You’ve found the perfect pair of shoes online. The design is stunning, the color is exactly what you wanted, and the price is a steal. You scroll down, check the size chart, and see “40.” But a moment of doubt creeps in. Is that a 9? A 10? An 8? If you’ve ever stood in your living room, holding a measuring tape to your foot while staring at a confusing EU-to-US conversion chart, you’re not alone. The question “What size is size 40 in women’s shoes?” is one of the most common puzzles in online shopping. It’s a deceptively simple question with a surprisingly nuanced answer, and getting it wrong can mean the difference between a new favorite pair of heels and a painful blister factory.

Let’s start by cutting through the noise. In the most straightforward terms, a women’s EU size 40 typically converts to a US size 9. But here’s the catch: that conversion is a starting point, not a universal truth. The reason for this confusion is that shoe sizing is not a standardized science like measuring a cup of flour. It’s more like a recommendation from a friend who has very different taste. Different countries use different systems—European (EU), United States (US), United Kingdom (UK), and Japanese (JP)—and even within those systems, brands and styles can vary wildly. So while you can confidently say that EU 40 is roughly a US 9, the real answer depends on the shape of your foot, the type of shoe, and the brand’s specific “last” (the mold used to shape the shoe).

Understanding the Sizing Systems: A Quick Translation Guide

To truly understand size 40, you need to know how it fits into the global sizing puzzle. The European system, which is used by most brands outside of the US and UK, is based on the length of the foot in centimeters. An EU 40 corresponds to a foot length of about 25.5 to 26 centimeters (roughly 10.04 to 10.24 inches). This is a more logical system than the US one, which is based on barleycorns (yes, an actual historical unit of measurement). Here’s a quick breakdown of how EU 40 translates to other common systems:

  • US Women’s: 9 (most common), but sometimes 9.5 in certain brands
  • UK Women’s: 6.5
  • Japanese (JP): 25.5 to 26.0 cm
  • Mexican (MX): 6.5
  • Australian (AU): 8.5

Notice the word “most common.” That’s your first clue that this isn’t a fixed rule. For example, if you usually wear a US 9.5 in running shoes, you might find that an EU 40 feels a bit snug. Conversely, if you wear a US 8.5 in heels, an EU 40 might feel a little loose. The key is to think of these conversions as a rough map, not a GPS destination.

Why Size 40 Can Fit Differently Across Brands and Styles

Here’s where things get interesting. Shoe sizing isn’t just about length; it’s about width, arch support, toe box shape, and heel drop. A size 40 in a pair of sleek stilettos from a high-end European brand will feel very different from a size 40 in a pair of chunky sneakers from an American athletic brand. European brands, especially those from Italy and France, tend to run narrower and sometimes shorter than their US counterparts. This is because European shoe lasts are often designed for a more pointed, elegant silhouette, while US lasts are frequently built for a more relaxed, roomy fit. So, if you have a wider foot or a high instep, a US 9 might be your standard, but an EU 40 could feel like a vise.

Let’s look at a real-world example. Say you’re eyeing a pair of classic leather loafers from a Spanish brand. You’re a solid US 9 in most brands, so you confidently order the EU 40. When they arrive, they’re tight across the top of your foot and your toes feel cramped. That’s because the brand’s last is narrower than average. On the flip side, if you order a size 40 in a pair of chunky platform sneakers from a trendy US-based brand, they might feel loose and sloppy. The lesson here is that you need to consider the shoe’s intended use and the brand’s reputation. Athletic shoes often have more generous sizing, while dress shoes and boots tend to be more true to the European standard.

How to Measure Your Foot for the Perfect Size 40 Fit

Don’t rely on memory or a one-size-fits-all chart. The most reliable way to know if an EU 40 is right for you is to measure your foot at home. It’s simple, takes five minutes, and will save you from countless returns. Here’s how to do it like a pro:

  • Gather your tools: A piece of paper, a pen or pencil, a ruler or measuring tape, and a wall.
  • Trace your foot: Place the paper against a wall, stand on it with your heel touching the wall, and trace the outline of your foot. Make sure you’re standing naturally, not leaning forward.
  • Measure the length: Use the ruler to measure from the very tip of your longest toe (which might not be your big toe) to the back of your heel. Do this for both feet, as one foot is often slightly larger.
  • Measure the width: Measure the widest part of your foot, usually across the ball of your foot. This is crucial for determining if you need a wide or narrow fit.
  • Compare to the brand’s chart: Once you have your measurements in centimeters, look for the brand’s specific size chart. If your foot measures 25.5 cm, an EU 40 should be a good length match. If it measures 26 cm, you’re right at the edge, and you might want to size up to a 41 if the shoe runs small.

A professional tip: always measure your feet at the end of the day. Your feet swell as you walk around, so a shoe that fits in the morning might feel tight by evening. Also, wear the type of socks or stockings you plan to wear with the shoes. If you’re buying winter boots, measure with thick socks. If you’re buying sandals, measure with bare feet.

Practical Tips for Buying Size 40 Shoes Online

Now that you understand the theory, let’s talk about the practice. Shopping for size 40 shoes online doesn’t have to be a gamble. Here are some actionable strategies to increase your chances of a perfect fit:

  • Read the reviews, especially from people who mention their foot shape. Look for comments like “I’m a standard US 9, and these fit perfectly” or “I have narrow feet, and these were too wide.” This is your best insight into how the shoe actually fits in real life.
  • Check the brand’s sizing guide on their website. Many brands provide specific advice, like “This style runs half a size small” or “We recommend ordering a size up for wide feet.” Ignore generic charts and use the brand’s own data.
  • Know your foot shape. Are you a narrow, standard, or wide width? Do you have a high arch or flat feet? Knowing this helps you predict how a shoe will feel. For example, if you have wide feet, you might need to size up to an EU 41 in a narrow European brand.
  • Consider the material. Leather, suede, and canvas will stretch over time. Synthetic materials like plastic or patent leather are less forgiving. If you’re buying leather shoes, a snug fit might be okay because they’ll mold to your foot. If they’re synthetic, you need them to be comfortable from the first wear.
  • Use the “two-finger rule.” When trying on shoes, there should be about a thumb’s width (roughly 1 cm) of space between your longest toe and the end of the shoe. This allows for foot swelling and prevents your toes from hitting the front of the shoe when walking.
  • Don’t ignore half sizes. If you’re between sizes, don’t force it. An EU 40.5 or 41 might be a better fit if you’re on the edge. Some brands offer half sizes in EU, so look for them if you need more precision.

Final Recommendations for Your Size 40 Shopping Journey

So, what size is size 40 in women’s shoes? In the simplest terms, it’s a US 9. But the real answer is more personal. It’s a conversation between your foot and the shoe. My advice is to start with the conversion, but always verify with measurements and brand-specific information. If you’re buying from a new brand, order two sizes if you’re unsure—the 40 and the 41—and return the one that doesn’t fit. Many online retailers offer free returns, so take advantage of that safety net. And never, ever buy a pair of shoes that are too tight hoping they’ll stretch. They might, but they might also cause pain and damage to your feet. Your feet are your foundation; treat them well. With a little patience and the right approach, you can confidently navigate the world of EU 40 shoes and find the perfect pair every time.