The average weight of an orange is about 7 ounces (200 grams in the metric system). This means that a bag with 20 oranges in it would probably weigh about 8.75 pounds which is equal to about 4 kilograms.
Of course there are several different types of oranges including the blood orange, navel orange, Persian orange, Valencia orange, and several other less common types. Different types of oranges can have different average weights.
The heaviest orange in history on record was a 5.5 pound (2.5 kilogram) orange from South Africa in June of 1981.