Model and input conventions
Convert each volatility percentage to a decimal, square it and multiply by its years to expiry. Subtract the near cumulative variance from the far cumulative variance, then divide by the difference in years. The square root of that forward variance rate is the annualized forward volatility. Both terms use calendar days divided by 365; the far expiry must be strictly later and the cumulative-variance difference cannot be negative.
Worked numerical example
A 30-day term at 20% volatility and a 60-day term at 30% give a forward variance rate of 0.14. Its square root is approximately 37.4166% annualized volatility for days 30 through 60. The result exceeds both entered volatilities because the later thirty days must account for the extra variance. If both terms have the same volatility, the extracted forward volatility equals that common input.
Read the scenario table
The table displays the start-to-near, near-to-far and start-to-far windows separately. Check that the first two cumulative variances add to the final value. Volatility percentages themselves do not add, so reading only their difference misses the maturity weighting. Use the forward-window length to ensure the question concerns the intended dates, especially when comparing a short event window with a much longer expiry.
Where the model stops
Compatibility matters: unrelated strikes, different annualization conventions or a mixture of realized and implied quantities can produce an uninterpretable result. The calculation is an additive-variance identity, not a complete market model for arbitrary options. A negative difference is rejected rather than converted to a positive number. This tool does not compute the Cboe VIX index, price variance futures or validate an exchange volatility surface.