# 4" Sphere Rule

### What is the 4-inch sphere rule?

The **4-inch sphere rule** limits opening size in a required **guardrail** or **railing**. Under **IRC R312.1.3**, a required guard cannot have an opening that lets a **4-inch diameter sphere** pass through.

This rule applies to **deck railings**, **balcony guards**, **landing guards**, and many **stair guards**. It also affects **baluster spacing**, **cable railing spacing**, and other guard infill layouts.

### 4-inch sphere rule at a glance

| Location                                            | Maximum opening  | Code basis               | Notes                                                       |
| --------------------------------------------------- | ---------------- | ------------------------ | ----------------------------------------------------------- |
| Guard on walking surfaces and landings              | 4 in. sphere     | IRC R312.1.3             | Standard guardrail opening rule                             |
| Stair guard infill on open side of stair            | 4 3/8 in. sphere | IRC R312.1.3             | Applies to infill along stair slope                         |
| Triangular opening at tread, riser, and bottom rail | 6 in. sphere     | IRC R312.1.3 Exception 1 | Limited stair geometry exception                            |
| Open riser stair opening                            | 4 in. sphere     | IRC R311.7.5.1           | Applies when stair is more than 30 in. above floor or grade |

### Why guard opening limits matter

The **4-inch sphere rule for railings** exists to reduce child entrapment risk. Openings larger than 4 inches can allow a small child’s head to pass through and become trapped.

That is why the code applies across materials. **Wood balusters**, **metal pickets**, **cable railing**, and other guard systems all need compliant opening sizes.

### Where the 4-inch rule applies

Use the baseline **4-inch opening rule** for:

* **Deck guardrails**
* **Balcony railings**
* **Interior landing guards**

On stairs, the rule changes slightly. **Stair guard openings** can allow a **4 3/8-inch sphere** through the infill area on the open side of the stair. The small triangular space at the stair tread, riser, and bottom rail can be up to **6 inches**.

### Baluster spacing and cable railing spacing

If you are checking **baluster spacing code**, the practical rule is simple: the clear opening must stay under the **4-inch sphere rule**. Many installers use spacing around **3 1/2 inches to 3 3/4 inches** for margin.

For **cable railing**, spacing alone is not enough. The opening must stay compliant both at rest and under load. If cables deflect too much, the opening can exceed the allowed size even when the initial spacing looks correct.

### Open riser stair code

The **4-inch sphere rule** also appears in **open riser stair code**. Under **IRC R311.7.5.1**, openings between treads cannot allow a **4-inch sphere** to pass when any part of the stair is more than **30 inches above** the floor or grade below.

If the stair is **30 inches or less above** the floor or grade below, that open riser limitation does not apply in the same way.

### Common code mistakes

The most common failures are:

* treating **4 inches** as center-to-center spacing instead of clear opening
* ignoring **cable deflection** under load
* applying the **4-inch rule** to stair exceptions that use **4 3/8 inches** or **6 inches**

### Frequently asked questions

<details>

<summary>What is the 4-inch sphere rule for railings?</summary>

The **4-inch sphere rule** requires that openings in a required **guardrail** do not allow a **4-inch diameter sphere** to pass through. It is a core **railing code** rule under **IRC R312.1.3**.

</details>

<details>

<summary>How far apart can balusters be?</summary>

Balusters must be spaced so the clear opening does not let a **4-inch sphere** pass through. In practice, many systems use about **3 1/2 inches to 3 3/4 inches** of clear spacing for margin.

</details>

<details>

<summary>Does the 4-inch rule apply to cable railing?</summary>

Yes. **Cable railing spacing** must stay within the **4-inch sphere rule** at installation and under load. Proper cable tension and post spacing are both part of compliance.

</details>

<details>

<summary>Is the 4-inch sphere rule different on stairs?</summary>

Yes. **Stair guard infill** can allow a **4 3/8-inch sphere**. The triangular opening at the tread, riser, and bottom rail can be up to **6 inches**.

</details>

### Related pages

* [When is Handrail Required?](/code-compliance/getting-started/when-is-handrail-required.md)
* [Maximum and Minimum Handrail Heights](/code-compliance/railing-code/maximum-and-minimum-handrail-heights.md)
* [Handrail Size Requirements](/code-compliance/railing-code/handrail-size-requirements.md)


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