Grid size by use
Primary maths (ages 5 to 11): 1 cm squares, so area and perimeter can be counted in whole units and young hands have room. Secondary maths and science: 5 mm squares with a heavy line every 10 mm (every 2 squares) or every 5, for graphs with a readable scale. US classes: 1/4 inch, the usual “quad ruled” notebook, with a heavy line every inch. Engineering and technical drawing: 2 mm, 1/8 inch or 1/10 inch, in a light colour that photocopies out. Art, pixel work and craft: whatever matches the project, from 5 mm for pixel art to 1 cm or larger for pattern blocks.
Metric sizes
1 mm (millimetre paper, heavy lines every 5 and 10), 2 mm, 5 mm and 10 mm are the standard metric grids. 5 mm is the general-purpose size: it gives 38 × 55 squares on A4 with a normal margin, enough for a full graph with labelled axes, and the squares are big enough to write a digit in. Make any of them on the graph paper maker by typing the size in millimetres.
Inch sizes
1/4 inch (6.35 mm) is the US classroom default and the usual notebook ruling. 1/5 inch (5.08 mm, “5 squares per inch”) is the engineering pad. 1/8 inch (3.175 mm) and 1/10 inch (2.54 mm) are for detail. Half-inch and one-inch squares suit young children and large diagrams. In the maker, switch the unit to inches and type the fraction as a decimal: 0.25, 0.2, 0.125, 0.1, 0.5 or 1.
Heavy lines and what they are for
A heavier line every few squares lets the eye count in groups: every 5 squares on 5 mm paper marks 2.5 cm, every 2 marks a centimetre, every 4 on quarter-inch paper marks an inch. For plotting data, set the heavy line to match your axis scale so that each labelled tick falls on a heavy line. For drawing, turn heavy lines off (set them to 0) so the grid is quiet.
Other grids
Dot grid at 5 mm is the choice for journals and lettering, where lines would compete with the writing. Isometric paper at 8 mm or 1/4 inch is for 3D sketches. Hex paper with a 6 to 8 mm side is for chemistry and game maps. Polar paper with 10 rings and 15° lines is for polar curves and directional plots. Engineering paper is 5 squares per inch with margins and a header.
Choosing the paper size
Pick what your printer holds: A4 outside North America, Letter inside it. A3 or Tabloid doubles the area for wall charts and large plots. A5 is for notebooks and planners. The grid size stays the same whatever the paper; only the number of squares changes, and the maker tells you how many.
Line weight and colour
The size of the squares is only half the choice. A 0.1 mm line in light grey gives a grid that fades behind pencil work and vanishes on a photocopy; a 0.3 mm line in a darker grey gives a grid you can read from arm’s length on a classroom wall. Pale blue is the old draughtsman’s trick for a grid that scanners ignore. Whatever you pick, print one sheet and look at it before printing a stack.