Both indexes look at the prices of hundreds of items we spend money on but CPI doesn't include housing costs and mortgage interest payments. Also RPI and CPI are calculated using different formulas. The way CPI is calculated is more accurate and comparable with how other countries measure inflation.
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Government recently changed how index-linking is calculated on index-linked saving certificates from RPI to CP
The CPI is generally lower than RPI.
A review of consumer price statistics in 2015 recommended that government and regulators should work towards ending use of RPI ASAP.
The UK government has reduced use of RPI for indexation.
Direct taxes, benefits, public sector pensions, nation savings, state pension all moved from RPI to CPI.
Student loans are now the exception.
Why??