Window marker spacing calculator
How many marks a window needs, at the spacing that has evidence behind it.
Markers on glass work by spacing rather than by quantity. A few decals on a large pane leave gaps a bird can fly through, which is the single most common mistake in do-it-yourself window treatment. This tool takes the spacing American Bird Conservancy publishes and works out what it means for the window in front of you.
- Spacing
- 2 in
- Dots needed
- 475
- 19 across, 25 down
- Minimum size
- 1/4 in
- diameter
Check your work the way ABC suggests: stand ten feet back. If you cannot see the pattern clearly from there, neither can a bird approaching the glass.
Where these numbers come from
The spacing is ABC’s, quoted directly: Ideal spacing follows the two-inch-by-two-inch rule. Patterns that have vertical, horizontal, or diagonal spaces of two inches or less will prevent most collisions.
So are the minimum dimensions of the marks themselves: If your pattern consists of stripes, they should be at least one-eighth of an inch wide. If you use dots or similar shapes, they should be at least one-quarter of an inch in diameter.
And so is the check at the end: You should be able to see the pattern clearly from 10 feet away.
The calculator does arithmetic on those three published figures and adds nothing of its own. It places a mark at each edge of the window and every two inches between, which is why a 36-inch span comes out as 19 columns rather than 18. Where a window is not an even multiple of two inches, it tightens the last gap rather than leaving one gap wider than the rule allows.
Two things the number does not tell you
The guidance is not unanimous on spacing.Audubon has passed on a suggestion to draw vertical lines four inches apart, and ABC’s position is that four-inch gaps are what small songbirds and hummingbirds fit through. We follow the tighter figure, and the disagreement is set out in full in what to do when a bird hits your window.
Do not stick anything to a thermopane window without checking with the manufacturer first. Cornell warns about this, and a sealed double-glazed unit can be damaged by what adheres to it or by the heat a dark film traps.
Which windows to treat first
Not all of them, and not at random. The windows that matter most are the ones that reflect vegetation or sky, and the ones a startled bird would fly toward when leaving a feeder. Working out which those are, and the evidence on feeder distance, is covered in the full article on window strikes and in where to put a feeder.
If a bird has already hit the glass and is on the ground, that is a different and more urgent question: how to help a bird that flew into a window covers the first hour.
This tool contains no affiliate links and recommends no product. It calculates from published guidance and links to the source for every figure it uses.