How do you calculate a gating ratio?

Divide the upstream (choke) area by the total downstream (ingate) area. With a 2.4 in² choke and 3.6 in² of ingate, the ratio is 0.67 — the choke is the smallest area, marking a pressurized system.

What is a pressurized vs. unpressurized gating system?

In a pressurized system the choke is the smallest area (ratio under 1 from choke to ingate, e.g. 1:2:1), keeping runners full and metal back-pressured. Unpressurized systems have the ingate as the largest area to slow metal and reduce turbulence, favored for aluminum and other oxide-prone alloys.

What gating ratio should I use for aluminum?

Aluminum and other light alloys typically use unpressurized ratios like 1:2:2 or 1:4:4 to keep fill velocity low and avoid oxide entrainment. Iron and steel often tolerate pressurized ratios such as 1:2:1 or 1:1.2:1 because they're less oxide-sensitive.

Why is the example ratio less than 1?

At 0.67, the upstream choke is smaller than the combined ingate area, so the choke controls flow and the system is pressurized. The runners and ingates stay full, maintaining back pressure throughout the pour.

What does the conversion multiplier do?

It rescales the raw ratio for your preferred reporting convention — leave it at 1 to report the true area ratio, or change it if your shop expresses gating ratios on a normalized scale.

Does a lower gating ratio always mean less turbulence?

Not by itself. The ratio sets where the flow is choked, but actual turbulence also depends on sprue height, runner design, and pouring rate. A well-chosen ratio reduces velocity at the ingate, but it must be paired with sound runner geometry.

Foundry & Forging calculator

Gating Ratio Calculator

The gating ratio compares the cross-sectional areas of a casting's gating system — typically choke to runner to ingate — and determines whether metal enters the mold under pressure or fills smoothly. Foundry methods engineers use it to control fill velocity, suppress turbulence, and keep oxide and air from being entrained in the melt. This calculator takes your upstream choke area and downstream ingate area and returns the ratio that defines your system as pressurized or unpressurized. Getting it right is the difference between clean fill and inclusion-riddled scrap.

What this calculator does

Calculate a gating ratio from sprue, runner, and ingate area or another approved gating-system basis.
Use it when a casting engineer or patternmaker checks whether a gating layout matches the intended pressurized or non-pressurized gating practice.
It divides upstream gating area by downstream ingate area and applies a reporting multiplier to express the gating system as a single ratio.

Formula used

Gating Ratio Calculator ratio = upstream gating area ÷ downstream ingate area
Converted gating ratio calculator ratio = ratio × reporting conversion multiplier

Inputs explained

Sprue choke (upstream) cross-section:
Total ingate (downstream) cross-section:
Reporting unit conversion multiplier:

How to use the result

Use it when designing or troubleshooting a gating system to set fill velocity and decide between pressurized and unpressurized layouts.
A single area ratio doesn't capture runner length, mold orientation, or pouring temperature; treat it as one design lever, not a complete fill model.

Current U.S. benchmarks

The producer price index for steel mill products stands at 348.53 (BLS, May 2026), up 6.7% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 3,569 primary metal manufacturing establishments employing about 354,911 workers (Census County Business Patterns, 2023).

Common questions

How do you calculate a gating ratio? Divide the upstream (choke) area by the total downstream (ingate) area. With a 2.4 in² choke and 3.6 in² of ingate, the ratio is 0.67 — the choke is the smallest area, marking a pressurized system.
What is a pressurized vs. unpressurized gating system? In a pressurized system the choke is the smallest area (ratio under 1 from choke to ingate, e.g. 1:2:1), keeping runners full and metal back-pressured. Unpressurized systems have the ingate as the largest area to slow metal and reduce turbulence, favored for aluminum and other oxide-prone alloys.
What gating ratio should I use for aluminum? Aluminum and other light alloys typically use unpressurized ratios like 1:2:2 or 1:4:4 to keep fill velocity low and avoid oxide entrainment. Iron and steel often tolerate pressurized ratios such as 1:2:1 or 1:1.2:1 because they're less oxide-sensitive.
Why is the example ratio less than 1? At 0.67, the upstream choke is smaller than the combined ingate area, so the choke controls flow and the system is pressurized. The runners and ingates stay full, maintaining back pressure throughout the pour.
What does the conversion multiplier do? It rescales the raw ratio for your preferred reporting convention — leave it at 1 to report the true area ratio, or change it if your shop expresses gating ratios on a normalized scale.

Last reviewed 2026-05-12.