A converter is only as good as the number you give it, so it is worth two minutes with a soft tape measure. Measure over thin clothing or bare skin, standing straight and relaxed.

1. Find your natural waist, just above the hip bones, where your pants usually sit.
2. Wrap the tape around it, level all the way round, and breathe out normally.
3. Read the number without pulling the tape tight. A snug tape gives you a size that feels tight by lunchtime.
4. Measure your inseam by laying a well-fitting pair flat and measuring from the crotch seam to the bottom of the leg.

Your body measurement and the size on the label are not always the same. Many brands add a little ease, so a pair labelled 32 often measures 33 or 34 inches around the waistband. If you have a pair that fits perfectly, measuring the waistband laid flat (and doubling it) is often more reliable than measuring yourself.

US and UK men’s pant sizes both use the waist in inches, so they are the same number. EU sizes run in steps of two, and as a rule of thumb the EU size is your waist in inches plus 16.

Odd waist sizes such as 31 or 33 sit between two EU sizes. Pick the smaller one for slim fits and stretch fabrics, and the larger one for rigid denim and relaxed fits.

US sizes are simply the waist in inches, often written as W32. Jeans add a second number for the inseam, such as L32, while chinos and trousers often come in short, regular and long lengths instead.

UK sizes use the same inches as the US, so a UK 32 is a US 32. The difference is usually in the length: UK trousers are more often sold as short, regular and long than with an exact inseam.

EU sizes are used in Germany, France, Italy and most of Europe. They are a number such as 48 or 50 that rises in steps of two. A 32 inch waist is usually an EU 48, which is where the plus-16 rule comes from. Italian tailoring brands tend to cut slimmer, so check their own chart before you order.

Letter sizes (S, M, L) cover a range of waists, so they are less precise for trousers. They work for joggers, shorts and elasticated waists, but for chinos and jeans a numbered size gives a much better fit.

Jeans are labelled with two numbers, such as W32 L32. W is the waist and L is the inseam, both in inches. A 30 to 32 inch inseam is a regular length for most men; under 30 is short and 33 or more is long.

The inseam you need also depends on the style. Slim and tapered jeans look best with little or no break at the shoe, so many men go one inch shorter than their straight-leg length. Wide and baggy jeans are usually worn with more length stacking at the ankle, so the regular or long length suits them.

Hemming is cheap, so if you are between lengths, buy the longer pair and have it shortened. You cannot add length back.

Check the brand’s own size chart. Sizing is not standardised, and some brands cut generously, so their 32 measures closer to 33 or 34. Reviews that mention “runs small” or “runs large” are worth reading.

Think about the fabric. Stretch denim and chinos give a little and relax with wear; rigid denim does not, and can shrink slightly after the first wash. Size up if you are between sizes in rigid fabrics.

Remember the cut. Slim and skinny fits feel smaller at the thigh and seat even in your usual waist size. If you have muscular legs, an athletic or tapered fit in your normal waist usually beats sizing up.

Buy for the waist and seat. Length is easy to fix and the waist can be taken in a little, but a seat that is too tight cannot be let out much. Get the top half right and adjust the rest.