Service tax interest calculation

18% or 15% (as the case may be) simple interest upto 30.09.2014.
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Variable interest rates will apply on/after 01.10.2014.
- Notification No. 12/2014-ST, dated 11.07.2014 (link)
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Sl. No.	Period of delay	Rate of simple interest
1.	Up to 6 months	18%
2.	More than 6 months and up to 1 year	18% for the *first 6 months* of delay, and 24% for the delay *beyond 6 months*
3.	More than 1 year	18% for the *first 6 months* of delay 24% for the period *beyond 6 months up to 1 year, and 30%* for any delay *beyond 1 year*

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As specified in the proviso to section 75, 3% concession on the applicable rate of interest will continue to be available to the small service providers, i.e. assessee having turnover not exceeding Rs. 60 lakhs during the preceding financial year or during any of the financial years covered by the notice.
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Time limits have been prescribed on monthly basis whereas interest is required to be computed on per day basis.
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For service tax late payment interest calculator w.e.f. 01/10/2014 - click here
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(Note: This calculator is not applicable where service tax became due before 01.10.2014 and was paid after 01.10.2014. For calculation of interest in such cases - click here)