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CoastMath charges your marketplace premium as a household expense, reduced by an advance premium tax credit estimated from income you expect. When the year’s taxes are computed, it recalculates the credit on your final modified AGI and settles the difference with that year’s tax bill or refund: a refund if you were owed more, a repayment if you received too much.
For 2026 the credit follows the original law: the enhanced credits of 2021–2025 expired, so no credit is available above 400% of the poverty line, and there is no cap on repaying excess advance credit.
Marketplace coverage is one policy for the household’s adults. You enter two annual premiums for the covered adults under 65 at their ages in the plan’s first year, in today’s dollars. Leave out a partner already on Medicare:
- Benchmark premium: the second-lowest-cost silver plan for the covered adults, before any credit. The credit is sized from this figure.
- Your plan’s premium: the gross premium of the plan you pick. This is what you pay, less the credit.
Both premiums grow each year with medical cost inflation (plan inflation when that field is blank). With age rating on, they also rise with each adult’s age along the federal default age curve, which triples a premium between ages 21 and 64. Children’s premiums aren’t added; dependents you enter only increase the household size used for the poverty line. A partner who isn’t on the policy still counts in the household.
Coverage runs from the start milestone’s month (or the plan’s start) through the month before the end milestone, and each adult leaves the marketplace in the month before their 65th-birthday month, when Medicare begins. The premium then drops to the remaining adult’s share.
Premium tax credit=Benchmark premium−Applicable percentage×MAGI
The credit can’t be negative and can’t exceed your plan’s premium, so choosing a plan cheaper than the benchmark can reduce the net premium to zero but never below it.
- MAGI is adjusted gross income plus the untaxed part of Social Security benefits. It includes IRA withdrawals, Roth conversions and realized capital gains; pre-tax payroll and HSA contributions reduce it.
- Poverty line: 2026 coverage uses the 2025 guideline for your household size. Later years index it with plan inflation.
- Applicable percentage rises with income as a share of the poverty line, from 2.10% to 9.96%, interpolated within each band. At 300% to 400% of the poverty line it is 9.96%.
- Below 100% of the poverty line the credit is zero and the full premium is charged, with a warning. CoastMath doesn’t model Medicaid.
- Poverty line, household of 1
- $15,650
- Poverty line, household of 2
- $21,150
- +$5,500 per additional person
- No credit above, household of 1
- $62,600
- 400% of the poverty line
- Applicable percentage
- 2.10%–9.96%
- By income as a share of the poverty line
| MAGI | $45,000 | |
|---|---|---|
| ÷ | Poverty line, household of 1 | $15,650 |
| = | Income as a share of the poverty line | 287.5% |
| Applicable percentageBetween 8.44% at 250% and 9.96% at 300% | 9.58% | |
| Benchmark premium | $12,000 | |
| − | Expected contribution (9.58% × $45,000) | $4,312 |
| = | Premium tax credit | $7,688 |
| Your plan’s premium | $11,000 | |
| = | Net premium for the year$11,000 − $7,688 | $3,312 |
- Premium tax credit
- Net premium you pay
Benchmark $12,000, plan $11,000. At $62,601 the credit falls from $5,765 to zero.
Show data
| MAGI | Credit | Net premium |
|---|---|---|
| $15,000 | $0 | $11,000 |
| $15,649 | $0 | $11,000 |
| $15,650 | $11,000 | $0 |
| $20,000 | $11,000 | $0 |
| $25,000 | $10,835 | $165 |
| $30,000 | $10,140 | $860 |
| $35,000 | $9,385 | $1,615 |
| $40,000 | $8,556 | $2,444 |
| $45,000 | $7,688 | $3,312 |
| $50,000 | $7,020 | $3,980 |
| $55,000 | $6,522 | $4,478 |
| $60,000 | $6,024 | $4,976 |
| $62,600 | $5,765 | $5,235 |
| $62,601 | $0 | $11,000 |
| $70,000 | $0 | $11,000 |
| $80,000 | $0 | $11,000 |
A marketplace sets your advance credit from an income estimate made before the year starts. CoastMath does the same:
- First plan year: the estimate is Last year’s modified AGI if you entered it (the field sits under Medicare premiums and shows while IRMAA surcharges are on); otherwise it is the year’s income before the withdrawals that fund spending.
- Later years: the estimate is the previous year’s final MAGI.
The premium paid during the year is your plan’s premium minus the advance share of the estimated credit, and that amount is what appears in spending. After the year’s withdrawals, conversions and gains are known, the credit is recalculated on final MAGI. The difference, final credit minus advance credit, is added to the year’s tax settlement: a positive difference is a refund, a negative one is owed in full. It isn’t part of total taxes, and the credit is never counted as income.
| Last year’s modified AGI (the estimate) | $40,000 | |
|---|---|---|
| Advance credit applied to premiums | $8,556 | |
| Premium paid during the year$11,000 − $8,556 | $2,444 | |
| Final MAGI after IRA withdrawalsWithdrawals for $40,000 of living costs, premiums and taxes | $47,181 | |
| Final credit on that MAGI | $7,301 | |
| = | Repaid with the year’s taxes$7,301 − $8,556 | −$1,255 |
One adult aged 60, benchmark $12,000, plan $11,000, no other income. The next year’s estimate starts from $47,181, so it matches and nothing is repaid.
Setting the advance percentage below 100% raises the premium you pay during the year and moves the rest of the credit to the tax settlement. At 0%, the whole credit arrives as a refund. See Withholding, refunds and balances due for how a settlement is funded.
Medicare costs after 65 are covered in Medicare premiums and IRMAA.