The Flesch-Kincaid Grade Level is a readability test to indicate how difficult a passage in English is to understand. In the 1970s, Dr. Rudolf Flesch collaborated with J. Peter Kincaid, a researcher working for the U.S. Navy. Their goal was to create a readability formula that could match U.S. school grade levels. The formula was developed by modifying the Flesch Reading Ease formula. It was recalibrated to reflect U.S. school grade levels, making it more useful for educational materials and government documents.

The formula for the (English) Flesch-Kincaid Grade Level is:
\[ \text{Flesch-Kincaid Grade Level} = 0.39 \times \left( \frac{\text{Total Words}}{\text{Total Sentences}} \right) + 11.8 \times \left( \frac{\text{Total Syllables}}{\text{Total Words}} \right) - 15.59 \] We apply a measure of Spanish Orthographic Length to adapt the formula to Spanish texts.
SOL refers to the number of letters in a word. It's used as a proxy for word complexity, similar to how syllable count is used in English. Spanish has more regular phonetic rules and syllable structures than English. As a result, we can measure word complexity with the number of letters instead of syllables. We match the score of SOL to this table:

SOL Score	Reading Difficulty	US Grade	Spanish Grade	Ages	SMOG Grade
13+	Very Difficult	College	Universidad	19+	13
12	Difficult	12th Grade	2º Bachillerato	17-18	12
11	Slightly Difficult	11th Grade	1º Bachillerato	16-17	11
10	Slightly Difficult	10th Grade	4º ESO	15-16	10
9	Moderate	9th Grade	3º ESO	14-15	9
8	Average	8th Grade	2º ESO	13-14	8
7	Fairly Easy	7th Grade	1º ESO	12-13	7
6	Slightly Easy	6th Grade	6º Primaria	11-12	6
5	Easy	5th Grade	5º Primaria	10-11	5
4	Very Easy	4th Grade	4º Primaria	9-10	4
3	Extremely Easy	3rd Grade	3º Primaria	8-9	3
2	Basic	2nd Grade	2º Primaria	7-8	2
1	Very Basic	1st Grade	1º Primaria	6-7	1