Do I pay both US self-employment tax and CPP?

No. And for a lot of readers the question never arises, which is the part most pages skip. US self-employment tax runs on your status rather than on where the people paying you sit: it generally reaches US citizens and US residents even when they live and work outside the United States. So the overlap with CPP is a US-person problem. If that’s you and you live in Canada, the Canada-US totalization agreement assigns your self-employment coverage to Canadian law, and section 1401(c) then exempts the same income from US self-employment tax, to the extent it falls exclusively under the Canadian system. What people get wrong is the proof, because the exemption isn’t automatic on the return.

Who owes US self-employment tax in the first place?

Generally, US citizens and US residents, wherever they live and work. IRC 1402(b) defines self-employment income as net earnings derived by an individual “other than a nonresident alien individual”, and the SSA describes who Social Security covers in the same terms. US residence runs on IRC 7701(b)(1)(A): a green card, substantial presence, or a first-year election. A Canadian resident who is not a US citizen and meets none of the three generally has no US self-employment tax on that income and no US return to report it on, which leaves nothing for the agreement to relieve.

“In general, U.S. Social Security covers self-employed workers if they are U.S. citizens or resident non-U.S. citizens, even if they live and work outside the United States.” SSA, Totalization agreements overview

Two carve-ins keep that from being a clean line. The exclusion in IRC 1402(b) reads “other than a nonresident alien individual, except as provided by an agreement under section 233 of the Social Security Act”, so a totalization agreement can pull in someone the Code would otherwise leave out. And Article V(8) of the agreement sends a person back to US law where they would be subject to Canadian law but “coverage is not effected under those laws”. Both are narrow, and both are reasons to settle status before working through the rest of this.

Which country’s system am I in?

Your country of residence, once both systems reach the same earnings. The agreement’s self-employment rule starts there: it applies to a person who would otherwise be covered under the laws of both countries, and it then assigns that person to Canadian law alone if they are resident in Canada for the purposes of those social security laws. It doesn’t turn on where the people paying you are based, where the money lands, or which currency you invoice in.

“Self-employed workers who reside in the U.S. are assigned U.S. coverage. Self-employed workers who reside in Canada are assigned coverage under the Canadian or Quebec system.” SSA, Agreement with Canada

That is the SSA’s summary of it. The operative text is Article V(6) of the agreement, and it carries three qualifications the summary drops: it engages only where a person would be covered under both countries’ laws; it keys residence to the relevant social security laws rather than to income tax residence; and Article V(11) lets the two Competent Authorities “by common agreement, make exceptions in the application of this Article in respect of any person or category of persons”.

“Where, but for this Article, a person would be covered under the laws of both Contracting States in respect of earnings from self-employment, that person shall, in respect thereof, be subject only to the laws of Canada if that person is considered to be resident in Canada for the purposes of the relevant provisions of those laws, and only to United States laws in any other case.” Agreement on Social Security between Canada and the United States, Article V(6)

One more piece of plumbing, because it changes which form you file. The agreement itself reaches only two Canadian programs, and Quebec sits outside it.

“For Canada, the Agreement applies to the Old-Age Security program and the Canada Pension Plan. The United States has an Understanding with Quebec that applies to the Quebec Pension Plan.” SSA, Agreement with Canada

Where you live	Which system covers you	Which certificate
United States	US Social Security and Medicare	From the SSA on request, no special form
Canada, outside Quebec	Canada Pension Plan	Form CPT56, from the CRA
Quebec	Quebec Pension Plan	Form QUE/USA 101, from Retraite Quebec

The employee rule is different and is not what this page covers. Article V(1) puts an employed person under the laws of the country where the work is done, and Article V(2)(a) then keeps a worker sent there by their own employer under the first country’s laws so long as the period of work “is not expected to exceed 60 months”. A Canadian employee temporarily working in the US is a separate analysis.

How do I actually prove the exemption?

You request a certificate of coverage, and then you keep attaching it. This is the step that gets missed, because people treat the agreement as something that just applies. The SSA is direct that a certificate is what establishes the exemption, and it comes from the Canadian side rather than from the IRS: the CRA if CPP covers you, Retraite Quebec if you’re in Quebec. Ask early, because the certificate carries an effective date that generally runs from when the work started rather than from when you applied, and never from before the agreement took effect.

