Alcohol Concern, the national agency on alcohol misuse, is urging licensees to publicise the size of its alcohol measures over the Christmas period.
There is currently inconsistency in the on-trade with some bars serving a 25ml standard measure and others offering 35ml or even 50ml as the normal spirit serve.
This has led to widespread confusion as to how many units each drink contains, making it hard for drinkers to monitor their alcohol levels.
Over the festive period, with drink-driving high on the agenda and the pub attracting customers who rarely visit at other times of the year, Alcohol Concern wants licensees to allow the customer to make an informed judgement.
A spokesperson for the group said: "As part of our ongoing bid to increase people's awareness and knowledge about alcohol volumes in drinks, we think bars should clearly indicate the size of the servings of wine and spirits they offer - for example, how many people know that a 'large' (250ml) glass of wine equates to a third of an average bottle of wine?
"Customers should always be given the option of buying a smaller measure without being made to feel small themselves.
"People need to be equipped to make informed choices about the number of units they're consuming - the choice should be with the drinker, not the supplier."
Spirit companies such as Guinness UDV and Bacardi-Martini promote the introduction of a 35ml spirit serve as the standard "single". At 1.5 units, it brings a spirit and mixer in line with a small glass of wine or a 275ml bottled lager or premium packaged spirit.
Bacardi-Martini has recently launched an upsizing campaign as part of its "Pourfection" initiative.
This is designed to drive sales across the entire spirits category and change the way spirits are served in the on-trade.
Marie Carter, senior trade marketing manager for Bacardi-Martini, said: "Recent research has proved that by increasing measures sales will increase rapidly as consumers order more of their favourite drink.
"By promoting 35ml as the standard serve we will eliminate confusion and simplify the spirits offering. It's good for customers, increases profitability and can help licensees lift their outlets above the competition."
Bacardi-Martini estimates that outlets selling around 28,000 25ml shots at £1.80 a year will increase yearly profits by £33,000 with a switch to 35ml.