After cause and path through, the third big job of por is exchange: one thing given (or counted) in return for another. Lo compré por veinte euros — I bought it for twenty euros. Te cambio mi libro por el tuyo — I'll trade my book for yours. Hablé por mi hermana en la reunión — I spoke on behalf of my sister in the meeting. Tres veces por semana — three times a week. The unifying picture is a small set of scales: something on each side, and por tells you what's being weighed against what.
This use is everywhere in everyday peninsular Spanish — every time you mention a price, a swap, a rate, or speaking on behalf of someone, you reach for por. It is also the por that learners most commonly confuse with cause-por (same word, very different sense) and with para (which never marks exchange). Sorting this out is a B1-level milestone in handling Spanish prepositions.
The core idea: something given in return
When the English sentence works with in exchange for, in return for, in place of, or even the bare for of a price, Spanish wants por. The thing on the por-side is what is given up, paid, or substituted.
Lo compré por veinte euros en el rastro, fue una ganga.
I bought it for twenty euros at the flea market — it was a steal.
Lo vendí por mucho menos de lo que me costó.
I sold it for a lot less than it cost me.
Te cambio mi bocadillo por tu manzana.
I'll trade you my sandwich for your apple.
The pattern is symmetrical: in lo compré por veinte euros, twenty euros is what I gave up to get the thing; in te cambio mi libro por el tuyo, my book is what I'm giving in exchange for yours. Por sits between the two items being weighed.
Money and prices
Prices are the prototypical exchange. The price tag goes on the por-side.
¿Por cuánto te lo vendieron al final?
How much did they end up selling it to you for?
No pago más de cincuenta euros por una camisa.
I won't pay more than fifty euros for a shirt.
Compré los dos libros por diez euros, estaban de oferta.
I bought the two books for ten euros — they were on offer.
Notice that para is impossible in these sentences. Lo compré para veinte euros is ungrammatical. Para never marks a price, because price is not a goal — it is what you give up to reach the goal. The goal would be the thing you bought; the price is what you exchanged for it.
The same logic extends to salaries, fees, and bills:
Trabaja por mil euros al mes, una miseria.
He works for a thousand euros a month — peanuts.
Me cobraron treinta euros por la entrada al museo.
They charged me thirty euros for the museum ticket.
Item-for-item swaps
The same construction handles bartering, trading, or substituting one thing for another.
Cambié mi coche viejo por uno híbrido de segunda mano.
I traded my old car for a second-hand hybrid.
¿Me cambias el turno del sábado por el del domingo?
Will you swap your Saturday shift with my Sunday one?
En la frutería me han dado un kilo de naranjas por el precio de medio.
At the fruit shop they gave me a kilo of oranges for the price of half.
The verb is often cambiar (to swap), but the structure also appears with intercambiar, trocar (literary), sustituir, reemplazar, and canjear (to redeem, e.g. points for a prize). All of them take por for the thing received or given in exchange.
Canjeé los puntos del supermercado por una sartén.
I redeemed my supermarket points for a frying pan.
Substitution: standing in for someone
A natural extension of exchange is substituting one person for another — doing something on someone's behalf or in their place. Por covers this too.
Hablé por mi hermana en la reunión, que estaba enferma.
I spoke on behalf of my sister at the meeting — she was ill.
¿Puedes firmar por mí? Tengo las manos llenas.
Can you sign for me? My hands are full.
Voy a votar por mi abuela, le di la papeleta yo.
I'm going to vote on behalf of my grandmother — I'm taking her ballot.
The thing being "exchanged" here is the person doing the action: my sister couldn't speak, so I'm giving my voice in her place; my grandmother couldn't go to vote, so I'm giving my action in hers.
This is also why por introduces the agent of a passive verb (covered on the Por for cause page): in fue escrito por María, María is being identified as the one doing the writing — she's standing in for the abstract role of "writer."
Idiomatic exchange: por una vez, por todas
A handful of fixed phrases use the exchange logic in slightly more abstract ways. They are worth learning as units.
Por una vez en la vida, hazme caso.
For once in your life, listen to me.
Vamos a resolver este asunto de una vez por todas.
Let's settle this once and for all.
Por mí, podéis hacer lo que queráis.
As far as I'm concerned, you can do whatever you want.
In por una vez en la vida, "one time" is being weighed against the implied "all the other times you didn't listen" — an exchange. In de una vez por todas, "this one occasion" is being given in exchange for all the future occasions where the matter would otherwise come up again. These phrases sound mysterious until you spot the exchange logic underneath.
Rates and ratios
The exchange idea naturally extends to rates — so much of X per so much of Y. English uses per (or sometimes a); Spanish uses por.
| Spanish | English |
|---|---|
| tres veces por semana | three times a week |
| dos por uno | two for one (a typical offer) |
| cien kilómetros por hora | a hundred kilometres per hour |
| ocho horas por día | eight hours a day |
| diez por ciento | ten per cent ("ten per hundred") |
| cincuenta euros por persona | fifty euros per person |
En este supermercado tienen dos por uno en yogures esta semana.
