Which screen mesh blocks mosquitoes, no-see-ums, flies, gnats, midges, and other small insects.
A mosquito screen works when the mesh opening is smaller than the insect trying to pass through. That sounds simple, but the right choice depends on the insect, the mesh opening, the screen fit, and the local conditions around the window or door.
This page is a practical reference, not a lab chart. Sizes vary by species and product, but the comparisons below are useful for choosing between standard, fine, and ultra-fine screen mesh.
| Insect Problem | Usually Enough | Better Choice | Why |
|---|---|---|---|
| Mosquitoes | Standard 18x16 mesh | Fine mesh if gaps or humidity are a problem | Mosquitoes are larger than standard screen openings. |
| Houseflies and larger flies | Standard 18x16 mesh | Standard mesh with a tight frame fit | Flies are much larger than residential mesh openings. |
| Fruit flies and small gnats | Standard mesh in many homes | Fine 20x20 mesh | Small insects make frame gaps and worn mesh more noticeable. |
| No-see-ums, biting midges, sandflies | Standard mesh is often not enough | Ultra-fine no-see-um mesh | These insects can be close to the size of standard mesh openings. |
| Thrips and very tiny plant insects | Residential mesh is usually not enough | Specialty fine insect netting | Some are smaller than common screen openings. |
| Mesh Type | Approx. Opening Size | Usually Blocks | Typical Use |
|---|---|---|---|
| Standard Mesh (18x16) | About 1.1-1.4 mm | Mosquitoes, houseflies, many gnats | General household windows and doors |
| Fine Mesh (20x20) | About 0.9-1.0 mm | Mosquitoes, flies, more small gnats and midges | Humid areas, coastal homes, small-insect pressure |
| Ultra-Fine Mesh (30x20 or tighter) | About 0.7-0.8 mm or smaller | No-see-ums, biting midges, sandflies | No-see-um areas, marshy regions, sleeping spaces |
These measurements vary slightly between brands, but they provide a reliable reference point.
The width of the insect matters more than its full body length. Long insects can still be blocked if their body width is larger than the opening.
| Insect | Approx. Body Width | Standard Mesh | Fine Mesh | Ultra-Fine Mesh | Notes |
|---|---|---|---|---|---|
| Mosquito | About 2.5-4 mm | Blocks | Blocks | Blocks | Standard screen mesh is designed around this use case. |
| Housefly | About 4-7 mm | Blocks | Blocks | Blocks | Usually only enters through open doors or frame gaps. |
| Fruit fly | About 1.5-3 mm | Usually blocks | Blocks | Blocks | Kitchen sources often matter more than the window screen. |
| Gnat | About 1.0-1.5 mm | Mostly blocks | Blocks better | Blocks | Loose frames, lifted adhesive, and worn corners are common entry points. |
| No-see-um / biting midge | About 0.6-1.0 mm | Sometimes | Better | Best | Standard mesh may not be enough in areas with heavy no-see-um activity. |
| Sandfly | About 0.8-1.2 mm | Sometimes | Better | Best | Fine or ultra-fine mesh is usually preferred in coastal regions. |
| Thrips | About 0.2-0.5 mm | Does not reliably block | Does not reliably block | May not block | Usually requires specialty netting; rarely the main residential screen issue. |
Standard mosquito screen mesh is usually enough for mosquitoes. If mosquitoes still appear indoors, the cause is often a gap around the frame, a door opening, a torn corner, or air movement pulling insects through a weak point.
No-see-ums are a different problem. They are much smaller than mosquitoes and may get through standard mesh, especially near coastal, marshy, wooded, or humid areas. For these insects, ultra-fine no-see-um mesh is usually the safer choice.
Flies are large enough that almost any intact residential screen should block them. If flies are entering, focus on open doors, torn mesh, loose magnetic closures, or gaps around the edge.
Even with the right mesh size, tiny insects can enter under specific conditions:
These issues are more significant than the mesh size itself in many homes. A fine mesh screen with lifted adhesive or weak corners will perform worse than a standard mesh screen that seals tightly.
Read next: How to Measure a Window for a Mosquito Screen.
Standard mesh (18x16) is enough for typical mosquitoes and flies.
Fine mesh (20x20) reduces small-insect entry during peak humidity hours.
Ultra-fine mesh is the safest choice, especially around sleeping areas.
Fine or ultra-fine mesh may be worth considering because insect pressure is higher. Local conditions can make small weaknesses more noticeable, even when the mesh itself is technically correct.
Check for micro-gaps, airflow drafts, and lighting that may be drawing insects toward the screens.
See: When screens aren't enough
For a fuller explanation of why some homes have more insect pressure than others, see Local Conditions Explained.
Choosing the right mesh is less about perfection and more about understanding the insects in your area.
If your current mesh should block the insect but you still see insects inside, check these items first:
For deeper guidance, you can continue with: