Top 12 Mistakes English Speakers Make
With French Numbers

Mistake #1

Rule: Age uses avoir (to have) + the number + the obligatory word "ans". Never use être (to be) with a bare cardinal. The pattern extends to other physical / experiential states: avoir faim (to be hungry), avoir froid (to be cold), avoir peur (to be afraid). See the full rule on the age in French page.

Severity: High; native speakers identify this immediately as anglicism.

Mistake #2

Rule: The "et" connector is mandatory at every X1 position from 21 through 71 (with the unique soixante et onze for 71). Vingt et un, trente et un, quarante et un, cinquante et un, soixante et un, soixante et onze. The exception: 81 (quatre-vingt-un) and 91 (quatre-vingt-onze) do NOT take et.

Severity: High; this is taught in every French A1 textbook and is a systematic rule.

Mistake #3

Rule: The "et" connector does NOT appear at 81 (quatre-vingt-un) or 91 (quatre-vingt-onze). This is the exception to the et-un rule above. Adding et at 81 / 91 is overcorrection. The reasoning: quatre-vingts is built on multiplication (4 x 20), not on a round-tens base, so the et that would link a unit to a round-ten base does not apply.

Severity: Medium; produced by learners who have over-internalised the et-un rule.

Mistake #4

Rule: Cent and vingt drop the plural-s when another counting word follows in the number. Quatre-vingts (80, alone) but quatre-vingt-un (81). Trois cents (300, alone) but trois cent un (301). The s only appears when the multiplied form is the last counting word in the number.

Severity: High; both adding and dropping the s in the wrong context are common.

Mistake #5

Rule: French uses thin spaces (or non-breaking spaces) for digit grouping, never commas. The comma is reserved for the decimal separator (3,14 = pi). Writing 1,000,000 in French text creates real ambiguity because a French reader scans the comma as a decimal point.

Severity: Medium; a typographic error, not a grammar error, but pervasive in poorly-edited French text.

Mistake #6

Rule: French decimals use the comma (virgule), not the point. Pi = 3,14. The euro-dollar exchange rate = 1,08. The decimal comma is the universal French and continental European convention. Writing 3.14 in French is an anglicism.

Severity: Medium; almost universal in poorly-edited French translations from English.

Mistake #7

Rule: The de-preposition (elided to d' before a vowel) is mandatory between any number ending in million / milliard / billion and the counted noun. Million is a noun and noun-of-quantity; nouns of quantity always require de before what they quantify. Same as une douzaine d'oeufs, une dizaine de personnes.

Severity: High; an immediately diagnostic mistake of intermediate French.

Mistake #8

Rule: France uses the long-scale numerical convention: million (10^6), milliard (10^9), billion (10^12), billiard (10^15). English uses the short-scale: million (10^6), billion (10^9), trillion (10^12). The mismatch produces a thousand-fold error in either direction. For very large numbers, switching to scientific notation or SI prefixes (giga, tera) avoids the ambiguity entirely.

Severity: Critical; consequential in finance, government, and scientific translation.

Mistake #9

Rule: French requires a non-breaking space between the number and the % sign. Same applies to other unit symbols: 100 m, 5 kg, 25 °C. The Imprimerie nationale style guide and the BIPM SI brochure both prescribe the spaced form. The unspaced form is an anglicism.

Severity: Low; a typographic error, but visible in formally-typeset French text.

Mistake #10

Rule: In the expression pour cent, cent is invariable. It does NOT take the plural-s, even when multiplied by another number. So cinq pour cent, dix pour cent, cent pour cent, deux cent pour cent. The pour cent rule is its own rule, separate from the standalone cent plural-s rule. Easier to remember: pour cent never takes the s.

Severity: Medium; an over-application of the regular cent rule.

Mistake #11

Rule: French numerical date format is DD/MM/YYYY, day-first. The American MM/DD/YYYY (month-first) creates real ambiguity for any French reader. Always day-first in French. Never write 04/17/2026 in a French context; it would be parsed as 4 November 2026 if the month is allowed to be the 17th (which it is not, of course, but the dispute is enough to confuse).

Severity: High in international correspondence; medium in everyday French text.

Mistake #12

Rule: Et demie / et demi has gender agreement based on the time noun. Heure is feminine, so dix heures et demie (10:30, with the e). Midi and minuit are masculine, so midi et demi and minuit et demi (without the e). Many learners assume the e is universal because heure is the most common time noun; the masculine forms (midi, minuit) catch them out.

Severity: Medium; a subtle gender-agreement error, common but not glaring.

How to Build Internal Habits Around These Rules

Memorising the rules is half the battle; the other half is internalising them so you produce the right form automatically. The avoir-for-age rule is the easiest to drill: every time you state your age in French, say j’ai first, then the number, then ans. After a hundred reps, the verb stops being a conscious choice. Same for j’ai faim, j’ai soif, j’ai peur; the personal-state pattern becomes second nature.

The de-preposition rule for million / milliard requires explicit attention because there is no English-language analogue to drive the habit. Make a deliberate practice: every time you write a number ending in millions or milliards, scan the next word; if it is a noun, insert de (or d’); if it is another number, do not. The conscious check eventually becomes automatic.

For the typographic conventions (digit grouping, decimal comma, % spacing), set up your text editor or word processor to use French locale defaults. French Word, French LibreOffice, and French keyboards all autoinsert thin spaces and use comma decimals by default. Configure your tools once and the typography will follow automatically.

Frequently Asked Questions