Health Contribution Base: What to Retain and Why Pro-rating Exposes You to URSSAF Risks

When your collective agreement refers to the “fixed gross base salary”, the health contribution is calculated on the contractual salary—and definitely not on the reduced remuneration due to an absence. Pro-rating this base amount results in underpayment…and exposes you to a URSSAF correction. Here’s the rule and the setting method.

This article is part of the file Payroll Law: The Employer’s Guide.

The Base to Retain: Contractual Fixed Gross Salary

When the collective text refers to “fixed gross base salary” (a common wording found, for instance, in certain collective agreements for service providers), the health contribution is calculated based on the contractual base salary, regardless of monthly variations.

Example: for a contractual base salary of €4,000 gross, the contribution is calculated on €4,000, even if an absence has reduced the paid remuneration to €3,300 (subject to the collective ceiling, often expressed as a percentage of Category A of the monthly ceiling of the Social Security).

Why? The term “fixed” contrasts with any variation: bonuses, increases, overtime or complementary hours, and temporary variations related to an absence. The text explicitly excludes these accessories (“excluding bonuses, excluding increases…”). A strict interpretation is required: if social partners had wanted a prorated base, they would have referred to “the remuneration subject to contributions” or a base “adjusted in case of absence.” In the absence of such provisions, you do not substitute the fixed gross base salary for the social base of the month.

URSSAF Risk of a Reduced Base

Using €3,300 instead of €4,000 leads to underpayment of both employee and employer contributions to the system. Moreover, the employer’s contribution to the funding of supplementary health coverage constitutes a benefit, a portion of which is re-integrated into the CSG/CRDS base. A reduced contribution can thus result in a social shortfall—and create a risk of correction during an audit. Retaining the reference contractual base ensures compliant social treatment.

Note: Check the insurer’s notice or the collective contract—a clause may modify the calculation in the event of contract suspension. In the absence of a contrary provision, calculating based on the non-prorated contractual base salary is the most robust option.

Setting the Bases: Collective Agreement by Collective Agreement

Do not look for a unique “standard” base. The texts may be hybrid (a single article may refer to “gross salaries subject to contributions” while excluding overtime, bonuses, allowances, benefits in kind, and professional expenses).

Method: define a reference base (PMSS, gross subject to contributions, or base salary), then apply inclusion/exclusion rules by payroll item, according to the collective agreement, the implementation act, and the insurance contract.

Frequently Asked Questions

Pro-rating in case of absence? No, base = contractual base salary.

What is the URSSAF risk? Underpayment → CSG/CRDS impact → possible correction.

Is there a base for all collective agreements? No: reference base + rules by item.