A worked example: ₹24,000 base salary plus overtime in West Bengal
Start with a ₹24,000 monthly base salary, a standard schedule of 208 hours, and 15 overtime hours at the standard 2x multiplier. The regular hourly rate is ₹24,000 / 208 = ₹115.38, so the overtime hourly rate is ₹115.38 × 2 = ₹230.77, and 15 hours of overtime at that rate produces overtime pay of ₹230.77 × 15 = ₹3,461.54. Added to the ₹24,000 base, that brings the month's total gross salary to ₹27,461.54.
The formula: run overtime pay first, then feed the higher gross into the professional tax slabs
The two calculators chain together directly: overtime pay = hourly rate × multiplier × overtime hours, and that result gets added to the base salary to form the month's gross salary, which is then looked up against the state's professional tax bands. West Bengal's bands run ₹0 up to ₹10,000, ₹110 from ₹10,001–₹15,000, ₹130 from ₹15,001–₹25,000, ₹150 from ₹25,001–₹40,000, and ₹200 above that.
The slab actually changes: ₹130/month becomes ₹150/month
On the ₹24,000 base salary alone, West Bengal's professional tax is ₹130/month (₹1,560 annually) — that salary sits in the ₹15,001–₹25,000 band. Once the ₹3,461.54 in overtime pay is added, the ₹27,461.54 gross salary crosses into the ₹25,001–₹40,000 band, and the professional tax rises to ₹150/month (₹1,800 annually). That's a real ₹20/month increase in professional tax caused entirely by the overtime pay pushing gross salary across a band boundary — not by any change in the tax rate itself.
Why this does not always happen
This slab jump isn't guaranteed — it depends on how close the base salary sits to a band boundary and on the state's own band structure. In Karnataka, for example, professional tax is a flat ₹200/month for any salary above ₹15,000, so overtime pay added to a salary already above that threshold would change nothing about the tax owed. The West Bengal example above shows a real slab change, but the same overtime pay applied to a salary already deep inside a band, or in a flat-rate state, would leave the professional tax untouched — both outcomes are worth checking with the real numbers rather than assumed either way.