Formula
Ncr = π² · E · I / (K · L)², where K·L is the effective (buckling) length. The column buckles about its weaker axis, so use the smaller I. Slenderness λ = K·L / i with i = √(I/A), and the critical stress σcr = Ncr / A.
Euler's formula is valid only for slender, perfectly straight, elastic columns. Real columns have imperfections and yield before that; codes such as Eurocode 3 use buckling curves (the reduction factor χ) that give a much lower design resistance, especially for medium slenderness. Use this tool to understand the influence of length and supports, not for design.
For learning and quick checks only. Real structures must be designed by a licensed engineer according to the valid standards.