The short answer
If tax was withheld from your pay, or you qualify for a refundable credit such as the Earned Income Tax Credit, you may be owed a refund even when your income was too low to require a return. You generally have three years from the original due date to claim it. Filing need not cost anything: IRS Free File is available at adjusted gross income of $89,000 or less, and volunteer sites prepare returns for most people earning $69,000 or less.
Why refunds go unclaimed
Income below the filing threshold means you are not required to file, but it does not mean you are owed nothing. Tax withheld from a paycheck is returned only when you file, and refundable credits are paid only when you claim them. The people most likely to miss out are part-year workers, students, older people with small jobs and families whose income fell.
The Earned Income Tax Credit is the largest single source of the money left behind. It is refundable, so it is paid even when no tax is owed, and the IRS estimates that about one in five eligible workers does not claim it - often because they did not know they qualified, or did not file at all.
The three-year deadline
To receive a refund you generally have to file within three years of the return's original due date. A return normally due in April has to be filed within three years of that April to release its refund; after that the money is kept, however clear the entitlement. Not having known about it does not extend the deadline.
The deadline applies to refunds, not to tax owed. If you owe for a year you did not file, the IRS can still assess it, and a refund for a later year may be held and applied to an earlier year's balance or to certain other debts, such as past-due child support.
Claiming a refund for a past year
IRS Free File covers only the current filing season. For an earlier year you use that year's forms and instructions, which remain on IRS.gov, and in many cases the return is filed on paper to the address given in those instructions.
If you no longer have your W-2s or 1099s, the IRS can supply the figures: a wage and income transcript, free from your IRS online account, lists what employers and payers reported for that year. Rebuild the return from it rather than from memory, because a mismatch with what the IRS already holds delays the refund.
Filing for free with IRS Free File
IRS Free File is a partnership between the IRS and tax software companies. If your adjusted gross income was $89,000 or less, it provides guided preparation and electronic filing of your federal return at no cost, and some offers include a state return. Start from IRS.gov rather than a company's own site, because the same companies' paid products do not follow the Free File terms.
Anyone, at any income, can use Free File Fillable Forms, electronic versions of the paper forms that can be filed online. They do little of the arithmetic for you, so they suit people who are comfortable preparing a return by hand.
Volunteer help: VITA and TCE
The Volunteer Income Tax Assistance programme prepares and files returns free for people who generally earn $69,000 or less, people with disabilities and people who speak limited English. The volunteers are certified by the IRS and handle the Earned Income Tax Credit and the Child Tax Credit routinely.
Tax Counseling for the Elderly offers the same service with a focus on people aged 60 and older and on pension and retirement questions. Both run mainly from late January to April at libraries, community centres and similar places; the IRS site locator lists those near you, and some stay open after the filing season for past-year returns.
What our calculator works out
Our EITC calculator estimates the credit from your earnings, filing status and number of qualifying children. It is a quick way to see whether a year you did not file might have carried a refund worth claiming - run it with that year's income in mind.
What this does not tell you
This article is general information, not tax advice. Whether you are owed a refund, and how much, depends on the return you file and on what the IRS already holds for that year.
The calculator applies the current year's EITC rules. Earlier years had different amounts and limits, so for a past year its result is only a sign that a return may be worth filing.
Official sources
What you can do next
List the last three years in which you worked and did not file. For each one, get the wage and income transcript, then have the return prepared at a VITA site or on that year's forms - the oldest of the three is the one closest to being lost.