People who think their pint is not the full measure are being urged to ask for a top-up by Trading Standards officers in Bath and North East Somerset.
A survey carried out by a partnership of 15 trading standards departments in the south west found 20% of pints had less liquid than recommended.
Landlords who delivered short measures were advised and revisits are planned.
"If you've asked for a pint make sure you get it," said Roger Pope, Principal Trading Standards Officer.
Also included in the survey was a comparison of soft drink and alcohol prices.
Although the average price per pint of soft drink was found to be similar to a pint of beer, a huge variation in prices was found.
The lowest was £1.50, but the highest was £4.73 for a pint of bottled orange juice.
Ice proved to be an expensive commodity as it pushed the price of a soft drink up by around 13% per pint (30 pence on average).