Readability formulas for the Portuguese language

There seems to be no readability formula for the Portuguese language developped in the litterature, as most of them have been developped for the English language. However, these formulas can be used for the Portuguese language to a certain extent, either by using a translated text, or by directly applying them, or even by applying a formula based on a closely-related language.
Let's have a look on these three different approaches.

By using a translated text

In order to be able to use two English-based readability formulas (namely the SMOG and the Flesh-Kincaid Index), Afonso Miguel Cavaco and Dulce Várzea, the authors of Contribution to the study of information leaflets readability in Portuguese pharmacies (2010, in Portuguese, with PDF version available), used the translated texts of the original material to apply the two formulas. They explain the translation process:

Two independent accredited translators translated the brochures into English, resulting in a consensus version for each leaflet. The Portuguese retroversions were evaluated to confirm the proximity of the translations with the original texts.

The schooling years have then been correlated between Portugal and United States to interpret the resulting score.

By directly applying them

By directly applying English-based formulas to Brazilian Portuguese, the authors of Readability Formulas Applied to Textbooks in Brazilian Portuguese (1996, PDF version) remarked that Flesch scores were systematically lower for Portuguese texts than for their English counterparts, mostly due to the difference of the number of syllables per word between the two languages. As the scores are related to the number of schooling years, other differences arose as the Brazilian educational methods differ from the US ones in some stages. Results obtained from other formulas (namely Kincaid, ARI and Coleman-Liau) showed a high correlation with the Flesch scores. They conclude by saying that the results obtained by these formulas are meaningful for Brazilian Portuguese.

By applying a formula based on a closely-related language

In their paper The Brazilian governmental websites and its readability to users with low literacy level (2007, PDF in Portuguese), Stefan Martins and Lucia Filgueiras applied the Fernándes Huerta formula, i.e. the Flesh Reading Ease formula adapted for Spanish, based on the fact that both languages [Portuguese and Spanish] have for origine the same latin base, and share various common formal structures such as morphosyntax and phonetics. They then discuss the results of that formula as if it was designed for Portuguese.

Mancko provides a free online tool to calculate some readability formulas for English, French, Portuguese and Spanish.

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Formules de lisibilité pour la langue portugaise (in French)
Fórmulas de legibilidade para a língua portuguesa (in Portuguese)
Fórmulas de legibilidad para la lengua portuguesa (in Spanish)