How to Count Your Telework Days as a Frontalier: the 40% Tax Rule vs the 49.9% Social-Security Rule

The most common counting mistake is keeping a single number for two different rules. Put side by side, the differences are structural, not cosmetic:

The Swiss State Secretariat for International Finance sets out the tax side on sif.admin.ch; France published the avenant through décret n° 2025-838; the social-security framework is documented by the CLEISS. Because the ceilings come from separate instruments, staying under one does not formally establish anything about the other. In practice, though, 40% is the lower bar: a year planned around the tax ceiling also lands inside the social-security one.

The tax rule is expressed as a share of annual working time — not calendar days, not weeks. Within that envelope:

• Every day teleworked from your home in France counts. Up to 40% of your annual working time can be performed there while your salary remains taxed as if you had worked on Swiss soil — that is the whole point of the avenant, in force since 24 July 2025 and applicable to income years from 1 January 2026 (sif.admin.ch).
• Temporary missions ride inside the same envelope, up to 10 days a year. The French publication text, décret n° 2025-838, counts temporary assignments in the residence state or a third state as telework « pour autant que leur durée cumulée n'excède pas 10 jours par année » — for as long as their cumulative duration does not exceed 10 days per year. Client visits, training days and conferences outside Switzerland belong in this sub-count.
• The denominator is your own annual working time. A full-time schedule and an 80% contract produce different day equivalents for the same 40%, so anchor the count to your contracted hours rather than a generic "two days a week".

What the 40% does in your specific canton — Geneva's source taxation versus the 1983-accord cantons — is a regime question, not a counting question; the reference list of conventions on impots.gouv.fr and your cantonal tax administration are the places to check it.

The social-security ceiling answers a different question: how much cross-border telework can you do without your affiliation switching from Switzerland to France? Since 1 July 2023, the EU/EFTA framework agreement on habitual cross-border telework — which both France and Switzerland have signed — allows telework in the residence state up to just under 50% of total working time (49.9% by convention) while keeping the worker affiliated in the employer's state.

The mechanics differ from the tax rule in one important way: this is not something you claim on a return. The agreement works as a standing derogation under Article 16 of Regulation (EC) 883/2004, and it is your employer who requests the corresponding A1 certificate attesting that Swiss legislation continues to apply to you. The standard route documented by the CLEISS covers the simple case: a salaried worker, a single Swiss employer, and telework as the only activity carried out from France. Anything outside that pattern — several employers in different states, self-employment on the side, a second activity in France — falls outside the standard route, and the right desks for it are the CLEISS on the French side and your employer's caisse de compensation on the Swiss side.

For your count, the consequence is simple: the 49.9% is measured against your total working time, so the same day log you keep for the tax rule gives you this percentage too — you are just reading it against a different line.

One day log, two readings — but do not let the shared log blur the rules, because they diverge exactly where it gets expensive:

• A 45% telework year is comfortably inside the social-security ceiling and clearly over the tax one. The reverse cannot happen: 40% is always the binding line.
• Mission days have a defined place in the tax count — inside the 40%, capped at 10 per year by décret n° 2025-838 — while their treatment in the social-security count depends on your overall pattern of activity; if missions are a regular feature of your job, ask the CLEISS or your employer's caisse rather than assuming.
• The consequences differ in kind. Crossing 40% changes where salary is taxed; crossing 49.9% changes which system you contribute to — different administrations, different paperwork, different money.

This article deliberately stays on the counting question. For the tax agreement itself — the cantonal regimes, the 2027 automatic data exchange, what actually happens when you cross the line — read our dedicated guide to the 40% telework tax rule.

Neither text prescribes a bookkeeping format, so what follows is method, not law — a way to keep numbers you can defend.

1. Count worked days, not calendar days. Leave, public holidays and sick days are not working time: they belong in neither the numerator nor the denominator. A morning at home and an afternoon in the office is 0.5 telework day.
2. Keep three columns, not one: days worked in Switzerland; days teleworked from France; mission days outside Switzerland. Mission days get their own column precisely because they carry their own 10-day sub-limit in the tax count.
3. Tally monthly. A December reconstruction from memory convinces nobody. A dozen month-end totals, kept as you go, reads like a record.
4. Compute both rates from the same log. Telework days (plus mission days within the allowance) divided by total worked days, read once against 40% and once against 49.9%.
5. Attach evidence as you go: calendar exports, badge or transport records, and the telework schedule agreed with your employer. Your employer's HR definition of your annual working time is the official denominator — align your count with it once, in writing.

None of this is legally required. All of it is what makes the difference between asserting your rate and being able to show it.

Take a full-time frontalier who, after leave and public holidays, works 220 days in the year. The arithmetic — and it is only arithmetic, since the official reference is your own contractual working time:

• 40% tax ceiling: 0.40 × 220 = 88 days. Two telework days a week for 44 working weeks uses exactly that.
• 10 mission days inside it: a year with 8 mission days leaves at most 80 ordinary telework days under the tax ceiling.
• 49.9% social-security ceiling: just under 110 days — roughly 109 days on this schedule. The 21-day gap between the two ceilings is the zone where your tax position moves while your social security does not.
• A buffer is cheap insurance. Aiming around 35% (~77 days here) absorbs an unplanned week at home without drama.

Run the same arithmetic on your own contracted days — an 80% contract, a mid-year start or a long sick leave all shift the day equivalents, which is exactly why the monthly tally matters more than any rule of thumb.

The counting method above tells you where you stand. What crossing a line means for you personally — canton by canton, contract by contract — is a question for the institutions, not for a blog:

• Tax: your cantonal tax administration on the Swiss side and your French tax office; the convention texts are indexed on impots.gouv.fr.
• Social security and the A1: your employer first, then the CLEISS on the French side.
• Employer policy: many Swiss employers now fix a contractual telework cap and ask for periodic attestations — your HR department's count and yours should never diverge by more than a conversation.

And if you would rather not run the tally by hand: AdminLanding's free frontalier telework tracker keeps the day log, the monthly totals, the separate mission-day count and your running annual percentage in one place — built around the 40% and 49.9% ceilings described here.