Percentagem vs. porcentagem
The words percentagem and porcentagem are absolute synonyms in Portuguese, meaning they describe exactly the same concept: a rate or proportion per hundred. The primary distinction is regional and etymological. Percentagem is the standard form in European Portuguese (Portugal) and is derived directly from the English word percentage or Latin per centum. Porcentagem is the most common form in Brazilian Portuguese, influenced phonetically by the phrase por cento. However, both forms are officially recognized in the Portuguese language dictionaries.
Percentagem
B1This word refers to a quantity or amount expressed as a fraction of one hundred. It is the dominant spelling used in Portugal and African Portuguese-speaking countries. It is also frequently used in Brazil, particularly in formal academic or economic writing.
A percentagem de desemprego diminuiu este ano.
(The percentage of unemployment decreased this year.)
Qual é a percentagem de lucro sobre este produto?
(What is the percentage of profit on this product?)
Ela dedicou uma grande percentagem do seu tempo ao projeto.
(She dedicated a large percentage of her time to the project.)
Os cientistas analisam a percentagem de oxigénio na atmosfera.
(The scientists analyze the percentage of oxygen in the atmosphere.)
Porcentagem
B1This word holds the exact same meaning as the previous one, denoting a ratio out of 100. This spelling is widely preferred in Brazil due to its phonetic similarity to the phrase por cento. While rarer in Portugal, it is grammatically correct everywhere.
A bateria do meu celular está com uma porcentagem baixa.
(My cell phone battery is at a low percentage.)
Os professores calcularam a porcentagem de alunos aprovados.
(The teachers calculated the percentage of students who passed.)
Existe uma alta porcentagem de cacau neste chocolate.
(There is a high percentage of cocoa in this chocolate.)
A porcentagem de acertos na prova foi surpreendente.
(The percentage of correct answers on the test was surprising.)
Summary
To summarize, use percentagem if you are targeting a generic international Portuguese audience or specifically Portugal. Use porcentagem if you want to sound native to Brazil, although percentagem is also acceptable there. The meaning never changes.