What is the percentage of tax on short term capital gain from shares if security transaction tax is not paid?

If the gain is short term and if securities transaction tax has not been paid at the time of sale of shares the gain will be taxed in accordance with the normal provisions at the slab rates applicable to you. That means the gains will be just added to your income and no flat rate of tax.

In the same way, if securities transaction tax has not been suffered at the time of sale of the shares, which are long term assets, the gain will be charged to tax in accordance with Section 112 of the Act. Under this section tax has to be charged on the capital gain computed with the benefit of indexation at 20 per cent. In such a case, if 10 per cent of the gain computed without the benefit of indexation is lower than 20 per cent of the gain computed with the benefit of indexation, the excess over 10 per cent of the gain computed without the benefit of indexation will be ignored in computing the tax liability.

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27%...depending on your income bracket

The short term capital gains on sale of shares not subject to STT is taxable at normal rates of tax i.e. 30% if the assesee is in the highest tax bracket plus education and higher education cess as well as surcharge if applicable.

short term capital gain on shares transaction without paying stt is subject to normal rate of income tax. it will not be treated as short term capital.

like... if you have the taxable income including short term capital gain below the basic exemption limit (i.e. 110000.00 for a male below 65 years of age and so on) then there will be no tax.

but if you have the net taxable income including the short term capital gain more then the basic exemption limit then the tax will be calculated accordingly.