How are seat projections calculated from swing?

Seat projections from swing use Uniform National Swing (UNS): the same vote change is applied to every constituency equally. The Curtice method refines this by applying differential swings and adjusting for local incumbency effects, producing more accurate constituency-level estimates.

Can a small swing produce a large seat change?

Yes. Under First Past the Post, a 1-2% national swing can shift dozens of seats because many constituencies are decided by small margins. The UNS model captures this non-linear relationship between votes and seats.

What is a hung parliament swing threshold?

Based on current polling and MRP modelling, the swing threshold for a hung parliament versus a Labour majority is approximately: Labour would need to be on 29%+ (from current 18-20%) with Reform below 23% (from current 27-29%) to win a workable majority. Any scenario with Labour below 28% or Reform above 24% is likely to produce a hung parliament. The swing calculator allows users to specify these scenarios and see the implied seat totals. The range of hung parliament outcomes (Reform largest party, Labour largest party, or near-equal) corresponds to relatively small national swing differences of 2-4 points, illustrating the current extreme sensitivity of UK seat projections to modest vote share changes.

Interactive Tool

UK Swing Calculator

Q: What is electoral swing?

Electoral swing measures the shift in vote share between two parties from one election to the next. A 5% swing from Labour to Reform means Labour loses 5 percentage points and Reform gains 5 points, changing the relative gap between them by 10 points.

Q: What is the current UK polling average?

As of September 2026, the polling average stands at Reform 28%, Labour 18%, Conservative 18.8%, Liberal Democrats 12.6%, Green 15%, SNP 3%. These figures shift weekly as new polls are published.

Q: How accurate is the swing calculator?

The swing calculator is a simplified UNS model and should be treated as illustrative. Real elections involve tactical voting, incumbency effects, regional variation, and differential turnout. For professional-grade projections, see MRP modelling from YouGov or Electoral Calculus.

Model what happens to vote shares and Westminster seats when swing moves between parties. Based on current polling averages for 2026.

How it works

Select which party loses votes and which gains them, then set the swing amount. The calculator applies a simplified Curtice-style uniform swing to the current VI base to project new vote shares and estimated seat changes. Current VI base: Reform 28 %, Lab 18 %, Con 18.8 %, LD 12.6 %, Green 15 %, SNP 3 %.

Configure Swing

FROM party (loses votes)

TO party (gains votes)

Swing amount: 3%

0.5%15%

Results

Set your swing parameters and click Calculate.

Methodology

Uniform National Swing

UNS applies the same vote change to every constituency. It is the simplest and most widely used swing model. The Curtice adjustment scales seat changes by the square-root relationship between vote share and seats under FPTP — capturing how marginal seats amplify small swings.

Limitations

UNS ignores incumbency, regional variation, tactical voting, and demographic shifts. Seat projections should be treated as rough estimates only. For high-accuracy modelling, see MRP polling, which models each constituency individually.

Frequently Asked Questions

What is electoral swing? +

Swing measures the shift in vote share between two parties. A 5% swing from Labour to Reform means Labour loses 5 points and Reform gains 5, changing their relative gap by 10 points.

How are seat projections calculated? +

The calculator uses Uniform National Swing: the same vote change is applied across all 650 constituencies. Seat estimates are derived from a simplified Curtice-style formula that maps vote share to seats under FPTP.

What does the current VI base represent? +

The VI (voting intention) base is the May 2026 polling average: Reform 28%, Labour 18%, Conservative 18.8%, Liberal Democrats 12.6%, Green 15%, SNP 3%. These figures are updated monthly.

Why can a 1% swing move many seats? +

Under First Past the Post, many constituencies are decided by tight margins. A small national shift can flip dozens of seats simultaneously. This non-linear relationship between votes and seats is a defining feature of FPTP.

How accurate is this tool? +

This is a simplified illustrative model. Real elections involve tactical voting, incumbency effects, regional variation, and differential turnout. For professional projections, see Electoral Calculus or YouGov MRP.

Related tools: Seat Projection Calculator • Poll Tracker • Polling Methodology Explained