Quarter inch graph paper

A quarter inch is 6.35 mm, so this grid sits between the 5 mm and 1 cm metric sheets. It is the ruling most US maths textbooks and standardised tests assume, which makes it the safest default for American classrooms and for anyone working in feet and inches.

Four squares to the inch also makes mental scaling easy: one square is a quarter unit, four squares is one unit. Print at 100% / "Actual size" — any scaling will break the measurement.