Prom dress sizing catches almost everyone out the first time. You order your usual size and it will not zip. That is not you. Formal gowns are cut to their own size charts, and those charts do not match the ones behind your jeans. They do not quite match each other, either.
This page explains why, shows you how to measure yourself properly, and walks through what to do when you fall between sizes or have to order without trying anything on.
Why formal sizes run small
Evening gowns are cut closer to the body than everyday clothes, with structure built in. Sherri Hill's size guide puts it directly. Formal wear is sized differently from everyday clothing, and most people wear a Sherri Hill size one to two sizes larger than their jeans or streetwear size. The company adds that this is normal and "isn't a reflection of your everyday size."
Fabric matters too. The same guide notes that stretch fabrics have some give, while structured, beaded and sequined fabrics have very little. A sequined sheath in your street size has nowhere to go.
The fix is simple. Ignore the number on your favorite jeans. Go by your measurements and the chart for the label you are buying.
How sizes differ from label to label
Each designer publishes its own chart, and the same size number can mean slightly different measurements. Here is what four widely carried labels list for three sizes, in inches of body measurement (bust, waist, hips), from their own websites in October 2026.
| Label | Size 8 | Size 16 | Size 20 |
|---|---|---|---|
| Sherri Hill | 36.5, 28, 39.5 | 42, 33.5, 45 | 46, 37.5, 49 |
| Jovani | 36, 29.5, 39.5 | 42, 35, 46 | 46, 39.5, 50 |
| Johnathan Kayne | 36, 28, 40 | 42, 34.5, 46 | 46, 39.5, 50 |
| Ashley Lauren | 36.5, 28, 39.5 | 42, 34, 46 | 46, 40, 50 |
At size 8 the four charts sit within about an inch and a half of each other. By size 20 the gap is wider. Sherri Hill lists a 37.5 inch waist, while the other three list 39.5 or 40. Two inches at the waist is the difference between a zipper that closes and one that does not.
The ranges differ as well. Sherri Hill runs from 000 to 26, Jovani from 00 to 24, Johnathan Kayne from 000 to 24, and Ashley Lauren's standard chart from 00 to 30. Our page on plus size prom dresses goes further into extended sizes.
How to measure yourself
You need a soft tape measure, a friend, and the bra or shapewear you plan to wear under the dress. Sherri Hill's guide recommends wearing those undergarments when you measure, and keeping the tape level and snug but not tight. Measure over them, not over clothes.
- Bust. Wrap the tape around the fullest part of your bust, under your arms and level across your back. Johnathan Kayne's chart points out that this is not your bra size. It is a body measurement.
- Waist. Measure around your natural waist, the narrowest part of your torso. To find it, Johnathan Kayne suggests standing straight and bending to one side. The crease that forms is your natural waist. Sherri Hill places it about one to two inches above the belly button.
- Hips. Stand with your feet together and measure around the fullest part of your hips and seat. Johnathan Kayne says this is usually about nine inches below the natural waist, and Jovani's guide says eight. Use whatever point is fullest on you.
- Hollow to hem. Measure from the hollow at the base of your neck, between the collarbones, straight down to where the hem should land. For a long gown, do it in the shoes you will wear. Stand tall and look straight ahead. Johnathan Kayne warns that looking down while someone measures makes the number too short.
Write the numbers down with the date. If months pass before you order, measure again.
Length usually is not chosen by size. Johnathan Kayne's chart lists the same 62 inch hollow-to-hem length for its long dresses in every women's size, which tells you the hem is expected to be adjusted. If you are tall, check that number against yours. Sherri Hill notes there is usually little extra fabric in a hem to let down, so adding length is hard.
Picking your size from the chart
Compare each measurement to the label's chart. Then choose the size that fits your largest measurement. That is Sherri Hill's rule of thumb, and it is the standard approach for a simple reason. A dress can be taken in, but it can barely be let out.
Here is how it works on Sherri Hill's chart. Say your bust is 37.5, your waist is 30 and your hips are 42. Your bust matches a 10. Your waist and hips match a 12. Order the 12, then have the bodice taken in so it sits close at the bust.
The shape of the dress changes which measurement matters most. Sherri Hill's fit guide says a mermaid gown has to fit bust, waist and hips together. An A-line or ball gown skirt leaves room through the hips, so focus on bust and waist. An empire waist depends mostly on the bust. If only your hips push you up a size, and the dress has a full skirt, ask the shop whether the smaller size could work.
When you fall between two sizes
Size up. Sherri Hill's FAQ says that when measurements fall across two sizes, most people choose the larger size and have the gown taken in. Its guide explains why. Seam allowances on gowns are small, so letting a dress out is limited, while taking it in at the side seams is routine.
Two cautions. Beaded areas take longer to alter, because beads have to be removed and re-sewn around any new seam. And a dress that is several sizes too big is a much bigger job than one that is a little loose. Sizing up means one size, not three.
Almost every gown gets some alteration anyway. Sherri Hill's guide says nearly every formal gown is finished by a seamstress, since off-the-rack dresses are cut to a standard chart, not to an individual body. Plan the time and the cost from the start.
Ordering online or by special order
When you cannot try a style on first, a few habits prevent most sizing mistakes.
- Use the chart for that label, and for that style when one is given. Jovani's size page says its guides are specific to the product.
- Give the seller your measurements, not just a size. Prom Headquarters, a South Carolina shop listed here, asks for measurements on every order, has its team confirm the size, and notes that every designer has a different size chart.
- Read the return policy before you pay. The same shop says orders placed through its vendors are special orders that cannot be exchanged or returned, which is common in formalwear.
- If you can, try the label on in person first. Sherri Hill calls trying a style on at an authorized retailer the surest way to know your size, and you can carry that knowledge to anything else from the same line.
- Be wary of copies. Sherri Hill warns that a "dupe" is not cut to its size chart, so a look-alike may not fit like the real dress. Our page on prom dress scams explains how to spot them.
Our comparison of buying online and in store covers the trade-offs in more detail. When you are ready to try things on, you can browse prom shops by state.