How to calculate tds

As per section 194C if any single payment exceeds rs 30K or if aggregrate of payments exceeds Rs 75k then TDs is requried to be made @ 1% if pymt made to individual or HUF & other than that @ 2%.

At the beginning of the year you will have to estimate if the aggregrate payments would exceed Rs 75K. If yes then start deducitng tax from the first bill onwards at teh prescribed rates.  however if the amount could not be estimated then start deducting tax at the time the aggregrate paymetns exceeds 75k and after that.  However if single payment exceeds Rs 30k then deduct tax at the prescribed rates.