British adults are underestimating the annual cost of a care home stay by an average of £16,646, research shows.
This is up from £12,000 last year, according to the survey from Which?.
The survey found 58% came up with a figure that falls well short of actual costs.
One in 10 underestimated the cost by £837 a week, or roughly £43,500 a year.
Londoners could be in for the biggest shock, as they underestimated weekly costs by £649, which works out as £33,748 per year.
People in the East Midlands were the closest to reality, underestimating by £152 per week.
The research also shows the regions experiencing the largest year-on-year increase in care home fees are the north-east (11.3%), Yorkshire & Humberside (10.9%) and the north-west (10.4%).
However, the least affected areas – the east of England and the south-east – still saw sizeable jumps at 7.7% and 7.9%, respectively.