Yes. A Solo 401(k) needs an EIN even if you’re a sole proprietor with no employees, and normally it’s your business’s own EIN, not a second one. The IRS Schedule C instructions say you need an EIN if you have a qualified retirement plan, and the Form 5500-EZ instructions say not to put a Social Security number on the employer line. The only gray area is opening day: some providers let you start with your SSN and ask for the EIN later. That’s their policy, not an IRS exception. Some providers also ask for a separate EIN for the plan’s trust, so check with yours.
Applying directly with the IRS is free and the number is issued immediately online. Still choosing a plan? The Solo 401(k) vs. SEP IRA comparison covers the trade-offs. Once you have the number, opening the account step by step is the next stop, and the Solo 401(k) contribution calculator shows how much you can put in at your profit.
Do you need an EIN? Quick answer by situation
| Your situation | EIN for the plan? | Why | Source |
|---|---|---|---|
| Sole proprietor, no employees, opening a Solo 401(k) | Yes (provider may let you start with an SSN) | A qualified retirement plan is on the IRS list of reasons to need an EIN; Form 5500-EZ rejects an SSN | Schedule C instructions, Line D; Form 5500-EZ instructions, Line 2b |
| Sole proprietor who already has an EIN for another reason | Yes, use the one you have | A business entity should have only one EIN | IRS EIN page |
| Single-member LLC | Yes: the LLC's EIN | An LLC needs an EIN to operate; the LLC rules for keeping your sole proprietor EIN are further down | IRS EIN page; When to get a new EIN |
| Multi-member LLC (partnership) or S corporation | Yes: the entity's own EIN | Partnerships, LLCs and corporations need an EIN to operate; electing S corporation status doesn't require a new one | IRS EIN page; When to get a new EIN |
| Business with employees | Yes, and eligible employees join the plan | A plan that covers anyone besides you (and your spouse) is no longer a one-participant plan | IRS one-participant 401(k) page; Form 5500-EZ instructions |
| You and your spouse in the same plan | One EIN for the plan | A one-participant plan can cover you and your spouse | Form 5500-EZ instructions |
Sole proprietor, no employees, opening a Solo 401(k)
- EIN for the plan?
- Yes (provider may let you start with an SSN)
- Why
- A qualified retirement plan is on the IRS list of reasons to need an EIN; Form 5500-EZ rejects an SSN
- Source
- Schedule C instructions, Line D; Form 5500-EZ instructions, Line 2b
Sole proprietor who already has an EIN for another reason
- EIN for the plan?
- Yes, use the one you have
- Why
- A business entity should have only one EIN
- Source
- IRS EIN page
Single-member LLC
- EIN for the plan?
- Yes: the LLC's EIN
- Why
- An LLC needs an EIN to operate; the LLC rules for keeping your sole proprietor EIN are further down
- Source
- IRS EIN page; When to get a new EIN
Multi-member LLC (partnership) or S corporation
- EIN for the plan?
- Yes: the entity's own EIN
- Why
- Partnerships, LLCs and corporations need an EIN to operate; electing S corporation status doesn't require a new one
- Source
- IRS EIN page; When to get a new EIN
Business with employees
- EIN for the plan?
- Yes, and eligible employees join the plan
- Why
- A plan that covers anyone besides you (and your spouse) is no longer a one-participant plan
- Source
- IRS one-participant 401(k) page; Form 5500-EZ instructions
You and your spouse in the same plan
- EIN for the plan?
- One EIN for the plan
- Why
- A one-participant plan can cover you and your spouse
- Source
- Form 5500-EZ instructions
Sources: IRS Schedule C instructions (Line D), Instructions for Form 5500-EZ (2025), IRS pages Employer identification number, When to get a new EIN and One-participant 401(k) plans, all opened October 20, 2026.
If your question is whether your freelance business needs an EIN outside the plan, see do freelancers need an EIN.
Why you need one: the plan, not necessarily the business
If you’re a sole proprietor with no employees, you may not need an EIN for the business by itself. The Schedule C instructions say when you do: “You need an EIN only if you have a qualified retirement plan or are required to file employment, excise, alcohol, tobacco, or firearms returns or are a payer of gambling winnings,” per the IRS Instructions for Schedule C, Line D. A Solo 401(k) is a qualified retirement plan, so having one is enough to need an EIN, even if nothing else about a one-person business requires it.