This supermarket has a two-for-one offer on yogurts this week.
El menú del día sale a doce euros por persona.
The set lunch comes to twelve euros per person.
Voy al gimnasio tres veces por semana, los lunes, miércoles y viernes.
I go to the gym three times a week — Mondays, Wednesdays, Fridays.
A peninsular note: for "three times a week," you'll also commonly hear tres veces a la semana in Spain. Both are grammatical and natural. Por semana is slightly more abstract / statistical; a la semana is the everyday conversational choice. The same alternation exists for al día / por día, al mes / por mes, al año / por año. Por hora and por persona, however, do not have an a-variant — they are fixed.
Exchange-por vs cause-por: same word, different sense
This is where learners stumble. Por in lo hice por dinero and por in lo compré por veinte euros feel similar — both involve money — but they are doing very different jobs.
| Sentence | Sense | Test |
|---|---|---|
| Lo hice por dinero. | cause / motive (I did it because of money) | rephrase: because of money? |
| Lo compré por veinte euros. | exchange (I paid 20 € in return for it) | rephrase: in exchange for 20 €? |
| Hablé por ti. | can be either: for your sake (cause) or in your place (substitution) | context disambiguates |
In practice, exchange-por almost always sits next to a quantity (a price, a count, a percentage) or a second item being swapped, while cause-por sits next to a reason or motive that is not a quantity. Once you start parsing what's on the por-side, the distinction becomes mechanical.
Trabaja por dinero, no por amor al arte.
He works for money, not for the love of it. (cause)
Cobra veinte euros por hora de clase.
She charges twenty euros per teaching hour. (rate)
Same verb (work / charge), same word (por), completely different relationship between the parts.
Exchange-por vs para: never confuse them here
Para never marks an exchange. Where por puts something on the scales, para points forward to a recipient or goal. Prices, swaps, and rates are the cleanest case where the two prepositions cannot be substituted.
| Por (exchange) | Para (recipient / goal) |
|---|---|
| Lo compré por veinte euros. (price) | Lo compré para mi madre. (recipient) |
| Trabaja por mil euros. (salary) | Trabaja para Telefónica. (employer) |
| Te cambio mi libro por el tuyo. (swap) | Tengo un libro para ti. (recipient) |
Pagué cincuenta euros por el regalo que compré para mi padre.
I paid fifty euros for the present I bought for my father.
That sentence is a beautiful diagnostic: por cincuenta euros (the price I gave in exchange) and para mi padre (the recipient the gift is destined for). One Spanish sentence, two prepositions, two completely different jobs. English uses for in both slots, which is why English speakers find this hard at first.
A note on tip and gratitude por
The cause meaning lurks behind a useful idiom that looks like exchange but isn't quite: gracias por, te debo una por, me has hecho un favor por. Here por introduces the reason for the gratitude, not the exchange — but the resemblance to "in exchange for" is what makes the construction feel intuitive in both languages.
Gracias por venir hasta aquí solo para ayudarme.
Thanks for coming all the way here just to help me.
Te debo una por lo de ayer.
I owe you one for what you did yesterday.
These are classed as cause-por in formal grammar (you are thanking because of the favour), but pragmatically they sit right next to the exchange family. Don't worry about the label — the form is the same.
Common Mistakes
❌ Lo compré para veinte euros.
Wrong — price/exchange takes por, not para.
✅ Lo compré por veinte euros.
I bought it for twenty euros.
❌ Te cambio mi libro para el tuyo.
Wrong — item-for-item swap takes por, not para.
✅ Te cambio mi libro por el tuyo.
I'll swap my book for yours.
❌ Trabaja para mil euros al mes.
Ambiguous-to-wrong — para suggests purpose (in order to earn 1000). For salary received, use por.
✅ Trabaja por mil euros al mes.
He works for a thousand euros a month.
❌ Voy al gimnasio tres veces en semana.
Wrong — rate takes por (or 'a la'), not en.
✅ Voy al gimnasio tres veces por semana.
I go to the gym three times a week.
❌ Hablé para mi hermana en la reunión.
Means 'I spoke for my sister's benefit' — for substitution ('in her place'), use por.
✅ Hablé por mi hermana en la reunión.
I spoke on behalf of my sister at the meeting.
Key takeaways
- Por marks exchange: prices (por veinte euros), swaps (cambio X por Y), substitution (hablé por mi hermana), rates and ratios (tres veces por semana).
- The thing on the por-side is always what is given up, paid, or counted to reach the other side.
- Para never marks exchange. It marks recipient or goal — a completely different relationship.
- Exchange-por and cause-por look similar but are distinct: the test is whether the por-phrase is a quantity / item (exchange) or a motive (cause).
- Peninsular Spanish often prefers a la semana / al día over por semana / por día in everyday speech; both are correct.
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