Work out seat projection instantly with clear inputs, formula shown and shareable results.
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.
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
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.
Because in a two-party plurality system with normally distributed constituency margins, a small national swing flips a disproportionate number of marginal seats.
In multi-party contests, where regional parties are strong, or where support is geographically concentrated rather than uniformly spread.