See < Given an $n$-simplex with vertices $v_i$, put $B_{ij}=\|v_i-v_j\|^2$ for $0\leq i,j\leq n$. Then put $B_{n+1,j}=B_{i,n+1}=1$ except $B_{n+1,n+1}=0$. The volume is then $$ V = \sqrt{(-1)^{n+1}\frac{\det(B)}{2^nn!^2}} $$
See < Given an $n$-simplex with vertices $v_i$, put $B_{ij}=\|v_i-v_j\|^2$ for $0\leq i,j\leq n$. Then put $B_{n+1,j}=B_{i,n+1}=1$ except $B_{n+1,n+1}=0$. The volume is then $$ V = \sqrt{(-1)^{n+1}\frac{\det(B)}{2^nn!^2}} $$