A running or leaking toilet almost always comes down to one worn rubber part. This guide covers the three rubber seals that fail most often — tank bolt washers, the tank-to-bowl gasket, and the spud gasket — how to identify which one is leaking, and which replacement washers work.
The 3 rubber seals in every two-piece toilet
1. Toilet tank bolt washers
Two or three brass or stainless bolts clamp the tank to the bowl, and each bolt seals with a flat or beveled rubber washer inside the tank. When these harden, water seeps down the bolts and drips behind the bowl. Replacement is a standard flat rubber washer — most tanks use a 5/16" or 3/8" ID washer with roughly 3/4"–1" OD and 1/8"–3/16" thickness. EPDM and natural rubber both work; EPDM lasts longest in constantly submerged service because it resists water absorption and chlorine.
2. Tank-to-bowl gasket
The large sponge-rubber donut between the tank outlet and the bowl inlet. If water gushes (not drips) between tank and bowl during a flush, this is the culprit. Tank-to-bowl gaskets are molded parts specific to your toilet brand (Kohler, TOTO, American Standard sizes differ) — match the model number rather than improvising with a flat washer. While you have the tank off, replace the tank bolt washers at the same time; they're the cheap part that causes the next leak.
3. Toilet spud gasket
Found on flushometer (commercial tank-less) toilets and some older residential models, the spud gasket seals the flush valve connection. Like tank-to-bowl gaskets, these are dimension-specific molded parts.
Which material for toilet washers?
| Material | Submerged service | Chlorine/chloramine | Verdict |
|---|---|---|---|
| EPDM | Excellent | Excellent | Best choice — this is what most OEM toilet washers are made of |
| Natural rubber | Good | Fair | Works, softer seal, shorter life in treated water |
| Neoprene | Good | Good | Solid general-purpose alternative |
| Nitrile | Fair | Poor | Skip it — nitrile is for oil, not chlorinated water |
How to size a toilet tank bolt washer
Remove one old washer and measure three dimensions: inner diameter (must match the bolt — usually 5/16" or 3/8"), outer diameter (large enough to cover the tank hole with margin), and thickness (1/8" to 3/16" compresses well without over-torquing the porcelain). Our measuring guide shows exactly how. Then find the size in our size chart — Rubber Washer Warehouse stocks every OD, ID and thickness combination in EPDM, so an exact-fit replacement is a couple of clicks away.
FAQ
Why does my toilet tank leak only after flushing?
That points to the tank-to-bowl gasket: it only sees water pressure during the flush surge. Constant slow drips at the bolts point to the tank bolt washers instead.
Can I use a regular rubber washer for toilet tank bolts?
Yes — a flat EPDM rubber washer with the correct ID, OD and thickness is functionally identical to the washers sold in toilet repair kits, and you can buy exactly the size you need instead of a kit of parts you won't use.
How tight should toilet tank bolts be?
Snug plus a quarter turn, alternating sides. The rubber washer does the sealing; over-tightening cracks porcelain.
What size rubber washer fits a toilet seat bolt?
Most toilet seat bolts are 5/16" and seal with a washer around 1" OD × 5/16" ID × 1/8" thick. Measure your bolt to confirm, then match the size in our catalog.