Normally it’s your business’s EIN
On the IRS return itself, a Solo 401(k) doesn’t need a “plan EIN” separate from your business’s. The Instructions for Form 5500-EZ, the return one-participant plans file when plan assets exceed $250,000, say to “enter the employer’s nine-digit employer identification number (EIN)… Do not enter a social security number (SSN)” on the line for the plan sponsor. There’s also a line for the plan administrator’s EIN, but the instructions say: “unless the administrator is the employer identified [on the employer line]… enter the word ‘Same’… and leave the remainder… blank.” In a typical Solo 401(k), you’re both the employer and the administrator, so you write “Same” and use one EIN for both roles.
When a provider asks for a separate trust EIN
The plan’s assets are held in a trust or custodial account, and some providers want that account to have its own EIN. The IRS’s EIN page lists a “retirement plan or individual retirement account (IRA)” among the entities that need an EIN to operate. Whether you need one depends on how your provider sets up the plan. Ask your provider before you apply, and don’t apply for a second number unless they tell you to.
| Your situation | EIN |
|---|---|
| Sole proprietor opening a Solo 401(k) | Yes: a qualified retirement plan requires one (Schedule C instructions, Line D) |
| Sole proprietor who changes business name or location | No new EIN |
| Sole proprietor who owns several businesses | No new EIN |
| Sole proprietor who incorporates, forms a partnership or declares bankruptcy | New EIN |
| Single-member LLC using the owner's EIN, no corporate or S corp election, no employees or excise taxes | Can keep using the sole proprietor EIN |
| Existing LLC that changes its tax election to a corporation or an S corporation | No new EIN |
Sole proprietor opening a Solo 401(k)
- EIN
- Yes: a qualified retirement plan requires one (Schedule C instructions, Line D)
Sole proprietor who changes business name or location
- EIN
- No new EIN
Sole proprietor who owns several businesses
- EIN
- No new EIN
Sole proprietor who incorporates, forms a partnership or declares bankruptcy
- EIN
- New EIN
Single-member LLC using the owner's EIN, no corporate or S corp election, no employees or excise taxes
- EIN
- Can keep using the sole proprietor EIN
Existing LLC that changes its tax election to a corporation or an S corporation
- EIN
- No new EIN
Sources: IRS Schedule C instructions (Line D), IRS page When to get a new EIN (opened October 20, 2026), IRS EIN page.
Opening it with your spouse
If your spouse works in the business and joins the plan, you still have one plan and one EIN. The Form 5500-EZ instructions define a one-participant plan as one that “covers only you (or you and your spouse)” when you own the entire business. Your spouse doesn’t need an EIN of their own. For how the two contribution limits work, see the spouse Solo 401(k) guide. If you hire an employee who meets the plan’s eligibility rules, the plan has to include them, and it is no longer a one-participant plan.
Where this EIN gets used
- Schedule C, Line D. Once you have the plan, this is the EIN you enter on your own tax return.
- Form 5500-EZ, if your plan’s assets exceed $250,000, the annual filing threshold for one-participant plans, per the IRS page on Form 5500-EZ.
- Opening the account with a provider. Some Solo 401(k) providers ask for your EIN upfront; others let a sole proprietor with no employees start without one and ask for it later. Most use the same business EIN you use everywhere else. If yours asks for a separate EIN for the plan’s trust, see the section above.
How to get one
Applying for an EIN is free. You can apply online with the IRS and usually receive the number immediately, per the IRS page on employer identification numbers, so there’s no need to pay a third party. The IRS warns: “Beware of websites that charge for an EIN.” If a site asks for a fee before you reach the IRS form, you’re on the wrong site.
Applying online, step by step
The IRS’s online EIN application takes a few minutes:
- Check that you can use it. The IRS tool is for a domestic organization, formed or created in the United States or its territories, and you apply as the “responsible party” in control of the entity.
- If you have an LLC, form it with your state first. The IRS says that applying before the entity exists may delay your application.
- Pick a time the tool is open. Its hours, Eastern Time: Monday to Friday 6 a.m. to 1 a.m. the next day, Saturday 6 a.m. to 9 p.m., and Sunday 6 p.m. to midnight.
- Finish in one sitting. You can’t save the application for later, and it expires after 15 minutes of inactivity.
- Print the confirmation letter once the EIN is issued.
Only one EIN is issued per responsible party per day. You can use it right away for most things, though some uses, like e-filing a return, can take up to 2 weeks.
