Toilet flapper size: an orange-size drain takes a 2-inch flapper, a grapefruit-size drain a 3-inch
Look at the drain hole at the bottom of the tank. About the size of an orange or baseball: 2-inch. About the size of a grapefruit or softball: 3-inch.
Fluidmaster's other test: a flapper about 3 inches across fits a 2" valve; about 4 inches across fits a 3" valve.
Flapper makers' size rules
- Fluidmaster: "Most toilets will have a 2-inch drain or a 3-inch drain." (Fluidmaster)
- Fluidmaster: "If the flapper measures 3 inches across, select a 2″ flapper (like the 502 Fluidmaster model). If the flapper measures 4″ across, then select a 3″ flapper (like the 5403 Fluidmaster model)."
- Korky: "Most toilets made before the year 2000 will need a standard 2-inch flapper." and "Some toilets made after the year 2000 will need a large 3-inch flapper." (Korky)
Toilet makers' part numbers
Kohler canister and American Standard Champion toilets use a seal, not a standard flapper. Buy the toilet maker's part number when it is listed.
| Toilet (tank) | Maker's part (as printed) | Source |
|---|---|---|
| TOTO Drake ST743S and ST743E | THU500S "Flapper w/ Chain" | TOTO parts manuals |
| Mansfield 160 tank | 630-0030 "Replacement flush valve seal"; the sheet lists a 2" flush tower | Mansfield spec sheet |
| American Standard Champion 4215A | 7301111-0070AP "Champion Seal" (comes with flush valve 7381967-400.0070AP) | AS 4215A parts sheet |
| Kohler single-flush canister | No flapper: canister seal GP1059291 | Kohler brochure |
Sources
- Korky, What size flapper do I need?: https://www.korky.com/toilet-repair-help/what-size-flapper-do-i-need (read 2026-10-27)
- Fluidmaster, Determining whether you have a 2" or 3" flush valve and flapper: https://fluidmaster.com/toilet-problems/determining-whether-you-have-a-2-or-3-flush-valve-and-flapper/ (read 2026-10-27)
- TOTO, Parts Manual CST743S Drake 1.6 gpf (PD-00237 v.10, Dec 2021): https://www.totousa.com/filemanager_uploads/product_assets/PartsManual/PD-00237_CST743S.pdf (read 2026-10-27)
- Mansfield Plumbing Products, 137-160 Alto 1.6 spec sheet (PDF, Rev. 12/25, linked from mansfieldplumbing.com): https://assets.edgenet.com/78b982d8-88ae-4e3e-a632-feb3a7ba8df6 (read 2026-10-27)
- American Standard, Replacement Parts, Two-Piece Champion Toilet (4215A Tank), PDF dated 7/24: https://lixil.cdn.celum.cloud/247226_Champion_Parts_Toilets_July2024_original.pdf (read 2026-10-27)
- Kohler, Toilets Service Parts brochure (PDF, 2020): https://resources.kohler.com/webassets/kpna/brochures/Toilet_ServiceParts_Brochure_FINAL.pdf (read 2026-10-27)