Up to a third of mortgage holders and 38% of tenants have never calculated the value of their belongings, according to a survey by Primary Direct.
And about a quarter of those who did take the time, only did so once.
The research also revealed an estimated 5.9 million mortgage holders were potentially underinsured.
In addition, 66% of home owners said they had bought home contents cover, leaving around 3.8 million property owners uninsured.
This is despite the fact that 55% of homeowners said they were very concerned about the threat of burglary. Other concerns expressed were fire (34%), burst pipes (18%) and being flooded (11%).
The survey of 1,000 people in March coincides with the launch of Primary Direct's new home contents policy, which automatically inflates the sum insured by 10% at Christmas and for 30 days before or after a wedding.