The two directions
Exclusive working starts from a taxable value and adds tax on top: a base of one thousand rupees at eighteen per cent becomes one thousand one hundred and eighty. Inclusive working starts from the total the customer pays and recovers the base first.
The common mistake is taking eighteen per cent of the inclusive total. That overstates the tax, because the percentage belongs to the base value, not the total. Dividing by 1.18 gives the correct base, and the difference is the tax.
Splitting the tax on an invoice
For a supply within the same state, the total tax is split equally between central and state components. For an inter-state supply, the whole amount is a single integrated tax instead. The total tax is the same either way — only the presentation on the invoice changes — but getting the split wrong causes reconciliation problems later.
Keep pricing decisions pre-tax
When you compare margins, discounts or supplier quotes, work on values excluding tax. Tax is collected on behalf of the government and passes through your books; leaving it inside a margin calculation makes products look more profitable than they are, and distorts comparisons between quotes given inclusively and exclusively.
Check the arithmetic both ways
A quick sanity check: after extracting tax from an inclusive amount, add the tax back to your computed base. If you do not land exactly on the original total, the working is wrong. Doing this once on a real invoice is faster than reconciling a return later.