Are debt/GDP ratios calculated with real or nominal GDP as the denominator? As the title suggests, I would like to know whether the numbers are generally calculated with real or nominal GDP. Besides that I would also like to know whether it matters and how significant of a difference this could make.
Usually, the debt/gdp ratio has no unit. For instance, if the debt is equal to 60% of the gdp, the debt/gdp ratio is 0.6. If you use real gdp as denominator and nominal debt as numerator, you end up with a number which is less clear to interpret. It is more relevant to have either both variables in nominal terms or both in real terms. In these two cases, the result is the same.

If you denote D and GDP respectively nominal debt and nominal GDP. As long as you take the same price index P, the ratio in nominal variables, D/GDP, and the ratio in real variables, (D/P)/(GDP/P) are mathematically identical.