Costume sizing is worse than clothing sizing, and the reasons are specific rather than accidental. Knowing them is most of the answer.
Costume sizes are not clothing sizes
A costume labelled M is not cut like a shirt labelled M. Costumes are made to a small number of size bands rather than a full size run, they are frequently made to Asian rather than European or North American measurements, and they are often cut generously in some dimensions and not at all in others — roomy in the body, tight in the shoulder or short in the sleeve.
The practical consequence: ignore the letter. It carries almost no information across manufacturers.
Use the measurement table, and measure yourself
Every serious listing has a table in centimetres or inches. That table is the product; the letter is a label. Take three measurements with a soft tape:
- Chest or bust — around the fullest part, tape level.
- Waist — at the natural waist, not at the belt line.
- Height — this one is routinely forgotten and is the most common cause of a costume that fits but sits wrong.
For children, add the shoulder-to-floor length. Children's costumes are sized by age far more often than by measurement, and a nine-year-old can be anywhere across three age bands.
When you are between two sizes
Size up, with one exception.
Costumes are worn over clothes. On a cold night in late October, that means a layer underneath — and the sizing table almost never accounts for it. A costume that fits exactly over a t-shirt does not fit over a jumper.
The exception is anything fitted that depends on the fit for its effect: a bodysuit, a printed skeleton suit, a catsuit. There, a size too large reads as wrinkles rather than anatomy, and you should take the smaller size and accept the cold.
Read what the reviews say about fit, not about quality
Reviews are unreliable about material and reliable about fit. "Runs small", "very short", "the sleeves are tight" — these are consistent observations across many buyers, and they are more useful than the table when the two disagree.
If several people say a piece runs small, believe them over the chart.
The parts that do not size
Masks, hats, capes and one-size accessories are genuinely one-size for most adults, which is why they are the safe choice when you are ordering late or buying for someone whose measurements you do not have.
Shoes are the opposite. Costume boots and shoe covers are sized crudely and fit badly. If an outfit needs particular footwear, plan to use your own.
Order early enough to be wrong once
All of the above reduces the chance of a mistake; none of it eliminates it. The only real protection is time. A costume ordered three weeks out can be exchanged. A costume ordered on 26 October is whatever size it turns out to be.
That is the whole argument for ordering early, and it is a stronger one than any sizing chart.
For children, fit is a safety question before it is a comfort one
Two failure modes account for most of it, and both are about length.
A costume that is too long trips. Hems, capes and trailing sleeves are the common culprits, and the hazard is worse on an evening spent walking between houses in the dark. The fix is unglamorous: measure shoulder to floor, and if the garment is long, hem it or pin it rather than hoping.
A mask that is too large rotates. The CPSC's advice has been consistent for decades — prefer make-up and a hat to a mask, precisely because a mask obscures vision, and the child wearing it is crossing roads. Where a mask is used, the eye openings should allow full peripheral vision and the fit should be secure enough that it does not slip.
The stakes are documented. A 2018 study in JAMA Pediatrics, drawing on forty-two years of US national traffic data, found pedestrian deaths 43 % higher on Halloween than on control evenings, with the relative risk for children aged four to eight ten times higher, concentrated between 5 and 8 p.m. Fit, visibility and a torch are not fussing.
Make-up: test it the day before
If you are going the make-up route — and for children it is the better route — the CPSC recommends a patch test: a small amount on the inner arm of the person who will wear it, a day ahead. Redness, swelling or a rash is a sign of a possible allergy. Finding out at seven on Halloween evening is the version to avoid.
Sources: US Consumer Product Safety Commission (costume, mask and make-up guidance); JAMA Pediatrics, 2018, "Pedestrian Fatalities Associated With Halloween in the United States".