"Spelt" vs. "spelled"

Solution 1:

From Cambridge Advanced Learner's Dictionary:

spell (FORM WORDS) /spel/ verb [I or T] spelled or UK AND AUSTRALIAN ENGLISH ALSO spelt, spelled or UK AND AUSTRALIAN ENGLISH ALSO spelt

This means that you should say "spelled" in US English and you can use both "spelt" and "spelled" in UK/Australian English.

Solution 2:

Spelled vs spelt:

• In American English, spelt primarily refers to the hardy wheat grown mostly in Europe, and the verb spell makes spelled in the past tense and as a past participle.

• In all other main varieties of English, spelt and spelled both work as the past tense and past participle of spell, at least where spell means to form words letter by letter or (with out) to make clear. Outside the U.S., the two forms are interchangeable in these uses, and both are common.

• But when spell carries the sense to temporarily relieve (someone) from work, spelled is the preferred form throughout the English-speaking world. This is a minor point, though, as this sense of spell is rarely used outside the U.S., where it is most common.

• Spelled is not a recent Americanism as many people assume (including some who have commented on this post). Both spelled and spelt are old, and examples of each are easily found in historical Google Books searches covering the 17th and 18th centuries. It is true, however, that spelt was ascendant everywhere through most of the 19th century. This ended when Americans permanently settled on spelled around 1900.

(grammarist.com)

Solution 3:

I believe the Guardian and Observer Style Guide has a better answer than just “it's different between US / UK / AUS”:

spelled is the past tense

“she spelled it out for him”

spelt is the past participle

“the word is spelt like this”