Swing Percentage Calculator

Work out swing percentage instantly with clear inputs, formula shown and shareable results.

About Swing Percentage Calculator

Two-party swing is the average of one party's gain and the other's loss, which is why it is half the sum of the two changes. Halving avoids double-counting the same transferred voters and gives the single figure used to compare constituencies.

Formula

Butler two-party swing

Swing = [(B current - B previous) - (A current - A previous)] / 2

Worked example

With the default values below, Swing Percentage Calculator returns Two-party swing of 5.150%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Party A previous vote share: 42.1
Party A current vote share: 37.4
Party B previous vote share: 35.6
Party B current vote share: 41.2

Results

Two-party swing: 5.150%
Change in Party A share: -4.70%
Change in Party B share: 5.60%
Direction: Swing from Party A to Party B

Step by step

1Values used
2Butler two-party swing
3Two-party swing= 5.150
4Change in Party A share= -4.70
5Change in Party B share= 5.60
6Direction= Swing from Party A to Party B

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Indicative estimate only. Fees, entitlements, limits and formulas vary by jurisdiction, statute, policy wording and the facts of the case. This is not legal, tax, insurance or financial advice — confirm with a qualified professional or the relevant authority.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Indicative estimate only. Fees, entitlements, limits and formulas vary by jurisdiction, statute, policy wording and the facts of the case. This is not legal, tax, insurance or financial advice — confirm with a qualified professional or the relevant authority.

Frequently Asked Questions

Why divide by two?

Because a voter moving from A to B shows up twice — as a loss for A and a gain for B. Averaging the two counts the movement once.

What is the alternative measure?

Total vote swing (Steed swing) uses only the change in one party's share, which is more appropriate in genuinely multi-party contests.