MAGI Phase-Outs for the New Deductions: Rounding Rules Compared
Each of the four new deductions on Schedule 1-A shrinks as income rises, but the formulas are not identical. Tips and overtime use one schedule, car loan interest another and the senior deduction a third. Small differences in rounding change the result by a few hundred dollars, so it is worth seeing them side by side.
What "modified AGI" means here
For all four deductions, modified adjusted gross income is your AGI from Form 1040 plus a few amounts that are normally excluded: foreign earned income and housing amounts excluded on Form 2555, and income from Puerto Rico or American Samoa that is excluded from U.S. tax. For most people who live and work in the U.S., MAGI equals AGI. Importantly, the new deductions themselves do not reduce AGI, so claiming one deduction never helps you qualify for another. That avoids circular calculations.
The four schedules
| Deduction | Cap | Threshold (single / joint) | Reduction | Rounding |
|---|---|---|---|---|
| Qualified tips | $25,000 per return | $150,000 / $300,000 | $100 per $1,000 of excess | Excess rounded down to whole thousands |
| Qualified overtime | $12,500 / $25,000 | $150,000 / $300,000 | $100 per $1,000 of excess | Excess rounded down |
| Car loan interest | $10,000 per return | $100,000 / $200,000 | $200 per $1,000 or part | Excess rounded up |
| Senior | $6,000 per person | $75,000 / $150,000 | 6% of excess | None |
Head of household filers use the single thresholds. Married filing separately is not allowed for tips, overtime or the senior deduction.
Why the rounding differs
The statute for tips and overtime says the deduction is reduced by $100 for each $1,000 by which MAGI exceeds the threshold. Schedule 1-A implements that by dividing the excess by 1,000 and dropping the fraction, so only complete thousands count. The car loan provision says $200 for each $1,000 "or fraction thereof," which pulls any partial thousand up to a whole one. The senior deduction is written as a percentage with no step function.
Worked comparison at the same income
Consider a single filer with $160,500 of MAGI, $20,000 of qualified tips, $3,000 of car loan interest and age 66.
- Tips: excess over $150,000 is $10,500, rounded down to 10. Reduction $1,000. Deduction $19,000.
- Car loan: excess over $100,000 is $60,500, rounded up to 61. Reduction $12,200, which exceeds $3,000. Deduction $0.
- Senior: excess over $75,000 is $85,500. 6% is $5,130. Deduction $870.
The same household gets very different outcomes from each deduction because the thresholds are so different. Car loan interest phases out fastest for single filers, while tips and overtime stay intact well into six figures.
Where the deductions disappear
With the full cap, the tips deduction is gone at $250,000 of MAGI above $150,000 — that is, at $400,000 single or $550,000 joint. Overtime at the $12,500 single cap is gone at $275,000; at the $25,000 joint cap, $550,000. Car loan interest at the full $10,000 cap ends at $150,000 single or $250,000 joint, but with smaller interest amounts it ends much sooner: $2,000 of interest disappears $10,000 above the threshold. The senior deduction ends exactly $100,000 above the threshold.
Marginal rate effects inside the phase-out
A phase-out acts like a hidden tax rate. For tips or overtime, each extra $1,000 of income removes $100 of deduction, raising taxable income by $1,100 instead of $1,000. In the 32% bracket that adds about $32 of tax per $1,000, a 3.2-point bump. For car loan interest, $200 per $1,000 is a 20% reduction rate, which in the 24% bracket adds 4.8 points. For the senior deduction, 6% per qualifying person, or 12% for a couple who both qualify, adds about 2.6 points in the 22% bracket. These bumps stop once the deduction reaches zero.
Joint returns and two incomes
Joint thresholds are double the single ones for every deduction except the tips cap, which stays at $25,000 per return. Two earners who each have tips share the cap; two earners who each have overtime share the $25,000 joint overtime cap. For the senior deduction, each spouse's $6,000 is reduced by the same 6% of joint excess.
Getting MAGI right
Use the AGI line from your return, not your gross pay. A worker with $80,000 of wages who contributes $8,000 to a traditional 401(k) has W-2 box 1 wages of $72,000, and that lower figure feeds AGI. Self-employed people subtract half of self-employment tax and retirement contributions before reaching AGI. Capital gains, IRA distributions, the taxable part of Social Security and unemployment compensation all add to it. If you are estimating early in the year, project the full-year figure rather than annualizing a single pay stub, because overtime and bonuses tend to cluster.
Managing MAGI
Because MAGI starts from AGI, the usual tools that lower AGI also preserve the deductions: traditional 401(k) and 403(b) contributions, deductible IRA contributions, HSA contributions, and for self-employed people retirement plans and the deduction for half of self-employment tax. People 70½ and older can use qualified charitable distributions. Deferring a bonus or capital gain into another year can help if it is near the edge. None of these choices should be made for the deduction alone, but they are worth considering if you are just over a threshold.
A household with several deductions
Consider a married couple filing jointly with MAGI of $215,400. One spouse is a server with $14,000 of qualified tips; the other is 66 and works part time. They bought a new US-assembled SUV and paid $2,400 of loan interest. Tips: MAGI is below $300,000, so the full $14,000 counts. Car loan: the excess over $200,000 is $15,400, which rounds up to 16 thousands and a $3,200 reduction — more than the $2,400 of interest, so that deduction is zero. Senior: the excess over $150,000 is $65,400, 6% is $3,924, leaving $2,076. Their Schedule 1-A total is $16,076. The example shows why it is worth checking each deduction separately rather than assuming they all phase out together.
Try it
Each calculator on this site applies its own rounding rule exactly as described here. The combined calculator applies all four at once from one MAGI figure. Start with the tips, overtime, car loan or senior tool if you only need one.
This guide explains the arithmetic and is not tax advice. The Schedule 1-A instructions control if they differ.