Koondamishüvitis, redundancy severance under Article 90 of the Töölepingu seadus (Employment Contracts Act), comes from two different payers stacked together. Your employer always pays 1 month's average salary, calculated from your last 6 months of pay, regardless of how long you've worked there. If you've completed at least 5 years of service, Töötukassa, the state Unemployment Insurance Fund, adds a further payment on top: 1 extra month for 5 to 10 years of service, or 2 extra months for more than 10 years.
This only applies to redundancy specifically. Termination for cause or proven misconduct carries no statutory severance at all, and an ordinary resignation with standard notice doesn't trigger severance either. The one exception is resigning because of the employer's own serious breach of contract, which can lead to a discretionary payment of up to 3 months' salary, decided case-by-case rather than by a fixed formula, so it isn't part of this calculator.
On tax: koondamishüvitis is treated as ordinary taxable income, subject to the same flat 22% income tax as regular salary, and funded pension (II pillar) contributions are withheld from it too if you participate at a non-zero rate. It's exempt from social tax and unemployment insurance premiums on both the employee and employer side, since it's a compensatory payment rather than ordinary wages for work performed.