Social security for the self-employed: when it starts and how it is calculated
Opening an activity as a self-employed person links you to social security, but payment does not start on the day the activity starts. This article summarises what the practical guide of the Instituto da Segurança Social (ref. 1009, version 1.14, of 16 May 2025) says about the start, the quarterly return and the method of calculation.
The euro examples in that guide belong to 2025. This text does not repeat them, because the 2026 amounts were not confirmed here. Always check the guide that is in force.
When cover starts
According to the guide, the first enrolment takes effect only on the first day of the 12th month after the activity starts. It can be brought forward in the quarterly return.
The Tax Authority reports the start of activity to social security, which registers the person automatically.
The quarterly return
The guide states that the quarterly return is filed by the last day of January, April, July and October.
How the guide describes the calculation
Under the guide's general rule, relevant income is 70% of income from services and 20% of income from the sale of goods. The monthly base is one third of the quarter's relevant income.
The same guide uses a rate and a minimum contribution in its examples. Those figures belong to the May 2025 edition. We do not treat them here as the 2026 figures.
Exemptions
The guide describes exemptions, for example when the activity is combined with employment and for pensioners, each with its own conditions and deadlines. Each case depends on those conditions. The detail is in section C of the guide, not in this summary.
Key takeaways
- According to the guide, the first enrolment starts on the first day of the 12th month after the activity starts, unless it is brought forward in the quarterly return.
- The quarterly return is filed by the last day of January, April, July and October.
- The guide's general rule uses 70% for services and 20% for sales of goods, and the monthly base is one third of the quarter's relevant income.
- Do not use this text for a figure in euros: check the ISS guide that is in force.
Frequently asked questions
Official sources
- Practical guide 1009 — New regime for self-employed workers (v1.14, 16 May 2025) (opens in a new tab)
Instituto da Segurança Social, I.P. ·
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