You've got the mood board. You've got the denim swatches. You've even got a brand name that sounds expensive and vaguely European.
Now comes the part that's super critical but somehow gets very little thought: your base size.
If you think you can just start with a UK 10 because that's what you wear, try again.
Picking your base size based on what fits you is like buying a house in a terrible location just because it's pink.
(I did this once. Don't ask.)
What is a base size?
Your base size is the starting point for developing your fit and grading your size range.
It's the size your first pattern or block is usually developed around, the size you fit on your fit model, and the size all the others grow or shrink away from using your grade rules.
Which means it matters quite a lot.
Let's say you always buy a size 8, so you decide you'll develop your jeans in a size 8. Except you're 4'11", you've got proportionally shorter legs, and you regularly struggle to find jeans where the knee actually hits your knee.
If you develop the whole fit around your body and call that your standard size 8, you've accidentally built your brand around you rather than your actual customer.
Your body isn't wrong. It's just one body. And your base size needs to represent something much bigger than the person who happened to start the company.
Step 1: Who is actually buying these jeans?
"Everyone" is not a target demographic. And "women aged 18–45" isn't massively helpful either.
What does your customer actually want from her jeans?
Rigid, vintage-inspired denim she expects to feel snug when she first puts it on?
A super-stretch skinny that feels comfortable straight away?
A low-rise baggy?
A high-rise straight?
Something that skims her body, or something that looks like she's stolen it from a 6'4" skateboarder called Josh?
These are very different products. They don't just need different styling. They might need different blocks, different fit considerations and potentially different approaches to sizing.
Hopefully, if you've got this far, you already have a fairly good idea of who your customer is. Now we need to turn "cool girl who likes oversized jeans" into something technically useful.
Step 2: Benchmarking
You know who your customer is, so you should already know where she's spending her money.
Start by digitally stalking those brands. Look at the jeans on the models.
Where does the waistband sit?
How fitted are they through the seat and thigh?
Where does the knee break?
How wide is the hem, and how much stacking is there at the ankle?
Are they fitted at the waist and relaxed through the hip, or is the whole thing deliberately oversized?
Then look at their size guides. What body measurements are they designing around? What size range do they offer? What size are the models wearing?
If their fit matches your vision, it's time to go shopping.
Buy a few pairs, lay them flat and get the tape measure out. Measure the waist, hip, front and back rise, thigh, knee, hem and inside leg. Look at the pocket size and position, the yoke and the waistband shape. Then turn them inside out and have a nose at the construction while you're there.
Look at the denim too. Is it rigid or stretch? Lightweight or heavy? Soft and drapey? A measurement on a rigid 14oz jean doesn't necessarily feel the same on the body as the same measurement in a soft stretch denim.
This is benchmarking. You're not copying somebody else's jeans. You're understanding the market your customer already shops in, and working out what "a size 12" or "a relaxed straight" actually means in that world.
If your customer lives for the oversized silhouette of Brand X and you give her a jean that fits like she's been vacuum packed into it, she isn't going to evolve with you. She's going to send it back.
Step 3: Finding your fit model
A fit model is not a runway model. It doesn't matter whether they can walk in 6-inch heels or look moody in a photoshoot.
I don't care about their look. I care whether their waist, hip and thigh measurements are where I need them to be, and whether their proportions make sense for the customer we're trying to fit.
Professional fit models are worth their weight in gold. There are agencies specifically for this, and experienced fit models can often tell you exactly where a garment feels uncomfortable, frequently in the same technical language as your factory or garment tech.
Your fit model needs to be:
Consistent. Professional fit models keep their measurements within an agreed range and know when something has changed. You don't want your fit model going on a huge fitness kick halfway through six rounds of sampling, leaving you wondering why every waistband has suddenly become enormous.
Vocal. They need to tell you how the jeans actually feel. "They're tight" isn't that helpful. Where are they tight? Does the waistband dig in when they sit? Is there restriction across the front hip? Can they get their hands in the pockets? Does the back rise pull down when they move?
Proportional. This is the really important bit for jeans.
Two people can have exactly the same waist and hip measurements and carry that volume completely differently. One might have a fuller bum and a smaller waist. Another might carry more through the hips. Their thighs might be completely different, and so might their rise proportions. The same pair of jeans can look and feel very different on those two bodies.
You're never going to find a magical human who represents every customer in your size range. They don't exist. You're looking for someone whose measurements and proportions give you a reliable representation of the customer you're designing for.
And remember this is for fit. If your target customer is Julie, 51, then Lily, 21, might not be the best model, as proportions, posture and body shape change so much as we age.
Look beyond the numbers on the agency card
When you're going through agency cards, don't just find someone whose waist and hip technically fall inside your spec and call it a day. Look at their general proportions.
What's their waist-to-hip relationship?
How are their thighs proportioned?
What's their posture like?
How is their volume distributed?
Are their legs particularly long or short compared to your intended fit?
