A window’s design pressure rating, or DP, tells you how much wind load it’s rated to carry, in pounds per square foot. A DP 50 window is rated for 50 psf of wind pressure pushing in and pulling out, and it was tested at a higher pressure than that to earn the number.
In hurricane country, DP is one of the most useful numbers on the label because it covers more than strength. Under the North American Fenestration Standard, a rated window also has to resist water and air leakage at set test pressures. The right DP for your home depends on wind speed, exposure, height and where each window sits on the wall.
If you’d like help reading the ratings on a quote, TurnKey Window can walk you through them. Ask for a free estimate or call (504) 226-6201.
Key takeaways
- Design pressure is the wind load a window is rated to withstand, in pounds per square foot.
- To earn a rating, a window is tested at a structural pressure 1.5 times its design pressure.
- A performance grade (PG) rating also requires passing air leakage and water penetration tests, not only the structural load.
- Windows near building corners see stronger suction than windows in the middle of a wall, so they may need a higher rating.
- DP measures wind pressure. It is not an impact rating, which comes from separate missile testing.
What design pressure means
When wind blows against a house, it pushes on the windward walls and pulls on others. Engineering Express explains that wind speed converts to wind pressure through a formula with many variables, so one wind speed produces many different pressures across a single building.
A DP rating tells you how much of that load a window can take. Andersen’s product performance sheet explains that the International Residential Code and International Building Code list NAFS as the way a window’s design pressure rating is shown, and that a DP rating depicts only the design and structural load performance. Water and air are covered by the broader performance grade, which comes up below.
Positive and negative pressure
Windows face pressure in two directions during a storm. Engineering Express describes positive pressure as wind pushing on the building and negative pressure as suction, and notes the suction forces are larger near building corners.
That’s why ratings are often listed as two numbers. Andersen’s sample label shows a design pressure of DP+50/-50, meaning 50 psf in each direction, and its structural tests run in both the positive and negative directions.
How design pressure is tested
DP isn’t a guess or a marketing figure. It comes from lab testing under the North American Fenestration Standard, known as NAFS or AAMA/WDMA/CSA 101/I.S.2/A440.
The Window and Door Manufacturers Association lists NAFS-26 as the current 2026 edition, jointly developed by FGIA/AAMA, WDMA and the Canadian Standards Association. Earlier editions are still referenced on older product literature.
The tests a rated window goes through
- Air leakage: the window is checked for how much air passes through at a set pressure.
- Water penetration: water is sprayed on the window while pressure is applied. Silverline’s structural data sheet notes the water test pressure is 15 percent of the performance grade, and Andersen’s PG 50 example was tested for water at 7.5 psf.
- Uniform structural load: the window is loaded at its design pressure, then at a structural test pressure of 1.5 times DP. A DP 40 window is tested to 60 psf.
- Additional tests: depending on the product type, operating force, forced-entry resistance and other durability tests also apply.
That 1.5 multiplier is a built-in safety margin. The rated number is what the window is expected to handle in service, and the test proves it can go further without failing.
DP versus performance grade
You’ll see both DP and PG on window literature, and they’re related but not identical. Andersen explains that a product only earns a PG rating when it meets all of the NAFS requirements, including ease of operation, air infiltration, water penetration resistance and forced entry resistance.
In other words, a PG 50 window has proven it holds up at 50 psf and stays dry and reasonably airtight at the matching test pressures. When a label shows a PG, it tells you more than a structural DP alone.
Performance classes on the label
NAFS groups products into four performance classes, each with a minimum grade. The AAMA excerpt of the 2008 edition lists these gateway minimums for windows and doors:
| Class | Minimum PG | Minimum DP (psf) | Minimum structural test (psf) | Minimum water test (psf) |
|---|---|---|---|---|
| R | 15 | 15.0 | 22.5 | 2.90 |
| LC | 25 | 25.0 | 37.5 | 3.75 |
| CW | 30 | 30.0 | 45.0 | 4.50 |
| AW | 40 | 40.0 | 60.0 | 8.00 |
A class minimum is only the entry point. A Class R window can carry a much higher grade, such as PG 50 or more, if it passes the tests at that level.
How to read the designation
The standard’s primary designator is a short code with the class, the grade and the maximum size tested. The 2008 excerpt gives the example “Class R, PG30, Size tested 800 x 1800 mm (32 x 71 in)” for a hung window, sometimes followed by a product type code.
The size part matters more than people think. The rating is tied to the size tested, WDMA p