This free French Republican calendar converter translates any Gregorian date into its equivalent under France's revolutionary calendar — complete with the décade day name, month meaning, and the famous sansculottide festival days — and converts back the other way too.
Pick a direction using the two tabs above. To convert a modern date, enter a Gregorian year, month and day (any date from September 22, 1792 onward) and click Convert. To go the other way, enter a Republican year, month, and day (or select the "Sansculottides" option for the five or six extra festival days at year's end), then convert. The result shows the matching date, the décade day name, and the month's seasonal meaning.
The French Republican (or Revolutionary) calendar was adopted by the National Convention on 5 October 1793 but made retroactive to 22 September 1792 — the day the First French Republic was proclaimed and, fittingly, the autumn equinox. Mathematician Gilbert Romme designed its decimal structure while poet Fabre d'Églantine invented the poetic month names. The calendar replaced the seven-day week with a ten-day "décade," and even divided each day into 10 decimal hours. Napoleon Bonaparte abolished it effective 1 January 1806, though it briefly reappeared for eighteen days during the Paris Commune of 1871.
The converter counts the exact number of days between your chosen date and the calendar's epoch, 22 September 1792 (Republican Year I, Vendémiaire 1), using Julian Day Numbers for precision across any date. Republican years run 365 days, except "sextile" (leap) years of 366 days, which fall — following the mathematical rule proposed by Gilbert Romme and confirmed by the real equinox during the calendar's active years — whenever the following year number is divisible by 4 (skipping century years not divisible by 400, mirroring the Gregorian leap rule). Each ordinary year has 12 months of exactly 30 days, grouped into three 10-day décades, followed by 5 (or 6, in leap years) extra "jours complémentaires" or sansculottides.
Formula: day-count = JDN(date) − JDN(22 Sept 1792), then that count is divided into Republican years (365 or 366 days each) to find the year, month, and day.
Let's convert 9 November 1799 — the date of Napoleon's coup d'état — into the Republican calendar:
| Step | Value |
|---|---|
| Julian Day Number of 9 Nov 1799 | 2,378,444 |
| Julian Day Number of epoch (22 Sept 1792) | 2,375,840 |
| Days since epoch | 2,604 |
| Years 1–7 consumed (incl. leap Years III & VII) | 2,557 days |
| Remaining day within Year VIII | 47 (0-indexed) |
| Month (day 47 ÷ 30) | Brumaire (month 2) |
| Day within month | 18 |
Result: 18 Brumaire, Year VIII — exactly matching the historical name of Napoleon's coup, "18 Brumaire," proving the math checks out against the record books.
Using the mathematical extension of Romme's leap-year rule, today's date converts to a specific Republican year, month and day — check the "Today's Republican Date" box near the top of this page for the live conversion.
It began retroactively on 22 September 1792 (the day the Republic was proclaimed) and was officially abolished by Napoleon effective 1 January 1806. It reappeared briefly for 18 days during the Paris Commune in 1871.
Every month has exactly 30 days, split into three ten-day "décades." The 5 or 6 leftover days at year's end are separate festival days called sansculottides, not part of any month.
They are the extra days added to complete the 365 (or 366, in leap years) day year: Virtue, Genius/Talent, Labor, Opinion, and Rewards, plus a sixth "Revolution" day in leap years.
It was unpopular with workers (one rest day per ten instead of seven), disrupted trade with other nations still on the Gregorian calendar, and clashed with the Catholic Church's calendar of feast days, which Napoleon was trying to reconcile with under the 1801 Concordat.
For its active years (I–XIV), leap years were meant to track the true astronomical autumn equinox at the Paris Observatory. In practice only Years III, VII, and XI were actually observed as leap, matching a simple 4-year rule during that period.
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