“Employers and self-employed workers must request a certificate of coverage to establish an exemption from U.S. Social Security contributions.” SSA, Agreement with Canada

The annual part is stated separately and is the bit worth underlining. In the SSA’s own words, self-employed workers “should attach a copy of the certificate of coverage to their U.S. tax return every year as proof of the exemption.” That means every return you file, rather than the first one alone. A certificate sitting in a drawer while it was omitted from three filed returns is the pattern that produces a query.

The SSA puts the date and the timing advice in one paragraph.

“The certificate of coverage you receive from one country will show the effective date of your exemption from paying social security system taxes in the other country. Generally, this will be the date you began working in the other country, but no earlier than the effective date of the Agreement. To avoid any difficulties, employers and self-employed workers should request a certificate as early as possible, preferably before work in the other country begins.” SSA, Totalization agreements overview

What would the US tax be if I got this wrong?

Three taxes on self-employment income, and they do not run on the same base. IRC 1402(b)(1) caps the base for the 12.4 percent old-age, survivors and disability tax at the contribution and benefit base less wages, and sets no cap on the 2.9 percent hospital insurance tax. IRC 1401(b)(2) then adds 0.9 percent above $250,000 on a joint return, half that filing separately, and $200,000 in any other case. So the rate is 15.3 percent up to the wage base and 2.9 percent above it, plus another 0.9 percent above your threshold, and those two boundaries can fall in either order.

“there shall be imposed for each taxable year, on the self-employment income of every individual, a tax equal to 12.4 percent of the amount of the self-employment income for such taxable year.” IRC 1401(a)

The hospital insurance tax is imposed in the same form at IRC 1401(b), at 2.9 percent under paragraph (1), with paragraph (2) adding the further 0.9 percent above the thresholds just given, and subparagraph (B) reducing those thresholds, but not below zero, by any wages taken into account. The relief from all three sits in the same section, at subsection (c), and it carries a condition: it exempts self-employment income from the section 1401 taxes to the extent that income is subject under the agreement exclusively to the other country’s social security system. That word “exclusively” is the test. Subsection (c) itself says nothing about a certificate, and the requirement to hold one comes from the SSA procedure quoted earlier. The two do different jobs, and both are live for a Canadian-resident US person: the statute sets the condition, and on the SSA’s wording you must request the certificate to establish the exemption.

“During any period in which there is in effect an agreement entered into pursuant to section 233 of the Social Security Act with any foreign country, the self-employment income of an individual shall be exempt from the taxes imposed by this section to the extent that such self-employment income is subject under such agreement exclusively to the laws applicable to the social security system of such foreign country.” IRC 1401(c)

What if I already paid into both?

Then one of the two was charged on earnings the agreement assigns elsewhere. Article V(6) puts self-employment coverage under one system, “only to the laws of Canada” or “only to United States laws in any other case”, and IRC 1401(c) grants the US exemption only to the extent the income is subject under the agreement exclusively to the other one. Both cannot be right on the same earnings. Which authority refunds what, and for which years, turns on where the money went, so treat it as a file-specific question rather than a rule.

• Whether you’re still a Canadian tax resident, because residence is the hinge, though Article V(6) keys it to the relevant social security laws rather than to income tax residence
• Working remotely for a US company as a Canadian, the employee-side version of this question
• How CPP and OAS are taxed if you live in the US
• How the totalization agreement works for employees, covering the 5-year detached worker rule, combining credits for benefit eligibility, and the WEP repeal
• Streamlined filing for self-employed and business owners, if you never filed US returns on your business income and need to catch up through the streamlined program
• Estimated tax payments and instalments, the quarterly payment obligations for self-employed cross-border filers

Worth pinning down what the certificate reaches. On the SSA’s own wording it establishes “an exemption from U.S. Social Security contributions”, and that is the whole of its scope. Income tax runs on separate rules and its own treaty articles, the certificate answers nothing there, and whether you have a US or Canadian return to file is a separate analysis.

What should I do next?

Settle your status first, because it decides whether any of this applies to you: US self-employment tax generally reaches US citizens and US residents, and the agreement can pull in a narrow class outside them. Then establish where you were resident during the period you did the work, since that decides which system covers it. Then request the certificate from the right authority, the CRA for CPP and Retraite Quebec for QPP, as early as you can, and build attaching it into your annual filing routine rather than treating it as a one-time task.

Related guides:

• Social Security and CPP: How the US-Canada Totalization Agreement Prevents Double Social Security Tax

This guide is general information, not tax advice for your specific situation. Which points apply, and how, depends on your facts.