Siren 4 Thermo Mid Zip Waterproof

Use the chart for your own size range. Merrell doesn’t recommend converting between men’s and women’s sizes, because every foot is different — shop in the men’s or women’s size you normally wear.

Many Merrell styles come in Medium (M) and Wide (W). The widths available for each style are shown in our size picker.

Between sizes? Size up if you wear thick socks or orthotics, or you're on your feet all day. Choose the smaller size if you like a snug, athletic fit.

How much room? About a thumb’s width at the toe — roughly ½ inch (1 to 1.5 cm) between the longest toe and the end of the shoe. Toes should wiggle freely without the foot sliding forward.

Thumb test: with your child standing, press the tip of your thumb down at the front of the shoe. You should be able to press down without hitting a toe.

Between sizes or one foot bigger than the other? Choose the bigger size. Merrell notes kids’ feet can grow up to a ½ size every 2–4 months, so re-measure often.

Time for a bigger size if toes are pressed against the front, the shoe leaves deep marks or blisters, or your child says their feet hurt. Too big if their heel lifts out when walking, or they trip or drag their feet.

1. Stand on a sheet of paper, with your weight on that foot.
2. Mark the back of your heel and the tip of your longest toe.
3. Measure between the marks in inches or centimeters.
4. Measure both feet and use the longer one.
5. Measure later in the day, when feet are at their largest, wearing the socks you’ll wear with the shoes.
6. For kids: have your child stand barefoot with their weight on the foot, then match the measurement to the kids’ chart.