Inheritance tax is calculated at 40% of the value of the possessions over and above the IHT thresholds. The rate may be reduced to 36% if 10% or more of the estate that is above the threshold is left to charity. So if the value of your estate is above the NRB, then the part of your estate above the threshold might be liable for tax at the rate of 40%.
As an example, if your estate is worth £525,000 and your IHT threshold is £325,000, the tax charged will be on £200,000 (£525,000 – £325,000). The tax would be £80,000 (40% of £200,000).