Quick Answer: Feet swell over the course of a day — commonly 4 to 8 percent in volume, and more in heat, after standing, or after flying. That is often close to half a shoe size. Shop for shoes in the late afternoon or evening, wear the socks or tights you will actually use, and test with the larger foot, because most people have a measurable difference between the two.

You tried them on in the morning. They were perfect — snug in the right way, no rubbing, a thumbnail of room at the toe.

By seven that evening, the same pair felt like they had shrunk. The toe box was tight, the strap was cutting, and you were walking differently to avoid the pressure.

The shoes did not change. Your feet did, and they do it every single day.

How much do feet actually change during a day?

More than shoe sizing has any way of accommodating.

Volume increases by roughly 4 to 8 percent over a normal day. Gravity pulls fluid into the lower limbs while you are upright, and the tissue of the foot holds it. That volume change is roughly equivalent to half a size, sometimes a full size in people prone to swelling.

Heat multiplies it. Warm weather causes vasodilation, which increases blood flow to the extremities. The same shoes will feel a half-size tighter on a 28C day than a 12C one, which is why summer sandals bought in spring so often disappoint.

Standing and walking multiply it further. A day on your feet produces significantly more swelling than a day at a desk. If you are buying shoes for a wedding, a conference or a day of walking, the relevant fit is the end-of-day fit.

Flying is the extreme case. Cabin pressure and prolonged sitting cause noticeable swelling in most people, which is why shoes that fit on departure can be genuinely hard to put on at arrival.

Why do the two feet not match?

Because almost nobody has symmetrical feet, and shoes are sold in pairs.

A difference of a half size between feet is common — one large-scale foot survey put meaningful asymmetry in the majority of adults. Yet shoes are sold as a matched pair in one size.

Always fit the larger foot. The smaller foot can be adjusted with an insole, a heel grip or a thicker sock. The larger foot cannot be made smaller, and a shoe that is tight on the larger foot will be the one that causes the blister, the bunion pressure and the numb toe.

Measure both, standing, at the end of the day. Sitting understates length because the foot lengthens under load — often by 3 to 5mm. If you have ever been measured sitting down and then found the shoes tight when walking, this is why.

Your size is not fixed for life. Feet lengthen and widen with age as the arch settles, and pregnancy commonly produces a permanent half-size increase. A significant number of people are wearing the size they were measured at twenty, which is the quiet cause of a lot of foot pain.

What should you actually check in the shop?

Five things, in order of how often they get missed.

Width before length. Most fit problems are width problems misdiagnosed as length problems, and sizing up to solve a width issue gives you a shoe that is too long and still tight. If the shoe is tight across the ball of the foot, you need a wider shoe, not a longer one.

A thumb’s width at the toe, standing. Around 1 to 1.5cm between the longest toe and the end. Note that the longest toe is not the big toe for everyone.

Heel slip of about 3 to 5mm is normal in a new flat shoe. A completely locked heel in a leather shoe on day one usually means it will be tight once your foot swells. In a heel, though, slip should be minimal, because the foot is being pushed forward rather than held.

Walk on a hard floor, not carpet. Shop carpet disguises almost everything — cushioning, sole flex and heel slip all read differently on it. Find the hard floor near the door and walk there.

Wear the right hosiery. Trying boots on with bare feet or thin trainer socks and then wearing them with thick socks is a guaranteed problem. Bring what you will wear.

5 Mistakes Everyone Makes Buying Shoes

1. Shopping in the morning. The most common and the most consequential. Go after 4pm, or after a walk, and you are fitting the foot you will actually have for most of the shoe’s life.

2. Trusting the size on the box. Sizing varies substantially between brands and between styles within a brand. Your size is a starting point for which box to open, not a fit.

3. Assuming leather will stretch enough. Leather stretches in width by a small amount with wear — a few millimetres — and essentially not at all in length. If it is short, it will always be short. Synthetic uppers stretch even less.

4. Ignoring the arch and the ball position. The widest part of the shoe should align with the widest part of your foot. If the shoe’s flex point does not sit where your foot bends, it will crease wrongly and rub, and no break-in period fixes it. This is the third mistake’s more expensive cousin, and it is the one that produces shoes you own for years and never wear.

5. Buying for the event rather than the day. A shoe bought for four hours of standing needs to fit at hour four, not hour zero. Try them on at the end of the day and then walk in them for longer than feels necessary in the shop.

FAQ: Shoe Fit Questions, Answered

Should I size up so they are comfortable in the evening?

Not usually — a shoe that is too long causes its own problems, including heel slip, toe sliding on descents and premature creasing. Fit correctly at the end of the day instead, and use a thin insole if they feel roomy in the morning.

How long should breaking in take?

For a well-fitting leather shoe, a few wears of two to three hours. If a shoe is still causing pain after a week of that, it does not fit — break-in softens leather and moulds the footbed, it does not change dimensions. Persisting past that point is how people end up with permanent problems.

What about buying shoes online?

Order in the evening, try them on in the evening, and walk on carpet only so they stay returnable. Measure both feet standing and check the brand’s own size chart rather than assuming your usual size — the variance between brands is larger than the difference between adjacent sizes.

TL;DR:

• Feet swell 4 to 8 percent during a normal day — often close to half a size, and more in heat or after standing.
• Shop in the late afternoon, wear the socks you will actually use, and fit the larger of your two feet.
• Width first, length second. Most tightness is a width problem that sizing up will not solve.
• Leather stretches slightly in width and not at all in length — break-in softens, it does not resize.

The reason this matters more than it sounds is that badly fitting shoes are the item people are most likely to keep and least likely to wear, so the money is lost twice over. Moving your shoe shopping to after work is free, takes no additional effort, and probably changes what you buy — which makes it the highest-return habit change available in a wardrobe.