as a stand-in for the cost-of-living adjustment.
Each year, the federal tax calculation decides how much of the year’s benefits is taxable, from 0% up to 85%, using the combined-income test for your filing status.
Before full retirement age, earnings above the annual exempt amount reduce benefits. In the year you reach full retirement age, only earnings before that month count, with the higher exempt amount. Fully withheld months increase your retirement benefit from full retirement age. For PIA-based survivor benefits, those months also adjust the survivor’s age reduction at survivor full retirement age; the reduced-worker limit still applies. Any increase in your own benefit reduces the survivor top-up in the same month.
CoastMath allocates a worker’s withholding proportionally across their own benefit and spousal payments on their record. In the first claiming year, benefits after employment ends are protected as an approximation of the monthly grace rule. Self-employment services are not measured.
With PIA basis, choose “Current record” under “My PIA reflects…” to estimate increases from future covered wages and self-employment earnings. Enter covered years to date. CoastMath infers equal historical earnings years and zeros for the rest of 35, then replaces low years with higher projected years. Increases take effect the following January and flow through spousal and survivor benefits. “Assumes I keep working” keeps the entered PIA unchanged.
Add one income stream per person with the Social Security income type. The fields that matter:
- Benefit basis: Benefit amount or PIA. See Benefits sized from your PIA for the second.
- Annual amount, with Benefit amount basis (or a monthly or other amount, chosen with Amount frequency): the benefit you expect at your claiming age, in today’s dollars. This is the amount CoastMath pays. It already includes any early-claiming reduction or delayed credits.
- Income recipient (plans with a partner): Primary person or Partner. The claiming age follows that person’s birth year and month.
- Claiming age: 62 through 70. Benefits start in the year the recipient turns that age. Leave it blank to start at plan start, or pick a Start milestone to override it.
- Growth rate, under Growth and tax settings: leave it blank to follow the plan’s Social Security COLA assumption, or inflation without one.
The built-in estimate
With Benefit amount basis, the estimate fills in the amount for you. From monthly PIA takes your primary insurance amount (your monthly benefit at full retirement age, from your Social Security record) and fills Annual amount with the claimed annual benefit. Saving keeps only that annual amount; changing the claiming age later doesn’t re-run the estimate.
Annual amount=Monthly PIA×12×Claiming factor×Expected benefit %
| Monthly PIA at full retirement age | $2,000 | |
|---|---|---|
| × | Months | 12 |
| × | Claiming factor at 6436 months before full retirement age of 67 | 80.0% |
| × | Expected benefitYour own planning assumption | 90.0% |
| = | Annual amount saved to the stream | $17,280 |
The claiming factor follows the statutory schedule: a reduction of five-ninths of 1% for each of the first 36 months before full retirement age and five-twelfths of 1% for each earlier month, and a credit of two-thirds of 1% for each month you wait past full retirement age, up to 70. Full retirement age is 66 for people born 1943–1954, rises by two months per birth year from 1955, and is 67 for people born in 1960 or later.
| Claiming age | Months from full retirement age | Share of PIA | Annual amount |
|---|---|---|---|
| 62 | 60 early | 70.0% | $16,800 |
| 63 | 48 early | 75.0% | $18,000 |
| 64 | 36 early | 80.0% | $19,200 |
| 65 | 24 early | 86.7% | $20,800 |
| 66 | 12 early | 93.3% | $22,400 |
| 67 | At full retirement age | 100.0% | $24,000 |
| 68 | 12 late | 108.0% | $25,920 |
| 69 | 24 late | 116.0% | $27,840 |
| 70 | 36 late | 124.0% | $29,760 |
From current annual income is a rough sketch for when you don’t have your record: it applies fixed replacement rates to one year’s income, capped at the $184,500 taxable maximum, adjusts for claiming age and caps the result at the maximum benefit ($4,152 a month at full retirement age in 2026). It doesn’t read your earnings history.
