Seat Projection Calculator

Work out seat projection instantly with clear inputs, formula shown and shareable results.

About Seat Projection Calculator

Two classical methods convert vote share into seats. The cube rule assumes seats move with the cube of the vote ratio, which captures the amplifying effect of first-past-the-post. Uniform swing applies the same percentage-point change everywhere and translates it through marginal seats. Averaging the two hedges their known biases.

Formulas

Cube rule

Projected seats = Previous seats x (Current share / Previous share)^3

Uniform swing

Projected seats = Previous seats + Swing x Total seats x 0.022

Worked example

With the default values below, Seat Projection Calculator returns Projected seats of 174. Change any input above and every figure here is recalculated in your browser.

Inputs used

Projected vote share: 38.5
Previous vote share: 34.2
Seats won previously: 128
Total seats: 403

Results

Projected seats: 174
Cube-rule projection: 183
Uniform-swing projection: 166
Swing in percentage points: 4.30%

Step by step

1Values used
2Cube rule
3Uniform swing
4Projected seats= 174
5Cube-rule projection= 183
6Uniform-swing projection= 166
7Swing in percentage points= 4.30

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 2 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 does the cube rule work at all?

Because in a two-party plurality system with normally distributed constituency margins, a small national swing flips a disproportionate number of marginal seats.

When do these methods fail badly?

In multi-party contests, where regional parties are strong, or where support is geographically concentrated rather than uniformly spread.