How many grams in a pinch of salt?

One senses a certain British reluctance when it comes to our conversion to metric measurements. The Department of Trade and Industry, for example, still refers nostalgically to "road traffic signs in miles" as "part of the British way of life". The latest bastion of the imperial system to fall is the measurement of fixed-rate pre-pack items, which will go metric from 1 October. Items sold loose in bulk have a stay of execution until 1 January 2000.

The duality of the new system is bad enough, but to make matters worse no one can agree on how many grams there are to the ounce. Ask the DTI and it will tell you 28g, ask The Guild of Food Writers and it will tell you it is 25g, and there are plenty of other conversion charts that reckon it is 30g. In fact, most of these charts even out to agree that 1kg equals 2.2lb (ish).

The Guild of Food Writers, an organisation that speaks for journalists and writers, has taken matters into its own hands to come up with a workable chart. The reasoning behind its 25g to the ounce is that 28g is unworkable, and it is imperial-thinking rather than metric. Roz Denny, the guild's vice chairman, argues that total conversion should have happened 20 years ago, and there is now an urgent need to be practical, to start thinking in metric, and working in metric.

While the DTI is recommending a conversion table, there is no obligation for shops to adopt it, as providing it is legally accurate they are free to draw up their own charts. For the shopper who has already converted to metric, life will be fairly trouble-free. It is the shopper who still works in imperial who is going to get hit.

If you ask for 1lb of mushrooms in a greengrocers, you may arrive home with anything between 400g and 500g, depending on whether weights are rounded up or down. Alternatively, if you adopt 25g as your base unit, you go shopping and think, oh yes, I need 1lb - or 16oz - of mushrooms, that's 400g. You get home and find that the recipe you want to cook says 1lb/450g of mushrooms. It is little things like this that are going to throw people.

Sainsbury's helpfully says, "If customers get confused all they have to do is ask a member of staff for help." Unfortunately, Sainsbury's seems to be rather confused itself. On the style sheet it hands out to the authors of its cookery books 400g is 13oz, and on the leaflets it hands out in its stores it is 14.2oz.

The more confused people become the less likely they are to want to convert to metric, if they have not already done so. Realistically, imperial measures are going to hang around well beyond 2000, so it is essential that everyone agrees on conversions as soon as possible. In the meantime, the next five years promise some of the messiest shopping ever.

The is nothing like a spate of metric conversion to make you feel old. If you have worked in pounds and ounces all your life, the confusion of a shift to metric should not be underestimated. Try telling most cooks over 30 that 2lb of potatoes is the same as 900g and they will as likely look at you as though you have arrived from Planet Zorg. Imperial cooks can visualise what a 3lb chicken looks like, or 1lb of cheddar or apples, and they may well assemble recipes in their mind on this visual basis. A tomato salad to feed five? About five tomatoes to the pound and I need two per person, so I need 2lb.

And it will be a sad cultural loss if future generations have no understanding of imperial measures whatsoever. There is a huge volume of cookery books in print in this country that go back hundreds of years, and these could be lost to our understanding if no effort is made to educate children. Already children are asking, "Mummy what's an ounce?"

It is interesting to note that the pint is the other unit that we shall continue to have the pleasure of using until the year 2000. Presumably the idea of walking into a pub and asking for 600ml of beer is too cold for comfort. There is a certain irony that the day on which units switch, much of the population will have such crashing hangovers that they will probably have difficulty remembering their own name.

Annie Bell is a cookery writer and the author of vegetarian cookery books