Isometric graph paper, A4
Three sets of parallel lines at 60° to each other, on A4. The construction grid for isometric projection, where all three axes stay at true scale.
Opens with these settings already loaded. Change anything you like, then download as PDF, PNG or SVG.
Why isometric projection keeps measurements honest
In isometric projection the three principal axes are drawn at 120° to one another and all three are scaled identically, so a distance measured along any axis means the same thing. That is the property perspective drawing gives up: a perspective sketch looks more natural but you cannot measure anything off it. For design sketching, packaging, furniture and anything where proportion has to survive being redrawn, isometric is the one that carries information.
Lines or dots
The line grid is easier to work on while you are constructing something, because you can follow an existing line rather than judging alignment between dots. The trade is that the grid stays visible in the finished sketch and photographs badly. A common working pattern is to construct on the line grid and then redraw the result on isometric dot paper once the geometry is settled.
Not the same as a 30/60 drafting grid
Isometric paper is sometimes confused with a general 30°–60° grid used for oblique or dimetric sketching. They are not interchangeable. On a true isometric grid the three axis directions are symmetric, which is what makes a cube read as a cube from any of its three visible faces. If your drawing is meant to be measurable, check that the grid you are printing has that threefold symmetry rather than two directions plus a vertical.
Printing it accurately
This only measures correctly if your printer doesn't rescale the page. In the print dialog set Scale: 100%, not "Fit to page" or "Shrink oversized pages". If you're not sure it worked, print the calibration page, measure the ruler on it with a real ruler, and it will tell you exactly what your printer is doing to the page.