An income interval is a defined range of dollar amounts, with a lower and an upper limit, used to sort earnings into groups. You see income intervals every time you look at a federal tax bracket table, a Census income report, or an eligibility chart for a benefits program. Grouping earnings into ranges lets agencies, researchers, and policymakers describe how income is distributed across millions of people without listing every individual figure.
What Makes Up an Income Interval
Every income interval has two boundaries. The lower limit is the smallest dollar amount that places an earner inside the group, and the upper limit is the largest. Subtract the lower from the upper and you get the class width. A bracket of $25,000 to $34,999 has a class width of roughly $10,000.
Intervals must be mutually exclusive, so no single income figure can land in two groups. If one bracket runs to $49,999, the next begins at $50,000. They also leave no gaps, so every dollar of income fits into exactly one bracket.
The midpoint sits halfway between the two limits. For a bracket of $25,000 to $34,999, the midpoint is $30,000. Midpoints matter when you are estimating an average from data that has already been grouped.
Closed, Open-Ended, and Variable-Width Intervals
A closed interval defines both endpoints, such as $50,000 to $74,999. Most brackets in a published dataset are closed, and they work well through the middle of the income distribution.
An open-ended interval leaves one boundary undefined. The bottom of a table might read “Less than $10,000,” and the top might read “$200,000 or more.” Open-ended brackets handle outliers, particularly very high earners whose exact incomes matter less for broad statistical trends. The tradeoff is precision: without a defined boundary, you cannot calculate an exact midpoint for that group.
Intervals can also be uniform or variable in width. Uniform intervals use the same class width throughout. Variable-width intervals adjust the range to match how the data is distributed, using narrower brackets where earners cluster and wider brackets where they thin out.
Income Intervals in Census Data
The U.S. Census Bureau uses variable-width income intervals throughout the American Community Survey, which collects annual data on income, employment, housing, and demographics. The ACS household income brackets are:
- Less than $10,000
- $10,000 to $14,999
- $15,000 to $24,999
- $25,000 to $34,999
- $35,000 to $49,999
- $50,000 to $74,999
- $75,000 to $99,999
- $100,000 to $149,999
- $150,000 to $199,999
- $200,000 or more
The lower brackets are narrower, in the $5,000 to $10,000 range, because small income differences at the low end have a large effect on a household’s economic situation. The upper brackets run $25,000 to $50,000 wide because a broader range captures enough earners to produce reliable estimates. The top bracket is open-ended.1U.S. Census Bureau. Table S1901 – Income in the Past 12 Months Policymakers use these figures to allocate federal resources, measure poverty, and identify populations that need economic support.2U.S. Bureau of Labor Statistics. American Community Survey
Federal Tax Brackets Are Income Intervals
Federal income tax brackets are the income intervals most people encounter directly. Under 26 U.S.C. § 1, the tax code groups taxable income into progressive tiers, each taxed at a different rate.3Office of the Law Revision Counsel. 26 USC 1 – Tax Imposed For tax year 2026, a single filer faces seven brackets:
- 10% on taxable income up to $12,400
- 12% on income from $12,401 to $50,400
- 22% on income from $50,401 to $105,700
- 24% on income from $105,701 to $201,775
- 32% on income from $201,776 to $256,225
- 35% on income from $256,226 to $640,600
- 37% on income above $640,600
The thresholds adjust each year for inflation.4Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026 Married couples filing jointly get wider brackets. Their 10% bracket covers taxable income up to $24,800, and the top 37% rate applies above $768,700.5Internal Revenue Service. Revenue Procedure 2025-32
The word to notice is “progressive.” Each rate applies only to the income that falls within that specific interval, not to your whole income. A single filer with $60,000 of taxable income pays 10% on the first $12,400, 12% on the next $38,000, and 22% only on the remaining $9,600 that sits above $50,400. Reading a bracket as if the top rate applies to every dollar is one of the most common mistakes people make when estimating their tax bill.
Income Intervals in Benefit Programs
Federal and state programs use income intervals to decide who qualifies for assistance. These intervals are usually tied to the federal poverty guidelines, which are updated each year and vary by household size.
Federal Poverty Guidelines
For 2026, the poverty guideline for a single person in the 48 contiguous states is $15,960 per year. A four-person household reaches the poverty line at $33,000.6U.S. Department of Health and Human Services. Detailed Guidelines 2026 Programs then set their eligibility at multiples of these numbers, such as 130% or 200%, and those multiples become the income intervals that determine who is in and who is out.
SNAP
The Supplemental Nutrition Assistance Program sets its gross income limit at 130% of the poverty level. For fiscal year 2026, a single-person household can earn up to $1,696 per month in gross income, and a four-person household can earn up to $3,483 per month.7USDA Food and Nutrition Service. SNAP – Fiscal Year 2026 Cost-of-Living Adjustments Income within the eligible interval moves you to the next step of the application; income above it generally disqualifies the household.
Social Security Wage Cap
Social Security taxes create a two-tier income interval by applying only to earnings up to an annual limit. For 2026, the taxable wage base is $184,500. You and your employer each pay 6.2% on earnings up to that figure. Any income above $184,500 is not subject to Social Security tax, though Medicare tax still applies.8Social Security Administration. Contribution and Benefit Base
Estimating Averages from Grouped Data
When you have income data that has already been grouped into intervals, you can still estimate the median and the mean using the interval boundaries and the number of earners in each bracket.
For the median, you find the interval that contains the middle observation (the median class) by adding frequencies from the bottom bracket up until you pass half the total count. You then estimate the exact median with: Median = L + ((n/2 − CF) / f) × w, where L is the lower limit of the median class, n is the total number of observations, CF is the cumulative frequency below the median class, f is the frequency of the median class, and w is the class width. The method assumes earners are evenly spread within the bracket.
For the mean, you multiply each interval’s midpoint by the number of earners in it, sum those products, and divide by the total number of earners: Mean = Σ(midpoint × frequency) / Σ(frequency). Open-ended top brackets like “$200,000 or more” force you to assume a midpoint, and that assumption can shift the mean considerably. This is one reason government income reports usually lead with the median rather than the mean.
Why the Choice of Intervals Changes the Picture
How the brackets are drawn affects what the data appears to say. Narrow intervals show more detail but produce noisy tables when the sample is small. Wide intervals smooth out the noise but can hide meaningful differences, since grouping $50,000 earners with $99,999 earners obscures real gaps in financial circumstances.
Open-ended top brackets add a specific complication. The median can still be calculated cleanly, but the mean depends on an assumed upper boundary, and different assumptions can move the reported average by tens of thousands of dollars. When an income statistic looks surprisingly high or low, checking how the intervals were built is often the first useful question to ask.