Swing needed to gain
The swing a party needs, on a uniform swing, to win a ward it does not hold.
How it's calculated
(Lowest winner's share − the party's best share) ÷ 2.
Worked example
Reform UK in Heworth after the 2026 by-election: (36.7 − 20.1) ÷ 2 = 8.3pp from Labour.
Source: Heworth ward, City of York — by-election of 15 January 2026 and May 2023.
Caveats
- Assumes a uniform swing; real swings vary by ward.
Where it's used
- Council Previews and Target Wards (coming soon)
Other definitions
Vote shareChange in shareSwing (two-party, Butler)MajorityUncontested seatCandidates per seat / per wardContestation rateWinner's shareEffective number of partiesWinner's bonus (seats vs votes)Incumbency and re-election rateWomen candidates and councillorsTurnout (three methods)Council control and No Overall Control (NOC)Majority at riskPsephology National Share EstimateBy-election vote trackerNotional resultIndex of changeSplit-ticket indicatorNational Equivalent Vote