A bonus or 13th-month payment is a one-off, non-monthly payment. The IRD's APIT system taxes regular monthly emoluments progressively, but treats 'other' (lump-sum) payments at a flat rate — in this version 12% — so the tax can be computed simply in the month the bonus is paid, without retroactively re-taxing the whole year.

That is why the Bonus tool uses a flat rate rather than your progressive slabs: the employer withholds 12% on the day the bonus is paid and that is the end of it.

A Rs 100,000 bonus: tax = 12% × 100,000 = Rs 12,000, leaving Rs 88,000 in your hand. The tool shows both figures.

Bonuses do not attract EPF/ETF — they are not regular earnings — and they are excluded from the EPF base in the main calculator too.

Use the flat rate for genuine lump sums (festival advance, 13th month, one-off performance bonus). Arrears of regular salary are different — some payroll systems tax arrears at your marginal rate; if in doubt, ask your payroll office how arrears were treated in the month paid.