Measure your feet on a hard, level floor while standing, since your foot spreads under body weight. Place a sheet of paper against a wall, stand with your heel lightly touching the wall, and mark the tip of your longest toe. Measure from the wall edge to the mark. Repeat for the other foot; many people have a small difference between sides.
For width, wrap a flexible tape around the broadest part of the forefoot without pulling it tight. You can also trace each foot while standing, keeping the pen upright rather than angled beneath the foot. Record the date, because measurements can shift with age, activity, and other changes in your body.
Use the larger foot as your reference when comparing measurements with a brand's sizing chart. Charts are not perfectly interchangeable, and length alone cannot describe the shape of a shoe. Look for information about width options, toe shape, and intended use, then check the retailer's fit and return guidance.
Repeat the measurement if the result seems surprising, and measure in the socks you expect to wear with the shoes. A home measurement narrows your search but does not replace trying footwear on. The final test is whether the shoe feels comfortable when you stand and move.