To form compound numbers from 21 onwards in Icelandic, combine the tens word, og (and), and the ones digit. The tens word never changes its form, and digits 5 and up do not change either.
| It is twenty-five degrees outside. | Það eru tuttugu og fimm gráður úti. |
| The box weighs forty-five kilos. | Kassinn vegur fjörutíu og fimm kíló. |
Agreement with Numbers 1–4
Just like standalone numbers, digits 1–4 at the end of a compound number (like 21, 32, or 54) must agree with the gender and case of the noun they describe.
Feminine Nouns
When measuring temperature with the feminine noun gráða (degree), a compound ending in 1 uses the feminine singular ein, and the noun itself is singular.
| The temperature is twenty-one degrees today. | Hitinn er tuttugu og ein gráða í dag. |
Neuter Nouns
For weight, we use the neuter noun kíló (kilo). A compound ending in 2 pairs with the neuter plural tvö.
| The dog weighs twenty-two kilos. | Hundurinn vegur tuttugu og tvö kíló. |
Masculine Nouns
Distances and volumes are typically measured with masculine nouns like kílómetri (kilometer), metri (meter), and lítri (liter). Digits 1–4 at the end of a compound take their masculine forms (einn, tveir, þrír, or fjórir).
| The route is twenty-one kilometers. | Leiðin er tuttugu og einn kílómetri. |
| The road is thirty-three kilometers. | Vegurinn er þrjátíu og þrír kílómetrar. |
If the noun is a direct object, the final digit (1–4) also changes to the accusative case to match. (Note: To specify what a volume consists of, use the preposition *af for "of".)*
| We need to buy twenty-two liters of juice for the party. | Við þurfum að kaupa tuttugu og tvo lítra af safa fyrir veisluna. |