Quick Answer: Evening, or after activity. Feet expand measurably over the course of a day — commonly by a quarter to a half size, and more in heat or after standing — so a shoe fitted at 9am can be genuinely tight by 5pm. The practical rule is to try on at the end of the day, in the socks you will actually wear, standing up, with about a thumb’s width between your longest toe and the front of the shoe.

You bought them in the morning, they felt perfect, and by the evening of the first proper wear your toes are complaining.

This is not the shoe breaking in wrong. It is the difference between the foot you shopped with and the foot you actually have most of the time. The size of that difference surprises people, and the second factor below — which has nothing to do with time of day — is one almost nobody accounts for.

How much do feet actually change over a day?

Enough to change the size you should buy.

Over a normal day of standing and walking, fluid pools in the lower legs and feet, and the foot’s volume increases. Measured changes commonly land in the range of a quarter to a half shoe size in length equivalent, with the larger effect in width and overall volume.

Several things push it further. Heat expands soft tissue and increases fluid retention, which is why summer feet run larger than winter feet. Long periods standing or sitting still — a flight, a desk day, a shift on your feet — increase it. So does exercise: after a run, feet are typically at their largest of the day.

The practical translation: a shoe fitted first thing on a cool morning is being fitted to your smallest foot of the year. It will fit for about two hours a day.

Try on at the end of the day, or right after a walk or workout. If you are buying running shoes specifically, trying them on after a run is the closest thing to a real test you can get in a shop.

What else changes fit besides the time of day?

Three things, and the first is the one people skip.

Your socks. A cushioned running sock versus a thin no-show sock is easily a third of a size difference in effective volume. Bring the socks you will actually wear with that shoe. Trying on running shoes in dress socks is close to useless.

Your feet have changed since the last time you measured. Foot length and especially width increase over adulthood as the ligaments in the arch gradually relax. Many people are wearing the size they wore at 22 and have needed a half size more for years. Pregnancy, significant weight change, and age all move this. Get measured on a Brannock device once a year — it takes two minutes and is free in most shoe shops.

Your two feet are different sizes. Almost everyone has a size difference between left and right, often a third to a half size. Fit to the larger foot, always, and use lacing adjustments or a thicker sock on the smaller side. Fitting to the smaller foot guarantees one foot is permanently cramped.

Add to that: different brands and even different models within a brand use different lasts, so a 8.5 in one shoe is genuinely not the same as an 8.5 in another. The number on the box is a starting point, not an answer.

How should you actually test the fit in the shop?

Stand up. Almost every fit mistake happens because people judge a shoe while sitting.

Thumb’s width at the front. Standing, with weight on the foot, there should be roughly 3/8 to 1/2 inch — about a thumb’s width — between your longest toe and the end of the shoe. Note that your longest toe is not always your big toe.

Width at the ball, not the toes. The widest part of your foot should sit at the widest part of the shoe. If the shoe is bulging at the sides or you can see your foot’s outline pressing through the upper, it is too narrow — and going up a length size to fix a width problem creates a shoe that is too long and slides.

Heel lock. Walk a few steps. Minor heel movement in a new shoe is normal and usually settles; obvious lifting means the heel counter does not match your foot shape. Try a runner’s loop lacing through the top eyelet before rejecting the shoe.

Walk on a hard surface. Shop carpet flatters everything. Find tile if there is any.

Five minutes, both shoes on. Anything that feels like a pressure point in five minutes will be a blister in an hour. New shoes should feel good immediately. The idea that they need breaking in applies to leather boots, not to modern athletic footwear — a running shoe that hurts in the shop will hurt on the road.

5 Mistakes People Make Buying Sneakers

1. Shopping first thing on a Saturday morning. The single most common cause of shoes that fit in the shop and pinch in real life. Go after 4pm, or after a walk.

2. Buying by the number instead of the fit. Sizing varies across brands by up to a full size. Try the size you expect, then try a half size up and down, and buy the one that fits.

3. Fitting to the smaller foot. Guarantees discomfort on one side for the life of the shoe. Fit the bigger one.

4. Assuming tight now means fitted later. Athletic uppers stretch very little. Mesh gives slightly; the length and the last do not change. What you feel is what you get.

5. Ignoring width entirely. Most people only think in lengths, and a substantial minority of feet need a wide fitting. If you consistently get little-toe irritation, forefoot pressure, or wear through the side of the upper, width is the problem and no length change will fix it.

FAQ: Sneaker Fit Questions, Answered

Does this apply to buying online?

Even more so. Order in the evening if you want, but the real move is to try them on at the end of the day when they arrive, on carpet so they stay returnable, wearing the right socks. Check the brand’s own size chart against a measured foot length in centimeters rather than trusting your usual size.

Should running shoes be a half size larger than casual shoes?

Usually yes. Feet swell during a run and toes need room on the downhill, so most runners land a half size above their casual size. That is a starting assumption to test, not a rule — some brands already build extra length in.

My feet feel different sizes at different times of year. Is that normal?

Completely. Summer feet run larger from heat and fluid retention; feet contract in cooler weather. As the weather turns cooler, shoes bought in July often feel slightly roomy — which is usually solved with a thicker sock rather than a new size.

TL;DR:

• Feet expand a quarter to a half size over a day, and more in heat or after exercise — shop in the evening.
• Bring the socks you will actually wear, and get measured properly once a year.
• Fit to your larger foot, leave a thumb’s width at the front, and check width at the ball rather than the toes.
• Athletic shoes do not break in. If it hurts in the shop, it will hurt on the road.

One afternoon fitting saves a year of a shoe you never quite reach for.