The one tailoring tweak that fixes boxy knitwear and makes it flatter your shape

I used to avoid a lot of knitwear because of one persistent problem: it made me look wider and shapeless, even when the sweater was soft, cosy and otherwise perfect. Over the years I’ve tried everything — tucking, belting, layering — but the simplest, most transformative fix is a tiny tailoring tweak that you can do yourself or ask a tailor to perform. It keeps the sweater’s relaxed feel while creating subtle shaping that flatters your body instead of swallowing it.

The one tweak: an internal waist tape (a gentle, invisible cinch)

Instead of hacking away at seams or adding obvious darts, I sew a narrow piece of elastic or grosgrain ribbon along the inside of the sweater at the waist level, anchored into the side seams. The tape pulls the fabric in just enough to suggest a waistline without changing the garment’s intended relaxed silhouette. It’s invisible from the outside, reversible, quick, low-cost and works on almost any knit — chunky, fine-gauge, boxy, cropped or long.

I call it the internal waist tape, and it’s become my go-to solution for boxy jumpers and oversized knitwear that lack proportion. I first tried it on a chunky fisherman's knit I adored but would rarely wear because it looked like a cocoon. The tape made it feel intentional — still cosy but clearly designed to sit with more shape.

Why this works

It maintains the sweater’s drape and intended ease while creating definition where you want it.

Because the tape is anchored to the side seams and not stitched through the front, there’s no external puckering or visible alteration.

It’s reversible — if you change your mind, you can remove it and the jumper returns to its original shape.

It’s adaptable across sizes and body shapes: you can tighten more for pronounced hourglass shaping or keep it gentle for subtle suggestion.

What you’ll need

Narrow elastic (5–10mm) or thin grosgrain ribbon — elastic gives more pull; ribbon is gentler and better for delicate knits.

A hand-sewing needle and matching thread, or a sewing machine if the knit is sturdy and you’re confident.

Measuring tape, pins, small safety pins or wonder clips.

Optional: knit stay tape (fusible) for very delicate knits — but avoid high heat if the yarn is synthetic.

Step-by-step: DIY internal waist tape

Here’s the simple process I follow. It takes about 20–40 minutes depending on the sweater and how neat you want the finish.

Try the sweater on and mark where you want the waist to sit. For most people this is at the smallest part of the torso. For longer jumpers I sometimes place it slightly higher to create a cropped proportion.

Take the sweater off and lay it flat. Measure the circumference at the marking. Subtract 2–6cm (1–2.5 inches) for gentle shaping — less for a subtle effect, more for visible tailored shaping.

Cut the elastic/ribbon to this shortened measurement plus about 2cm (for seams).

Pin the ends of the tape to the inside of the side seams at the waist position. If you want the tape to be completely invisible, position it about 1–2cm behind the side seam edge so any slight bulk is hidden.

Secure the tape with small stitches into the seam allowance only. If the knit is very loose, catch under the fabric edge rather than through the front to avoid visible holes. For stretch fabrics, use small zigzag stitches by machine if you prefer.

Try it on, adjust the tension if needed, then finish stitching along the tape at the side seams so it doesn’t shift.

When to choose elastic vs ribbon

Elastic: Best for chunky knits, heavier fabrics or when you want a bit more shaping. Choose low-profile elastic to avoid bulk.

Grosgrain ribbon: Ideal for lightweight or loosely knit jumpers where elastic might distort the fabric. Ribbon gives a gentler hold and is less likely to stretch the yarn.

When to see a tailor

If the jumper has complicated construction (panelling, seams in unusual places) or you’re working with very expensive knitwear — think cashmere by brands like Everlane, Naadam, or smaller artisan labels — I recommend a professional. A good tailor will replicate the internal tape method cleanly and can match thread and technique to the knit. Ask them to anchor the tape to the seam allowance and not to create external stitching.

How this compares to other fixes

Method	Pros	Cons
Internal waist tape	Invisible, reversible, maintains drape, quick	Less dramatic shaping than full tailoring
Side-seam take-in	Permenant, can create more tailored silhouette	Irreversible, can alter drape, needs skill
Adding darts	Precise shaping	Visible exterior depending on knit, can distort pattern
Belting/tucking	No sewing, versatile	Can disrupt silhouette and bulk at waist

Styling notes and outfit ideas

Once the jumper has a subtle waist, it becomes much easier to style. I love pairing a tape-altered chunky knit with:

High-waisted tailored trousers and loafers for a polished daytime look.

A midi skirt and ankle boots for a balanced feminine silhouette.

Layered over a shirt with the shirt tails peeking for structure and interest.

For boxy cropped knits, shortening the tape a little more creates a flattering cropped proportion that pairs perfectly with high-rise jeans. For oversized boyfriend knits I keep the tension gentle to preserve the cosy feel but add a suggestion of waist to avoid a shapeless block.

What I tested and why it’s reliable

I’ve tried this tweak on more than twenty jumpers across different weights, yarns and brands — from Zara’s chunky knits to more delicate cashmere from smaller UK brands. The results were consistently flattering: the sweater looked less like a tent and more like an intentional piece of our outfit. The key is subtlety: the tape should guide the fabric, not force it into an unnatural shape.

If you’re new to altering clothes, start with an inexpensive or vintage sweater to practice. If you’re working on cashmere or a sentimental piece, practice the placement first with pins and test the tension before stitching. And if in doubt, a local tailor will do this quickly and affordably — it’s one of those small investments that makes an outfit feel curated rather than accidental.

If you want, I can walk you through the process for a particular jumper you own — tell me the fabric and style and I’ll suggest the ideal placement and tension.