In the word "referring", we need to double r while in the word "considering", doubling r is not acceptable. why? Is there a rule to follow?
So words ending in r, with the stress not on that syllable don't require doubling:
Consider, butter, batter, founder, flounder, etc.
Monosyllabic words usually double the r (They necessarily have the stress on their only syllable):
bar -> barring, otherwise you'd get baring, which comes from bare.
spar -> sparring. Sparing is from spare.
fur -> furry, not fury, which is different.
stir -> stirring
However, with some words ending in l, eg. travel, unravel, Americans write traveling, and others write travelling, etc.
This is not always consistent.
If a word doubles a letter for the -ing form, it also doubles for the -ed form, and -al, -y.
referring, referred, referral.