Although it is seldom possible to predict with certainty whether a given Spanish noun is of masculine or feminine gender, Spanish has numerous guidelines that can usually be followed.

The most well-known rule or guideline is that nouns ending in -o are masculine and those ending in -a are feminine, but there are numerous exceptions to this gender rule, especially for those ending in -a. Many of them are listed below.

Here are some other guides to gender determination. Many words have definitions in addition to those listed:

Nouns ending in certain suffixes are usually feminine. They include -ción (usually the equivalent of "-tion"), -sión, -ía (usually the equivalent of "-y," although not in the diminutive sense), -za, -dad (often used like "-ty") and -itis ("-itis").

• la nación (nation)
• la intervención (intervention)
• la hospitalización (hospitalization)
• la ocasión (occasion)
• la tensión (tension)
• la economía (economy)
• la taxonomía (taxonomy)
• la probreza (poverty)
• la felicidad (happiness)
• la caridad (charity)
• la mastitis (mastitis)
• la meningitis (meningitis)

Nouns of Greek origin ending in -a are usually masculine.

• el problema (problem)
• el drama (drama)
• el poema (poem)
• el tema (subject)

Nouns ending in an accented vowel are usually masculine.

• el sofá (sofa)
• el tabú (taboo)
• el rubí (ruby)

Nouns with certain endings are usually masculine. These include -aje (usually the equivalent of "-age"), -ambre and -or. An exception is la flor (flower).

• el corage (courage)
• el mensaje (message)
• el espionaje (espionage)
• el hambre (hunger)
• el calambre (cramp)
• el calor (heat)
• el dolor (pain)
• el interior (interior)

Infinitives used as nouns are masculine.

• el fumar (smoking)
• el cantar (singing)
• el viajar (traveling)

Months and days of the week are masculine.

• el enero (January)
• el martes (Tuesday)

Letters are feminine while numbers are masculine.

• la d (d)
• el 7 (seven)

 

The gender of abbreviations typically matches the gender of the main noun.

• la ONU (United Nations; in Spanish, the O stands for Organización, which is feminine)
• los EE.UU. (United States; estado is masculine)
• las FF.AA. (armed forces; fuerza is feminine)
• la NASA (NASA; the word for agency, agencia, is feminine)

Words or phrases typically retain the gender of a missing or shortened noun.

• la moto (motorcycle; the word is a shortened form of la motocicleta)
• la disco (disco; the word is a shortened form of la discoteca)
• un Toyota (a Toyota. The masculine may be used here as a short form of un coche Toyota, as coche, the word for "car," is masculine. However, una Toyota may refer to a Toyota pickup truck, because the common word for "pickup" is the feminine camioneta.)
• la Alcatraz (the word for "prison," prisión, is feminine)

Compound nouns formed by following a verb with a noun are masculine.

• el rascacielos (skyscraper)
• el dragaminas (minesweeper)
• el guardarropa (clothes closet)

Two-word nouns, which are unusual in Spanish, carry the gender of the first noun.

• un kilowatt hora (kilowatt-hour)
• el sitio web (website)

With the exception of la plata (silver), names of the chemical elements are masculine.

• el flúor (fluorine)
• el cinc (zinc)

Names of rivers, lakes and oceans are masculine because el río, el lago and el océano, respectively, are masculine.

• el Danubio (the Danube)
• el Amazonas (the Amazon)
• el Titicaca (Titicaca)
• el Atlántico (the Atlantic)

Names of mountains are usually masculine, because el monte (mountain) is masculine. An exception is that the Rockies are usually referred to as las Rocosas or las Montañas Rocosas).

• los Himalayas (the Himalayas)
• el Cervino (the Matterhorn)
• los Andes (the Andes)

Names of islands are usually feminine because la isla (island) is feminine.

• las Canarias (Canary Islands)
• las Azores (Azores)
• las Antillas (West Indies)

Names of companies usually are feminine, because la compañía (company) is feminine, as are sociedad anónima (corporation), corporación (corporation) and empresa (business). This rule is not consistently followed, however.

• la Microsoft (Microsoft)
• la ExxonMobil (ExxonMobil)

The default gender for foreign words adopted into the language is masculine, but a feminine gender is sometimes acquired if there's a reason for doing so. Thus foreign nouns that end in -a sometimes become feminine, as do some words related in meaning to a Spanish feminine word.

• el marketing (marketing)
• la web (the Web or World Wide Web; the feminine is usually used because the Spanish words red and teleraña, words for "web" and "network," respectively, are feminine)
• los jeans (jeans)
• el rock (rock music)
• el software (software)
• el show (show)
• el champú (shampoo)
• el bistec (beefsteak)
• la pizza (pizza)