Expected benefit (%) is a planning haircut applied after the claiming factor, for example 80% if you want to plan for reduced benefits. It isn’t a statutory calculation.
With Benefit basis set to PIA, enter PIA per month at full retirement age ($) from your Social Security record, in today’s dollars, and Expected benefit paid (%). A claiming age is required. The projection computes the benefit from these every time it runs, so changing the claiming age changes the benefit as well as the start date. The form shows the first-year benefit at the claiming age as a preview.
Own benefit, in today’s dollars=Monthly PIA×12×Claiming factor for the first payment month×Expected benefit %
| Monthly PIA at full retirement age | $2,000 | |
|---|---|---|
| × | Months | 12 |
| × | Claiming factor at 7036 months of delayed credits past 67 | 124.0% |
| × | Expected benefit | 90.0% |
| = | Paid in 2036, the first full yearWith 0% inflation | $26,784 |
The claiming factor uses the month payments actually start: the claiming age in the recipient’s birth month, or a Start month or Start milestone you set instead. A start milestone must fall on a fixed age or date. The benefit then grows from plan start like any Social Security stream; see When payments start and how they grow.
Spousal benefits
When both partners have one active PIA-basis stream, a partner whose PIA is less than half of the other’s also receives the difference as a spousal benefit, listed as a separate income line named after their stream with “(spousal)”.
- It starts when both partners have claimed. Claiming your own benefit also files for the spousal benefit, but it can’t begin before the other partner claims.
- If it starts before your full retirement age, it’s reduced by twenty-five thirty-sixths of 1% for each of the first 36 months early and five-twelfths of 1% for each earlier month. Waiting past full retirement age adds no credits to it.
- Your expected benefit percentage applies to it, and it stops when the other partner dies.
| Half of the higher PIA | $1,500 | |
|---|---|---|
| − | Lower earner’s own PIA | $1,000 |
| = | Monthly spousal amount at full retirement age | $500 |
| × | Spousal factor, 36 months early | 75.0% |
| × | Months | 12 |
| = | Spousal benefit for the year | $4,500 |
| + | Own benefit: $1,000 × 12 × 80% | $9,600 |
| = | Lower earner’s Social Security in 2030 | $14,100 |
Survivor benefits for a PIA-basis couple are calculated from both PIAs when the survivor option is on; see Spousal and survivor Social Security.
A claiming age starts payments in the recipient’s birth month of the year they reach that age; with no birth month on file, in January. Set birth months in Plan detailsBirth month and Partner and estatePartner birth month. A Start month on the stream overrides the birth month. If the claiming age is already behind you when the plan starts, the benefit is paid from the plan’s start.
The amount you enter is in plan-start-year dollars. Each January after the first plan year, it grows by one year of the plan’s Social Security COLA assumption, or of its inflation assumption when no COLA is set. A Growth rate on the stream replaces that with a fixed rate; enter 0 for a benefit fixed in nominal dollars.
Growth compounds from the plan’s start, including the years before you claim, so a benefit estimate stated in today’s dollars is carried forward to the dollars of the year it is paid.
| Annual amount entered (2026 dollars) | $24,000 | |
|---|---|---|
| × | Five January increases, 2027–20311.025 to the fifth power | 1.131408 |
| = | Full-year rate in 2031 | $27,153.80 |
| × | March through December | 10 of 12 months |
| = | Paid in 2031 | $22,628.16 |
| → | Paid in 2032, after one more increase | $27,832.64 |
The Social Security view reruns the whole plan with each person’s own benefit claimed at every whole age from 62 to 70, in their birth month. With two partners whose ages can change, it runs every pair, up to 81 projections, plus the plan as saved. Everything else stays as you entered it: other income, spending, accounts, returns, inflation and tax settings. Taxes, withdrawals, the earnings test and spousal, survivor and former-spouse benefits are recalculated in each run.
