Sling Spline Size: How to Measure the Plastic Rod (And Stop Guessing)

Learn how to measure sling spline size (plastic rod) with a simple checklist. Avoid guessing between 3/16, 7/32, 1/4, and more.

If you’re replacing patio slings, the most confusing “small part” is often the most important one: the sling spline (aka plastic rod). Pick the wrong diameter and you’ll usually feel it immediately:

• Too small → the sling may not lock into the rail securely.
• Too large → it’s frustrating (or impossible) to slide into the rail smoothly.

The good news: measuring spline size is quick when you follow a simple measure + confirm routine.

What is sling spline (plastic rod)?

Spline is the flexible plastic rod that slides into the sewn pocket along the sling fabric edge. Once inside the rail channel, it helps hold the sling in place and maintain tension.

Why 3/16 vs 7/32 matters (more than people think)

On paper, 3/16″ = 0.1875″ and 7/32″ = 0.21875″—that’s only about 1/32″ difference. In a sling rail channel, that small change can be the difference between:

• a spline that can slip or feel “loose,” and
• a spline that bites properly into the channel without fighting you during installation.

Also, spline choice can depend on frame groove size and even fabric thickness used for the sling.

Common sling spline sizes (quick reference)

Many outdoor sling chairs use medium 7/32″ as the most common size, but not always.

Typical options you’ll see:

• Small: 3/16″ (≈ 0.18″)
• Medium: 7/32″ (≈ 0.22″)
• Large: 1/4″ (0.25″)
• XL: 5/16″ (≈ 0.31″)

How to measure sling spline size (plastic rod diameter)

Use this process to avoid guessing:

Step 1) Expose the spline

You can usually access it at the end of the sling rail or by removing a small section of the old sling. If your sling is already off, you can pull the spline from the sling pocket.

Step 2) Measure the diameter (not the length)

• Best tool: digital caliper (fast + accurate).
• Backup method: match it against drill bits (find the drill bit that matches the diameter snugly).

Step 3) Match to the nearest standard size

Compare your measurement to common sizes (3/16, 7/32, 1/4, 5/16). American Slings notes spline size depends on the manufacturer’s frame size and/or sling fabric thickness—so matching the original is the safest approach for replacements.

Step 4) Confirm before ordering

If you’re between sizes or unsure, don’t gamble—confirm with a photo and your measurement (especially for older frames, curved slings, or rail variations).

When should you replace the spline?

Replace it if:

• it’s brittle, cracked, flattened, or kinked,
• it feels loose in the rail,
• it won’t slide smoothly during installation,
• you’re replacing the sling and want a clean, reliable install.

Spline is designed to sit in the sling seam/pocket and slide into the chair rail—if it’s worn, it can fight the install or reduce hold.

Does fabric thickness affect spline size?

It can. Different sling fabrics and pockets may change how the spline seats in the rail, and American Slings explicitly notes spline size can depend on fabric thickness and frame size.
That’s why the best rule is: measure the original spline whenever possible.

Installation reminder (so sizing doesn’t go to waste)

When installing a new sling, you typically insert the spline into the sling pocket/hem and then slide the sling into the rail channel, keeping everything straight and untwisted.
(If you need the step-by-step, link internally to your Installing Chair Slings page and/or the 5-step install guide.)

If you want the easiest path: measure once, confirm once, then order spline by the foot (or by the roll for bulk jobs).

Useful Links

https://www.americanslings.com/product/sling-spline/

Sling Spline by the Roll

Measuring Your Plastic Rod

Installing Chair Slings

How Do You Replace the Fabric on a Patio Sling Chair?