When most builders are determining joist spans for decks, they refer to floor joist spans in their local US building codes. However, decks have to withstand moisture and inclement weather and may be framed with incised treated lumber which means that those joist ratios won’t be accurate.
Now the 2015 IRC makes it easy to calculate joist spans for US builders with a new joist span table R507.5: Deck Joist Spans for Common Lumber Species. Referring to the table makes it easy to size deck joists accurately.
The new table represents common lumber varieties and covers cantilevered joists too.
It’s important to remember that deck joists are part of your wall system and that building codes should represent the minimum requirements so strive to build beyond the code.