Grid Drawing Calculator
Scale a reference photo onto your paper with the grid method: grid layout, scale factor, and the cell size to rule on your drawing.
Use a 2H pencil for the grid lines so they're light enough to erase later, and number the rows and columns the same way on both the reference and the drawing.
How it works
The grid method breaks a reference photo into a grid of equal squares, then redraws that same grid (bigger or smaller) on your paper so you can copy one square at a time instead of the whole image at once. Enter your reference's width and height, the cell size you want to draw on the reference (a pencil-ruled 1 in grid works on most printed photos), and the width you want the final drawing to be. The calculator works out how many columns and rows that makes, how much bigger your drawing is than the reference, and what cell size to rule on your paper so the two grids line up.
Worked example: an 8 in by 10 in reference photo, gridded at 1 in cells, scaled up to a 16 in wide drawing. That's 8 columns and 10 rows, 80 cells total. The scale factor is 16 divided by 8, or 2, so your drawing grid uses 2 in cells instead of 1 in cells, and the finished drawing comes out 20 in tall. Check that height against your paper before you start ruling lines, since a wide reference can turn into a drawing that's taller than the sheet you have.
FAQ
What if the reference doesn't divide evenly into whole cells?
It happens with almost any real photo. The calculator will still give you a cols and rows count with a decimal, which just means one edge row or column is a partial square. Draw that partial square the same as the others, at whatever fraction of the cell it covers, and it won't throw off the rest of the grid.
Does the cell size on the reference have to match the cell size on my drawing?
No, and it usually shouldn't. The reference grid stays small (1 in is typical) so you can see enough detail inside each square to copy it accurately. The drawing grid is that same cell size multiplied by your scale factor, which is why the calculator gives you a separate number for the paper.
Should I draw the grid directly on my reference photo?
Only if it's a printout or photocopy you don't mind marking up. For a phone screen or a photo you want to keep clean, lay a sheet of clear acetate over it and rule the grid on that instead, or use a photo-editing app to overlay grid lines before printing.
Do I still need to know how to measure proportions if I use a grid?
The grid handles most of the proportion work for you, but it's worth practicing proportion by eye too so you're not dependent on a grid for every drawing. The two skills reinforce each other.
For more on working from photos and getting proportions right, see how to draw from reference photos, how to measure proportions when drawing, and construction drawing with simple shapes.