---
title: "Window Collision Calculator: Strip Spacing | Bird Harbor"
description: "How many strips or dots a window needs to stop bird strikes, spaced by ABC’s 2 × 2 in or 2 × 4 in rule or the Swiss guideline, with a printable template."
url: https://birdharbor.me/window-collision-calculator/
---

# Window collision calculator

Measure a pane of glass and choose a pattern. You get the number of strips or dots, exactly where each one goes, and a template to print, spaced so birds see the glass as a barrier.

## How the spacing works

Birds see reflections of sky and trees, or a clear path through, and fly at the glass. A pattern on the outside breaks the reflection, as long as the gaps are too small to fly through. The calculator spreads the fewest markings evenly, so every gap, including those at the frame, stays within your rule.

**ABC 2 × 2 in** is the American Bird Conservancy’s current advice for homes: vertical, horizontal or diagonal spaces of 2 in or less, with stripes at least 1/8 in wide and dots at least 1/4 in across, white for the best contrast. **ABC 2 × 4 in** is its older rule: horizontal lines at most 2 in apart, vertical lines at most 4 in; ABC now warns that small songbirds, hummingbirds included, can fly through 4 in gaps. The **Swiss Ornithological Institute** asks for vertical lines at least 5 mm wide at most 10 cm apart, horizontal lines of 3 mm at most 3 cm apart (or 5 mm at 5 cm), and dot grids covering at least 25% of the glass; for lines it suggests at least 15% coverage to be on the safe side. Its spacing is treated here as centre to centre, the stricter reading.

## Which windows first

ABC suggests starting with windows that birds have already hit, large windows and glass doors that reflect sky or greenery, and windows near feeders, baths or fruiting plants. Apply markings outside: from inside they disappear behind reflections.

## Questions

**Do hawk silhouettes work?**

Only when packed closely. In Daniel Klem’s experiments, cited by ABC, falcon silhouettes worked only when spaced two to four inches apart, which is a pattern by another name.

**What can I use for the strips?**

ABC lists tempera paint, tape and decals on the outside of the glass, insect screens, and commercial products. Whatever you use, you should be able to see the pattern clearly from 10 ft (3 m) away.

## Sources

- American Bird Conservancy: [Solutions for Homes](https://abcbirds.org/strategies/solutions-for-homes/) (the 2 × 2 in rule, minimum stripe and dot sizes, white patterns, visible from 10 ft, outside the glass, which windows to fix first); Christine Sheppard and Glenn Phillips, [*Bird-Friendly Building Design*](https://abcbirds.org/wp-content/uploads/2015/05/Bird-friendly-Building-Guide_LINKS.pdf), 2nd edition (ABC, 2015), pages 45–49 (Klem’s silhouette tests, the 2 × 4 rule, lines at least 1/4 in wide).
- Hans Schmid, Wilfried Doppler, Daniela Heynen and Martin Rössler, [*Bird-Friendly Building with Glass and Light*](https://www.otop.org.pl/uploads/media/bird-friendly_building_engl.pdf), 2nd edition (Swiss Ornithological Institute, Sempach, 2013), page 17 (line widths and spacing, dot-grid coverage); PDF hosted by OTOP, BirdLife Poland.