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Why French says 'augmenter de' where English says 'increase by'

When a price goes up by ten euros or a child grows by five centimetres, English uses by to give the size of the change. French uses de: augmenter de dix euros, grandir de cinq centimètres, réduire de vingt pour cent. The same pattern works with many verbs of growth and reduction, such as augmenter, diminuer, réduire, grossir and maigrir.

English speakers often reach for par, the usual translation of "by", and say something like augmenter par dix euros. That sounds wrong to French ears. Another common slip is to confuse the amount of the change with the final value. French keeps these two apart with two different prepositions.

Word list
augmenterA2 to grow le nombre augmente de vingt = the number grows by twenty croîtreA2 to grow croître de deux centimètres = to grow by two centimetres grandirA2 to grow grandir de cinq centimètres = to grow five centimetres pousserA2 to grow pousser de dix centimètres = to grow ten centimetres accroîtreB1 to increase accroître les ventes de 10 % = to increase sales by 10% augmenterB1 to increase les prix augmentent de 10 % = prices rise by 10% augmenterB1 to increase augmenter le prix de cinq euros = to raise the price by five euros chuterB1 to fall chuter de dix mètres = to fall ten metres diminuerB1 to decrease diminuer le prix de cinq euros = to lower the price by five euros réduireB1 to decrease réduire les dépenses de 10 % = to cut spending by 10% reporterB1 to postpone reporter la réunion d'une semaine = to put the meeting off by a week retarderB1 to postpone retarder le départ de deux heures = to delay departure by two hours croîtreB2 to increase la production a crû de 5 % = output grew by 5% déplacerB2 to reschedule déplacer le rendez-vous d'une heure = to move the appointment by an hour desserrerB2 to loosen desserrer sa ceinture d'un cran = to loosen one's belt by a notch diminuerB2 to decrease la température diminue de deux degrés = it drops by two degrees grossirB2 to gain weight grossir de trois kilos = to put on three kilos maigrirB2 to lose weight maigrir de trois kilos = to lose three kilos progresserB2 to make progress progresser de trois points en maths = to improve by three points in maths reculerB2 to reverse reculer de deux mètres = to reverse two metres repousserB2 to postpone repousser la réunion d'une semaine = to push the meeting back a week serrerB2 to tighten serrer la vis d'un tour = to tighten the screw by one turn vieillirB2 to get old vieillir de dix ans = to age ten years décroîtreC1 to decrease la population a décru de 2 % = the population fell by 2% resserrerC1 to tighten resserrer sa ceinture d'un cran = to tighten one's belt by a notch
How it works

The pattern is simple: verb + de + amount. The amount tells you how much something changed. It does not tell you where it ended up. For the starting point and the end point, French uses passer de … à …, with à marking the final value.

Le prix a augmenté de dix euros : il est passé de cinquante à soixante euros.
(The price increased by ten euros: it went from fifty to sixty euros.)

So augmenter de cent euros means "go up by a hundred euros", while passer à cent euros means "go up to a hundred euros". Use de for the difference and à for the result.

Before a vowel, de becomes d': d'un kilo, d'une semaine. The same de also appears with comparatives, where it gives the size of the gap.

Mon frère est plus grand que moi de dix centimètres.
(My brother is ten centimetres taller than me.)
The words
Going up

Augmenter works in two ways. Without an object it means "to rise" (les prix augmentent de dix pour cent). With an object it means "to raise". In both cases the amount follows de.

Mon patron a augmenté mon salaire de cent euros.
(My boss raised my salary by a hundred euros.)

Croître ("to grow") is a more formal word, common in news about the economy. Accroître is its version with an object: "to increase something".

La production a crû de cinq pour cent cette année.
(Output grew by five percent this year.)
L'entreprise veut accroître ses ventes de dix pour cent.
(The company wants to increase its sales by ten percent.)

Grandir is used for people growing taller, and pousser for plants, hair and nails.

Mon fils a grandi de cinq centimètres cet été.
(My son grew five centimetres this summer.)
Les tomates ont poussé de dix centimètres en une semaine.
(The tomatoes grew ten centimetres in a week.)

Grossir means "to put on weight".

J'ai grossi de trois kilos pendant les vacances.
(I put on three kilos during the holidays.)
Going down

Réduire and diminuer with an object mean "to cut" or "to lower". Diminuer also works without an object ("to drop"), and so does the formal décroître (la population a décru de deux pour cent).

Nous avons réduit nos dépenses de vingt pour cent.
(We reduced our spending by twenty percent.)
La température a diminué de deux degrés pendant la nuit.
(The temperature dropped by two degrees during the night.)

Maigrir is the opposite of grossir.

Elle a maigri de quatre kilos.
(She lost four kilos.)
Moving in time

When something is delayed or put off, retarder and reporter follow the same pattern. Here de gives the length of the delay.

Le vol a été retardé de deux heures.
(The flight was delayed by two hours.)
On a reporté la réunion d'une semaine.
(We put the meeting off by a week.)
Watch out
Don't translate "by" with par

Par belongs to multiplication and division: multiplier par deux means "multiply by two". For an amount added or taken away, use de.

Don't drop the preposition

English can say "grow five centimetres" or "lose four kilos", but French still needs de: grandir de cinq centimètres, maigrir de quatre kilos.

Keep the object and the amount apart

With verbs that take an object, the object comes first and the de phrase comes after it: diminuer le prix de cinq euros.

Remember à for the end point

If you mean "to sixty euros", say passer à soixante euros, not de soixante euros.

Other verbs of change, such as baisser (baisser de dix pour cent) and raccourcir (raccourcir une jupe de deux centimètres), work exactly the same way.

Summary

With verbs that make something bigger, smaller, earlier or later, French gives the size of the change with de, where English uses "by" or no preposition at all: augmenter de dix euros, maigrir de trois kilos, reporter d'une semaine. Never use par here, and keep à for the final value: passer de cinquante à soixante euros.