What Size Rubber Washer for a Toilet Tank? (Complete Guide)

If your toilet tank is leaking or wobbling, the issue is often a worn or incorrectly sized rubber washer. Choosing the right size matters—too small and it won’t seal, too large and it won’t fit properly.

This guide breaks down the most common toilet tank washer sizes, how to measure them, and how to pick the right one the first time.

Common Toilet Tank Rubber Washer Sizes

Toilet washers vary depending on where they are used. Below are the three main types you’ll encounter:

1. Tank-to-Bowl Bolt Washers (Most Common Repair)

• Standard size: Fits 5/16-inch bolts (most US toilets)

• Inner Diameter (ID): ~0.31" (8 mm)

• Outer Diameter (OD): ~1.0"–1.1" (25–28 mm)

• Shape: Flat or slightly tapered (cone-shaped)

Use:
Placed inside the tank around the bolts to prevent leaks where the tank connects to the bowl.

👉 Tip: If you're in North America, 95% of toilets use 5/16" bolts.

2. Flush Valve Shank Washer

• ID: ~2.3"–3.5"

• OD: ~3.1"–3.5"

• Thickness: ~0.3"–0.4"

Use:
Sits inside the tank under the flush valve, sealing the valve to the tank.

3. Tank-to-Bowl Gasket (Main Seal)

This is the large rubber gasket between the tank and bowl.

For 2-inch flush valves (most common)

• ID: 2-1/8" to 2-1/4"

• OD: 3-1/16" to 3-5/16"

For 3-inch flush valves (newer toilets)

• ID: ~3"

• OD: ~4-1/4"

Use:
Creates a watertight seal between the tank outlet and the bowl inlet.

How to Choose the Right Washer Size

Getting the correct size is easier than it sounds. Here are three reliable methods:

1. Check Your Toilet Model

Look inside the tank for a stamped model number. Search the manufacturer’s specs to find the exact washer size.

2. Measure the Old Washer

Use a ruler or caliper and measure:

• Inner Diameter (ID)

• Outer Diameter (OD)

• Thickness

👉 Always match the inner diameter first—that’s what determines fit.

3. Measure the Hardware

• Bolt washers: Measure bolt diameter (usually 5/16")

• Flush valve gasket: Measure the flush valve opening (2" or 3")

Pro Tips (From Plumbers)

• ✔ Most homes use a 2-inch flush valve, so 2" gaskets are the default choice

• ✔ Newer high-efficiency toilets often use 3-inch valves

• ✔ “Universal kits” usually fit standard 2" systems and 5/16" bolts

• ✔ If your toilet is over 15 years old, always double-check sizing before buying

Common Mistakes to Avoid

• ❌ Using the wrong gasket size (2" vs 3")

• ❌ Reusing old compressed washers

• ❌ Overtightening bolts (can deform washers and cause leaks)

• ❌ Mixing metric and imperial sizes without checking fit

When Should You Replace a Toilet Washer?

Replace washers if you notice:

• Water leaking under the tank

• A loose or wobbling tank

• Visible cracks or flattening on the rubber

• Frequent need to retighten bolts

Final Thoughts

For most residential toilets, you’ll need:

• 5/16" bolt washers

• 2-inch tank-to-bowl gasket

But measuring your existing parts is always the safest way to guarantee a perfect fit.