# What is Forecast Standard Deviation and is there a formula for it?

I'm familiar with statistics and errors for measurement such as Mean Absolute Percentage Error, MAE, MSE, MAD, RMSE, SMAPE (plus others) and their mathematical formulas but so far I've not found a single formula for FSD (Forecast Standard Deviation) used for valuating house prices in real estate. What exactly is FSD expressed in a mathematical formula?