What exactly is the Barnett formula?

In the world of budget buzzwords, the Barnett formula stands out as a curious addition to nation’s lexicon following today’s announcment. In simple terms, this formula is a population-based spending multiplier, used to determine how changes in public expenditure in England ripple out to Scotland, Wales, and NI. When a department's budget changes, the formula multiplies this change by two factors, a ‘comparability percentage’ and a ‘population proportion’, to get a final figure that can be added or subtracted to the grant for those specific regions.This process provides a degree of predictability in public spending across the nations, however issues have been raised in its application, as it can perpetuate regional disparities that could impact long-term economic growth.