Rollpie Matrix Calculator is a free matrix calculator that runs in your browser with no install. It adds, subtracts, and multiplies matrices A and B on the spot. Rows and columns go from 1 to 6, and the cells accept integers, decimals, and fractions such as 3/4.
Touch a cell of the result and the row of A and the column of B that produced it light up in color, while the Breakdown panel lays out the formula a₁₁·b₁₁ + a₁₂·b₂₁ + … with the actual numbers and the answer. You can fill a matrix with zeros or the identity, swap A and B, copy the answer, or move the answer into A to keep calculating. When the shapes do not fit, a button appears that reshapes B for you.
The board is a canvas: zoom with the wheel and drag to move it. Double-click a panel title to switch how the panels are laid out. In the movable layout you drag panels wherever you like and keep several open at once; in the fixed layout a single panel sits in the top right.