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The Ogam Wheel Calendar is the same as the Celtic Tree Calendar, but with a rune added to each date to indicate the moon phase. This moon phase rune is calculated by means of a device called the Ogam Wheel, which was brought to us by Charles Moyer.

The Ogam Wheel uses a variant of the Younger Futhark with 16 runes. The first rune F indicates a new moon, while the last rune O indicates a full moon. The intervening 14 runes indicate the intervening 14 days running forward for the waximng moon and backwards for the waning moon. This creates a 30-day lunation cycle. In each of the six Celtic Tree Months of Beth (Birch), Fearn (Alder), Uath (Hawthorn), Tinne (Holly), Muin (Vine) and Ruis (Elder), one of the two U runes is sacrificed and so is ommitted from the sequence of days so creating a 29-day lunation.

The Ogam Wheel follows a 19-year cycle. The position of the year in this 19-year cycle is called its Golden number, but this is not the same as the golden number used to calculate the date of Easter. It is presently either 10 greater or 9 less than this. To maintain the 19-year cycle, both U runes are sacrificed in Beth (Birch) of the year with golden number 4.

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