Ruler
From Roll20 Wiki
The Ruler tool lets you click and drag to measure distances on the table. While you measure, you can choose to show your measurement line with other players so you can discuss what you're measuring, or you can limit the measurement line visibility to just yourself and your GM.
The grid settings and default Ruler behavior can be changed in the Game Default Settings, or each page of the campaign can have unique settings, changed in Page Settings (accessible via the Page Toolbar)
Changing Settings (GM only)
You can change ruler and grid settings for each page by accessing its Page Settings. Among other things, you can change the scale, the distance units (feet, meters, etc.), and how diagonals are measured.
There are four options for measuring diagonals:
1. D&D 4E Compatible is the default setting. This measures a diagonal move as 1 unit. This simplifies the counting of squares at the expense of realism.
2. Pathfinder/3.5E Compatible measures a diagonal move as 1.5 units (rounding down). Thus, when 1 unit equals 5ft, diagonal moves alternate between 5ft and 10ft increments (i.e. 5ft, 15ft, 20ft, 30ft, etc.). This is slightly more complicated to count, but models reality more closely.
3. Euclidean measures diagonals using the Pythagorean Theorem. A diagonal move equals about 1.414 units using this method. This is also called the "real distance" or "as the crow flies".
4. Manhattan measures a diagonal distance as the sum of its horizontal and vertical distances. Effectively, a diagonal move equals 2 units using this method. This is also called "Taxicab geometry" or "rectilinear distance".