Runs are ranked by end net worth after tax: pre-tax balances left at the end are reduced by the heir income tax rate in your estate settings, or 24% when none is set. Runs in which the plan succeeds rank ahead of runs in which it fails. Lifetime Social Security is the household’s total received, after the earnings test; the cumulative chart shows where a later claim catches up with an earlier one.
With Benefit amount, the amount you entered is taken as the benefit at the planned claiming age and rescaled by the change in the claiming factor. With PIA, CoastMath sizes each benefit from the new claim date, so spousal and survivor amounts follow both ages.
| Annual amount entered, claiming at 67 | $24,000 | |
|---|---|---|
| × | Claiming factor at 70 ÷ factor at 67 | 1.24 |
| = | Annual amount at 70 | $29,760 |
| → | At 62: × 0.70 | $16,800 |
A person’s age isn’t varied when their benefit has already started, when they have more than one Social Security stream, when the stream starts at a milestone without a fixed date (a net worth or condition milestone), or when no age from 62 to 70 is still ahead.
Every year, CoastMath runs the federal combined-income test on the household’s total benefits. Combined income is all other income in adjusted gross income, including capital gains, dividends, withdrawals and Roth conversions, plus half of the year’s benefits. Up to a lower threshold nothing is taxable; between the thresholds up to 50% is; above the upper threshold up to 85% is.
- Single, head of household
- $25,000 / $34,000
- Married filing jointly
- $32,000 / $44,000
- Married filing separately, lived together
- $0
- Up to 85% taxable from the first dollar
- Married filing separately, lived apart all year
- $25,000 / $34,000
Taxable benefits above the upper threshold=min85% of benefits,min(50% of benefits, tier amount)+85% × (combined income − upper threshold)
Between the thresholds, the taxable amount is the lesser of 50% of benefits and 50% of combined income above the lower threshold. The tier amount is $4,500 for single filers and $6,000 for joint filers.
| Other income | $30,000 | |
|---|---|---|
| + | Half of benefits | $20,000 |
| = | Combined income | $50,000 |
| Lesser of half of benefits or $6,000 | $6,000 | |
| + | 85% of combined income above $44,00085% × $6,000 | $5,100 |
| = | Taxable benefitsBelow the 85% cap of $34,000 | $11,100 |
| → | Adjusted gross income | $41,100 |
- Taxable benefits
- 85% of benefits
Each extra dollar of other income adds 50 cents, then 85 cents, of taxable benefits until 85% of benefits is taxable.
Show data
| Other income | Taxable benefits |
|---|---|
| $0 | $0 |
| $5,000 | $0 |
| $10,000 | $0 |
| $15,000 | $2,500 |
| $19,000 | $4,500 |
| $20,000 | $5,350 |
| $25,000 | $9,600 |
| $30,000 | $13,850 |
| $35,000 | $18,100 |
| $40,000 | $22,350 |
| $43,706 | $25,500 |
| $50,000 | $25,500 |
| $60,000 | $25,500 |
| $70,000 | $25,500 |
The taxable share is added to adjusted gross income, so it can raise the rate on other income and push capital gains into a higher bracket; see How CoastMath calculates income tax. Because the test runs on the final income for the year, a withdrawal or Roth conversion that raises combined income also raises taxable benefits in the same year.
With Married filing separately, check Lived apart from spouse for the entire tax year under Plan details to use the single thresholds; otherwise the zero threshold applies in every year.
State tax starts from federal adjusted gross income, which already contains the taxable share of benefits. For California, CoastMath subtracts that share, so benefits aren’t taxed by the state. See State and local income tax for how each state is modeled.
Each benefit appears as income under its stream’s name in the year’s cash flow, and funds spending before any withdrawal. The taxable share isn’t shown as its own line; it is part of the year’s federal taxable income and tax. A survivor benefit adjustment, when you turn one on, appears as a separate income line; see Spousal and survivor Social Security.