What if I already have a business EIN?
Then you’re normally set for the plan too. The Form 5500-EZ instructions treat the plan administrator’s EIN as the employer’s whenever one person fills both roles, so there’s no separate application to make for the return. If a provider asks you for “the plan’s EIN,” ask whether they mean your business’s EIN or a separate EIN for the plan’s trust. Some providers require the latter.
If you can’t find it
The IRS’s EIN page suggests checking the notice the IRS sent when you applied, your business bank, any agency where you applied for a license, and past business tax returns.
If none of those works, call the IRS Business and Specialty Tax Line at 800-829-4933 (Monday to Friday, 7 a.m. to 7 p.m. local time). Once they’ve verified your identity, they can give you the number over the phone. Don’t apply for a new one instead: the IRS says a business entity should have only one EIN.
When you’d need a new EIN
Changes to your business can change your EIN. Per the IRS page When to get a new EIN:
- Sole proprietors need a new EIN if they incorporate, form a partnership or declare bankruptcy. A new business name or location doesn’t require one, and neither does owning several businesses.
- A single-member LLC can keep using the owner’s sole proprietor EIN if it doesn’t elect corporate or S corporation treatment and has no employees or excise taxes.
- Electing S corporation status doesn’t by itself require a new EIN for an existing corporation or LLC.
If you do get a new EIN, check which number your plan provider has on file. For how an LLC changes your taxes, see LLC vs. Sole Proprietorship.
Form 5500-EZ: dates and penalties
Per the Instructions for Form 5500-EZ:
- The $250,000 test combines plans. If you maintain more than one one-participant plan, their assets are added together. If the total exceeds $250,000 at year-end, each plan files, even the smaller ones.
- Due date: the last day of the 7th month after the plan year ends. For a calendar-year plan, that’s July 31 of the following year, or the next business day if it falls on a weekend or holiday.
- More time: Form 5558 gives a one-time extension of up to 2½ months. If the plan year matches your tax year, extending your income tax return also extends the 5500-EZ automatically; keep a copy of that extension with the plan’s records.
- Closing the plan: a final return is required for the year all assets are distributed, even if the plan never reached $250,000. That includes rolling everything into an IRA.
- Penalty: $250 a day, up to $150,000 per plan year, for filing late.
Form 5500-EZ due dates and late penalty
| Plan year ends | Due date |
|---|---|
| December 31, 2025 | July 31, 2026 (a Friday) |
| December 31, 2026 | August 2, 2027 (July 31 is a Saturday, so the next business day) |
| June 30, 2026 | February 1, 2027 (January 31 is a Sunday, so the next business day) |
December 31, 2025
- Due date
- July 31, 2026 (a Friday)
December 31, 2026
- Due date
- August 2, 2027 (July 31 is a Saturday, so the next business day)
June 30, 2026
- Due date
- February 1, 2027 (January 31 is a Sunday, so the next business day)
Last day of the 7th calendar month after the plan year ends; if it falls on a Saturday, Sunday or legal holiday, the next day that isn't one (Instructions for Form 5500-EZ, 2025). The weekday adjustments are our arithmetic from the calendar.
| Days late | Penalty at $250 a day |
|---|---|
| 30 | $7,500 |
| 90 | $22,500 |
| 365 | $91,250 |
| 600 or more | $150,000 (the cap per plan year) |
30
- Penalty at $250 a day
- $7,500
90
- Penalty at $250 a day
- $22,500
365
- Penalty at $250 a day
- $91,250
600 or more
- Penalty at $250 a day
- $150,000 (the cap per plan year)
$250 a day, up to $150,000 per plan year (Code section 6652(e), Instructions for Form 5500-EZ). Our arithmetic.
Common mistakes
- Paying for an EIN. The IRS says: “You never have to pay a fee for an EIN.”
- Applying twice. If you already have more than one EIN, call the Business and Specialty Tax Line and the IRS will tell you which one to use.
How we got these figures
The $250,000 threshold, the due date rule, the 2½-month extension, the $250-a-day penalty and the rules for the EIN fields come from the 2025 Instructions for Form 5500-EZ. The application hours, the one-EIN-per-day rule and the lost-EIN phone line come from the IRS EIN pages, and the IRS one-participant plan page. We opened all of them on October 20, 2026. The tables are our own arithmetic from those rules. For how we pick and check sources, see How We Research.
