Accurate proportions can make or break a sketch! You don’t need them to be perfect, but you do want your measurements to be fairly correct so that your subject is recognizable.
Good news! You don’t need a ruler and professional-level math skills to accomplish this. You already have a handy instrument: your pen!
HOW TO DO THIS:
Hold out your pen in front of you and use your finger on the pen to mark the width or length of a certain line or edge on your subject. Then move your arm (still at the same length from you!) and hold up your pen in front of another line or edge, and see how that measurement compares with the first one. This way you can compare relative proportions and then sketch them accurately.
For example, if you watch the video above, you will see that the width of the left-hand wall of the building is half a pen length, and that the height is two half-pen lengths (or a full pen length). So that shows that the width of the left wall is roughly one half of the building’s height. It tells me that when I draw the height of the building, I need to make sure it is twice as tall as the width of the left-hand wall.
Do this trick throughout your sketch as you draw the basic outlines, and soon you will have accurate proportions without needing to do math or use a ruler!