The Boundary Commission for England published its initial proposals for new constituency boundaries on 8 June. It includes changes to the number of constituencies in different counties, regions and nations, which affects the weight of votes in different areas.

This Insight looks at how voter weight would change in different areas if the proposals go ahead. See our other Insight for analysis of how many seats will change, and which seats will change the most.

Boundary reviews

Constituency boundaries are reviewed periodically to ensure constituencies are roughly a similar size and reflect local ties between areas.

The current boundaries were designed based on electorates in 2000 and have been used since 2010. Our briefing papers cover constituency boundary reviews and the number of MPs and parliamentary boundary reviews and public consultations.

Current constituencies don’t have equal voter weight

Current constituencies do not have equal electorates. There were some differences when they were first designed, and changes in population since that time have increased these differences.

This means that voters in different constituencies do not have equal weight: voters in constituencies with smaller electorates have greater weight as they make up a larger fraction of the total.

The boundary review is designed to even out electorates, which would create constituencies with more equal weight. As a result, the number of constituencies in England is proposed to rise by ten, with the total in Scotland falling by two and the number in Wales falling by eight. Here we look only at the proposals for England.

Measuring changes in voter weight

We have analysed how the proposed changes affect the relative weight of a single vote in different parts of England. We have made a simple calculation of voter weight based on the size of the electorate in each constituency.

For example, the current Blackpool North and Cleveleys constituency currently has a small electorate of 63,150. Most of its voters would be in the future Blackpool North and Fleetwood constituency, with a larger electorate of 75,396. So each voter would make up a smaller fraction of the new constituency’s electorate than the old – a fall in voter weight of 16%.

By contrast, the current Manchester Central constituency has an electorate of 94,952. After the proposed revision, the electorate would be 76,609. Voters in the new constituency will see their vote make up a larger proportion of the constituency’s total than before– a rise in voter weight of 24%.

In constituencies with unchanged or nearly unchanged boundaries, such as Crawley or Lincoln, voter weight will not change. Around ten million voters would have less than a 1% change in the weight of their vote under the proposals.

Voter weight rises for more people than it falls. This is because the number of constituencies in England is proposed to increase from 533 to 543. For 6.3 million voters, vote weight would fall by more than 10%, while for 12.4 million, vote weight would increase by more than 10%.

Visualising the change in voter weight

The two maps below show changes in voter weight by small area. This is a way of seeing which areas of the country would have larger or smaller constituencies under the proposals. The first map is a standard geographical map, while the second map is a cartogram showing areas scaled in size relative to their populations and grouped by county. This shows changes in urban areas more clearly, such as the variation between different parts of London.

Geographical map showing changes in voter weight as a result of the boundary review proposals. While each region has some areas that gain and lose in power, the largest rises are in the South East, East and South West regions, and the largest falls are in the North East, West Midlands and North West regions.

Cartogram map showing changes in voter weight as a result of the boundary review proposals. Each hexagon represents a small area (MSOA) with a population of around 8,000 people, and hexagons are grouped by county and conurbation. While each region has some areas that gain and lose in power, the largest rises are in the South East, East and South West regions, and the largest falls are in the North East, West Midlands and North West regions.

Data table showing the ten counties and conurbations with the largest increase in voter weight (led by Somerset, Oxfordshire and Buckinghamshire) and the ten with the largest fall in voter weight (led by Northumberland, Cumbria and Tyne and Wear).

Based on the results of the 2019 General Election, voter weight in constituencies with a Conservative MP would go up by 3.6% on average. This is compared to an average increase of 1.6% in Labour-held seats and 4.0% in Liberal Democrat seats.

It’s important to reiterate that the changes shown here are designed to reduce existing inequalities between constituencies. The maps are not evidence of an unfair deal, for example, for the voters of Cumbria and Northumberland. Instead, they show that constituencies in those areas currently have smaller electorates than others, so their votes currently have disproportionate weight.

The analysis doesn’t consider that voters in some areas are more likely to actually vote than in others (differential turnout). This would influence the weight of votes in an actual election.

Marginal constituencies and voter power

Our analysis also doesn’t consider other measures of voter power, like the marginality of a constituency. In practice, voters living in constituencies where election results are close are more likely to influence the result of an election than those where the winning candidate has a large majority.

Constituencies with the largest majorities at the 2019 election would have the largest increases in voter weight on average if the boundary proposals go ahead. Areas in the most marginal constituencies, with a majority of under 5%, would have a 1.9% fall in voter weight on average.

This pattern is broadly similar for seats held by both the Conservatives and Labour. Voters in Conservative seats gain more weight on average, but in areas forming the very safest seats (the 126 seats with a majority of over 40%), voter weight increases by 4.7% in Conservative seats and 8.6% in Labour seats.

These Labour seats cover mostly dense urban areas like West Ham, Croydon North and Sheffield Central which have had large increases in population since constituency boundaries were last revised before the 2010 election.

Download the data

The data is presented at output area level (averaged to MSOA level on the cartogram map), based on a best-fit analysis of which constituency the output area forms part of, according to current and proposed constituency boundaries.

The data file attached below gives full information at output area level, i.e. for 170,000 small areas in England. Download data in Excel (10 MB)

About the author: Carl Baker is a statistician specialising in health and geography at the House of Commons Library. Elise Uberoi is a statistician specialising in elections and Parliament at the House of Commons Library.