Quorum requirements for a referendum to be valid

Sri Lanka
- Mandatory referendum - turnout quorum
- Mandatory referendum - approval quorum
- Optional referendum - turnout quorum
- Optional referendum - approval quorum
For a referendum to be valid, there are two possibilities: (a) voter turnout exceeds 2/3: bill passes if majority of valid votes (at least 0.5 x (2/3) = 1/3 of all possible voters) support the Bill; (b) voter turnout is below 2/3: bill passes if at least 1/3 of all possible voters support the Bill.
Hence, the approval quorum is 1/3 of the electorate regardless of the actual turnout.
Constitutionof the Democratic Socialist Republic of Sri Lanka
(Adoptedin 1978, last amended in 2001)
http://www.priu.gov.lk/Cons/1978Constitution/Chapter_13_Amd.html
CHAPTERXIII - Submission of Bills to People by Referendum, Article 85.
(3) Any Bill or any provision in any Billsubmitted to the People by Referendum shall be deemed to be approved by thePeople if approved by an absolute majority of the valid votes cast at suchReferendum:
Providedthat when the total number of valid votes cast does not exceed two-thirds ofthe whole number of electors entered in the register of electors, such Billshall be deemed to be approved only if approved by not less than one-third ofthe whole number of such electors.
Turnout quorum: 2/3 of the electorate; Approval quorum: 1/3 of the electorate (see also source).