Three steps
1 · Split into syllables first
Stress is counted in syllables, not letters. ca-mión is two beats and the stress is on the second — you cannot judge it without knowing where the split falls.
2 · Count back from the end
Last = aguda, second-to-last = llana, third-to-last = esdrújula, further back = sobresdrújula.
3 · Look up the table
Six rows below. Only two of them take a mark, plus the esdrújula family — everything else is the default and stays bare.
Six cases, only four take a mark
The badge on each card is this page's verdict. Click a word to hear it (the small grey text after it is the syllable split; the accented syllable is the one actually stressed).
Stress on the last syllable, landing on an ending the rule treats as unstressed — the rule cannot produce this reading, so the mark says it.
This is exactly what the default gives (consonant-final → stress on the last syllable), so nothing is drawn.
The commonest shape in Spanish (79% of this corpus) and the default reading — no mark needed.
Stress is not final but the word ends in a consonant: it clashes with the default, so it must be marked.
Spanish stress almost never reaches the third syllable from the end, so when it does, it is always marked.
Only arises when pronouns are attached to a verb. Also always marked.
Try it: type a word
This only applies the rule. A real dictionary also marks a few words for another reason (sí / mí and friends), which is the dictionary's job, not this rule's.
Second use: telling homographs apart
Some words become a different word without the mark. Here it marks identity, not stress.
Third use: breaking a diphthong
A mark on the weak vowel (i / u) declares a hiatus: two syllables, not one glide. So día is two beats (dí-a), while dia would be read as one.
Three places Mandarin speakers slip
Marking every stressed syllable
The mark is for the exception, not for the stress. casa is stressed on ca, but that is the default, so it goes bare. Adding a mark is the error.
Adding a mark to single-syllable words
Single syllables never take one: sol, pan, mar, tres. Writing sól or pán is the rule over-applied. The only exceptions are homograph markers such as sí / mí / tú.
Leaving out a required mark
Three groups always require it: every esdrújula (teléfono), llana ending in a consonant (árbol, lápiz), and aguda ending in a vowel / n / s (café, camión). Missing ones are the most common spelling error of the three.
The numbers on this page
Rules: all six rows come from the RAE orthography and need no measurement. The “default stress” half is the same judgement the front-end syllabifier uses, and the build compares the two across 270 words.
Corpus spread: our corpus has 162 word types — llana 128 (79%%), aguda 32, esdrújula 2. Llana being the large majority is exactly why the default takes no mark: the majority needs no sign.
Marked words: only 8 in the whole corpus (sofá, méxico, jamón, café, teléfono, día, tía, corazón). Of those, 8 are required by the rule; the rest break a diphthong or distinguish a homograph.