SteelGratingCalc
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Tool 03 · Drainage Design

Steel Grating Open Area Percentage Calculator

Calculate the open area percentage for any bar grating specification. Open area determines drainage capacity, ventilation flow, light transmission, and debris retention. Enter bearing bar and cross bar dimensions to get your result instantly.

Bearing Bars

Top face width = thickness T

Clear space = pitch − thickness

Cross Bars

Round rod diameter or square side

Formula

OpenArea% =

[(P_bb − T_bb) / P_bb]

× [(P_xb − d_xb) / P_xb]

× 100

P_bb = Bearing bar pitch (c/c)

T_bb = Bearing bar thickness

P_xb = Cross bar pitch (c/c)

d_xb = Cross bar diameter or side

Application Guide by Open Area

< 40% — Minimal Drainage

Structural floors, heavy vehicle areas. Not recommended for water-intensive environments.

40–55% — Standard Industrial

Most common range. Suitable for walkways, platforms, mezzanines. Handles moderate drainage.

55–70% — Good Drainage

Suitable for wet processing areas, trench covers, oil & gas decks.

> 70% — High Flow

Drainage grates, screens, filtration applications. Maximum water and air throughput.

Open Area Results

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open

Open Area Percentage

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Solid Area

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BB Open %

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Suitability Rating

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Bearing bar clear gap:

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Cross bar clear gap:

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Get a Quote

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Frequently Asked Questions

What open area percentage is required for good drainage?
A minimum of 45% open area is generally recommended for drainage applications. Most standard 25×5mm gratings at 30mm pitch achieve 60–65%, which provides excellent drainage and reduces debris accumulation.
How does bearing bar pitch affect open area?
Wider pitch directly increases open area. For 5mm thick bearing bars: 22mm pitch gives ~77% open from bars alone, 30mm pitch gives ~83%, and 40mm pitch gives ~87.5%. Cross bars then reduce this further.
What is the typical open area for 25×5mm at 30mm pitch with 6mm round cross bars at 50mm pitch?
Approximately 62–64%. The bearing bars (5mm thick at 30mm pitch) account for 83.3% open in one direction, and the 6mm cross bars at 50mm pitch account for 88% in the other direction. Combined: 0.833 × 0.880 = 73.3%... adjusted for bar overlap areas the result is ~62–64%.
Does serrated (anti-slip) grating affect the open area percentage?
No. Serrations are notched into the top edge of bearing bars and do not change the cross-sectional area or pitch, so open area remains identical to plain grating of the same specification.