When should I use fewer instead of less?

Use fewer for things you can count individually, and less for quantities you measure as a mass. Fewer biscuits, less flour. Fewer people, less noise.

The test is whether the noun has a sensible plural. If you can say three of them, it is countable and takes fewer. If you cannot — you would not say three flours meaning quantity — it takes less.

That covers most cases, but there is a well-defined set of exceptions where less is correct even though the noun looks countable, and knowing them prevents overcorrection.

Use less with money: less than twenty pounds, not fewer. Use it with time: less than three weeks. Use it with distance and weight: less than five miles, less than two kilos. In each case the plural noun is standing in for a single measured quantity rather than a set of separate items. Fewer than twenty pounds would mean twenty individual coins.

The supermarket sign reading 10 items or less is the classic complaint, and by the ordinary rule it should be fewer. Whether it is worth objecting to is a separate question — the usage is understood by everyone and has a long history.

A related pair follows the same logic: number for countable, amount for uncountable. A number of errors; an amount of work.

Be aware the distinction is a comparatively recent prescription. Writers used less with countable nouns for centuries before an eighteenth-century grammarian expressed a preference, which is worth remembering before correcting anyone.

Percentages and fractions follow the measurement logic too: less than 50% of respondents, because the percentage is a proportion rather than a count. And with one, always use less: one less thing to worry about is standard, despite thing being countable.