Found a recipe written for a tin you do not own? This rescales every ingredient by surface area, so the batter sits at roughly the same depth and bakes as intended.
The maths is simple: round area is pi times the radius squared, square is the side squared, rectangular is length times width. Scale every ingredient, including eggs, by the multiplier and round eggs to something sensible. Keep the oven temperature the same. Baking time is the thing that shifts: a wider, shallower tin spreads the batter thin and bakes faster, a smaller or deeper one bakes slower, so start checking well before the original time and use a skewer. This works for tin depth left roughly equal. If you are going much deeper or much shallower, treat the time as a fresh guess.
How do you convert a round tin to a square tin?
Compare surface areas. A square tin holds a little more than a round tin of the same width across, so calculate by area rather than guessing.
Does the baking time change with tin size?
Yes, even at the same temperature. A wider tin bakes faster, a deeper one slower. Keep the oven the same and check early with a skewer.
What about the ingredient amounts?
Multiply each by the figure this tool gives. If a recipe is in cups, the cups to grams converter and the baking substitutions guide help.