Could someone tell me why tax changes every month?

Sofie

My tax code seems to be a moveable feast. On earnings of £24790 I paid:
April 2011 187T - 354.80
May 2011 187T - 355.00
June 2011 BR CUMUL - 449.00 (I can understand this is a change because I qualified for a full state pension)
July 2011 - BR CUMUL - 386.40
August - K159 NON CU - 412.80
September 2011 -K159 NON CU - 412.80

I also get a small pension by a previous employer approx £1000 per annum.

Thanks for any help anyone can give.

ceeforcat

First change - 20p increase - tax is charged on whole pounds on a year to date basis. If your income was £1000.60 per month, you would pay tax in the first month on £1000. In the second tax month you would pay on £2001 less what you paid in the first month.

June 2011 - BR (temporary code while pension deduction sorted out) operated on all of your earnings to date i.e. also collecting amount underpaid in April and May.

July 2011 - correct tax for month of July,

August and September - tax due for each month on code K159 not taking into account underdeductions as a result of BR code previously operated.

Note : as a result of K159 (if it is the correct code finally) not being operated April to July you will be underpaid for the year to the tune of £26.40 x 4 = £105.60

Sofie

Thank you very much Ceeforcat. That's really helpful.