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Immunisation protects a funded position from small rate moves by matching the duration of assets to the duration of liabilities. Blending a short and a long bond in the right proportions hits the target duration exactly.
Duration match
w_short = (D_long - D_liab)/(D_long - D_short); w_long = 1 - w_short
No. It neutralises small parallel shifts only; twists in the curve and large moves still bite, so convexity should also be matched.
Durations drift as time passes and yields move, so immunised portfolios are re-matched periodically.