What size hat do I need?

Measure around your head about half an inch above your ears and across the widest part at the back. Most adult women fall between 54 and 58cm, and hat sizes are quoted in that measurement.

Most unworn hats were the wrong size rather than the wrong style, and the measurement takes thirty seconds.

How to measure. Run a soft tape around your head, sitting it about half an inch above the ears at the front and over the widest point at the back — roughly where a hat would naturally sit. Keep it snug but not tight. That circumference is your hat size. Most adult women measure between 21¼ and 22¾ inches — 54 to 58cm, since plenty of listings give it that way — and S/M/L labelling usually maps to roughly 21⅝, 22½ and 23¼ inches.

Which hats are forgiving and which are not. A knitted beanie stretches and will cope with half an inch or so either way. A structured felt hat, a cap or a pillbox has almost no tolerance: slightly too big and it lifts in wind, slightly too small and it leaves a line across your forehead and gives you a headache after an hour or two.

Allow for hair. If you usually wear your hair up, or you have a lot of it, measure with it as you would normally wear it under the hat. That difference is easily half an inch, which is the difference between a size.

The practical tie-break. If you are between sizes on a non-stretch hat, size up — a slightly loose hat can be fixed with millinery tape inside the band, and a tight one cannot be fixed at all.

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