It can feel a bit clinical talking about humans as sets of measurements, but a professional fit model is basically a high-precision member of your engineering team. Most experienced models know exactly which parts of their body sit slightly off the standard they're working to, and you can account for those quirks.
If their legs are slightly shorter than your intended customer's, you don't shorten every pair of jeans in your brand to fit them. You make sure the length is deliberately long on them during the fitting.
Ask about their experience and availability, too. Are they in it for the long haul? Once you've found a brilliant fit model who understands your product and your brand, you want to keep them. That consistency becomes incredibly valuable as your range grows.
"That's okay, I'll be the fit model!"
Oh good.
Except... if you're the fit model, you're biased. You'll subconsciously wiggle and shift the garment until it looks good, because it's your baby.
I've received countless fit photos from brand owners wearing a sample in an Instagram pose that makes it completely impossible to see the actual fit. One leg forward, hip popped, back arched, hand in pocket.
Lovely photo. Absolutely useless to me.
And even if you promise to stand like you're having your passport photo taken, you still might not be representative of your customer.
If, by complete coincidence, your measurements and proportions genuinely make you a suitable fit model, fine. But "I'm conveniently available and would quite like the samples in my size" isn't a fit-model selection process.
Get a professional if you can. Or at the very least, find a consistent human with the right measurements and proportions who doesn't mind you staring at their crotch while holding a tape measure.
Step 4: Your base size needs to make sense within your range
Traditionally, brands develop around a size somewhere near the middle of their core size range, and there's a sensible reason for that.
Grading is the technical process of taking your base pattern and creating the other sizes around it. The further you move away from the size you've physically developed and fitted, the more chance there is of the proportions doing something you didn't intend.
So if your range is UK 6–16, developing around a 10 or 12 might make sense. But I wouldn't pick one just because somebody told you "all brands fit on a 10". I'd look at your actual size range, your customer, where most of your sales are likely to sit, and how far you're asking the grading to travel in each direction.
If you're selling a much broader range, one fit size might not be enough. You might develop your core size, grade up, and then physically fit another size further up the range to check what's actually happening.
You can make an Excel spreadsheet say whatever you like. Unfortunately, a human body may have other ideas.
Step 5: Don't confuse body measurements with jean measurements
This is where brands can get themselves into a right muddle.
Your body measurements describe the customer you're designing for. Your garment measurements describe the actual jeans. They are not the same thing.
If your target customer's waist measures 76cm, that doesn't mean the waistband on every pair of jeans you make should measure 76cm. How much ease you add or take away depends on the style, the denim, the stretch, the rise and how you want the jean to fit.
A rigid relaxed jean and a stretch skinny might be designed for exactly the same body measurements and have very different garment measurements.
That's why your core body sizing needs to stay consistent while your garment specs change with the design. Otherwise, every time you decide to make something oversized, you risk accidentally changing what your sizes mean.
Step 6: Document or die
Once you've settled on your base body measurements, write them down and save them in ten different places. Tattoo them on your arm if you have to.
These measurements become part of the technical foundation you use to develop your blocks and your size range.
Your blocks are your DNA fits, and a denim brand may eventually have more than one. Maybe you need a rigid block and a stretch block. Maybe your relaxed fits need a different starting point from your fitted jeans. That's fine. What matters is that there's a reason behind them.
You don't change your underlying body sizing on a whim because you saw a cool oversized fit on Pinterest. If you want a baggy jean, you change the garment. You don't redefine the human underneath it.
Consistency is what helps customers learn your brand. They might still pick different sizes depending on whether they're buying a fitted jean or deliberately sizing up in a baggy style, but there should be some logic behind it. They shouldn't need to consult an astrologer every time they add something to their basket.
"Can't I just let the factory decide?"
You can. And sometimes the factory's existing block or grade is a perfectly sensible starting point.
But I want you to know what you're agreeing to.
Factories work with lots of brands, often across different countries, customers, price points and fit expectations. Their "standard" block isn't automatically your customer's block. Their standard size chart isn't automatically your size chart. Their standard grade isn't automatically your grade.
So rather than asking "Do you have a standard size 10?", I want to know:
What body measurements is this block meant to fit?
What are the actual garment measurements?
What grade rules are being used?
What sort of customer and product has this block been used for before?
Then we can decide whether it's useful.
"The factory always uses this" is information. It isn't a fit strategy.
Your base size is the beginning, not the answer
Choosing a base size isn't about deciding whether your label starts at an 8, 10 or 12.
You're deciding the body you're designing around, the fit model you'll use, the blocks you'll develop and the point your whole size range grows from.
Get that foundation right and every new jean has somewhere sensible to start. Get it wrong and you can spend a fairly spectacular amount of money correcting the same fit problems over and over in slightly different trousers.
That's exactly the sort of technical Groundhog Day I'd rather help you avoid.
If you're developing a denim brand and you're not sure what your base size should be, whether your fit model is right, or whether the factory block you've been given makes any sense for your customer, that's exactly the kind of thing I can help you figure out.
Let's fix your fit.