Theory Size Guide
Theory is a New York-based luxury brand renowned for precision-cut tailoring, elevated basics, and refined silhouettes. Specialising in modern minimalism with impeccable craftsmanship, Theory offers sophisticated wardrobe essentials for the contemporary professional. The brand is celebrated for clean lines, stretch-infused fabrics, and a commitment to timeless style over trends.
Theory predominantly serves those seeking elevated workwear, travel-ready pieces, and luxury basics. The brand's aesthetic emphasises understated elegance with attention to detail in every stitch. Theory uses US sizing, though pieces are available at UK retailers including Selfridges and Net-a-Porter.
Does Theory Run Big or Small?
Theory runs true to size in general, but with important fit characteristics across different product categories:
Women's Dresses: Most dresses are structured or column-shaped with minimal stretch in crepe or silk styles. Fit tip: True to size — size up if you want room in the bust or hips.
Women's Trousers & Skirts: Often mid- or high-rise, tapered or straight-leg with fabrics like Precision Ponte or Good Wool featuring mild stretch. Fit tip: True to size at the waist, but expect a snug fit on curvier hips.
Women's Blazers & Jackets: Designed for clean layering with fitted sleeves that may feel narrow across the back if you have broad shoulders. Fit tip: Size up for bust and shoulder room, or if layering with knits underneath.
Women's Tops & Knitwear: Slim through the body, often cropped or semi-fitted with jersey and rib styles offering some give. Fit tip: True to size, but size up if you're between sizes or prefer a more relaxed drape.
How to Measure
Accurate measurements are essential for selecting the right Theory size. Follow these steps to ensure the best fit:
- Prepare Your Supplies: You'll need a soft measuring tape, comfortable clothing you can move in, and ideally a mirror or someone to assist. Measure in the afternoon or evening when your body is at its largest — feet and limbs naturally swell slightly throughout the day.
- Measure Your Bust: Wrap the measuring tape snugly (but not tightly) around the fullest part of your chest. Keep the tape parallel to the ground and parallel to your shoulders. Record this measurement in both centimetres and inches.
- Measure Your Waist: Find your natural waist by bending to one side — your waist is where your body naturally creases. Measure at this point with the tape snug but not tight. This is critical for trouser and dress fit.
- Measure Your Hips: Stand with your feet hip-width apart. Measure around the fullest part of your hips and bottom, keeping the tape parallel to the ground. For Theory's tailored fit, hip measurement is particularly important.
- Measure Your Inseam (for Trousers): Wear the shoes you'll pair with the trousers. Measure from your inner thigh to your ankle, or measure from your crotch to the floor and subtract approximately 1 inch (2.5 cm). Theory trousers often come cropped or full length.
- Record All Measurements: Write down all measurements immediately. Note whether you're measuring yourself or having someone help — assisted measurements are typically more accurate. Keep these measurements handy when shopping online.
Women's Clothing Size Chart
Theory women's sizes range from US 00–14 (UK 4–18). The brand uses US sizing throughout, though UK conversion is provided below. All measurements are approximate and can vary by specific style and fabric type.
| Theory (US) | UK | US | EU | Bust (inches) | Waist (inches) | Hips (inches) |
|---|---|---|---|---|---|---|
| 00 | 4 | 0 | 32 | 31–32 | 23–24 | 33–34 |
| 0 | 6 | 2 | 34 | 32–33 | 24–25 | 34–35 |
| 2 | 8 | 4 | 36 | 33–34 | 25–26 | 35–36 |
| 4 | 10 | 6 | 38 | 34–35 | 26–27 | 36–37 |
| 6 | 12 | 8 | 40 | 35–37 | 27–29 | 37–39 |
| 8 | 14 | 10 | 42 | 37–39 | 29–31 | 39–41 |
| 10 | 16 | 12 | 44 | 39–41 | 31–33 | 41–43 |
| 12 | 18 | 14 | 46 | 41–43 | 33–35 | 43–45 |
| 14 | 20 | 16 | 48 | 43–45 | 35–37 | 45–47 |
Men's Clothing Size Chart
Theory men's sizing typically ranges from XS to XXL, with US labelling throughout. Men's pieces emphasise clean tailoring and refined fits suitable for both professional and casual settings.
| Size | UK | US | EU | Chest (inches) | Waist (inches) | Shoulder Width (inches) |
|---|---|---|---|---|---|---|
| XS | 34 | 34 | 44 | 35–36 | 29–30 | 16–16.5 |
| S | 36 | 36 | 46 | 36–38 | 30–31 | 16.5–17 |
| M | 38 | 38 | 48 | 38–40 | 31–32 | 17–17.5 |
| L | 40 | 40 | 50 | 40–42 | 32–33 | 17.5–18 |
| XL | 42 | 42 | 52 | 42–44 | 33–34 | 18–18.5 |
| XXL | 44 | 44 | 54 | 44–46 | 34–36 | 18.5–19 |
Theory Footwear Size Conversion
Theory offers a curated selection of footwear designed to complement its minimalist aesthetic. Shoes typically run true to size. Use this conversion chart to find your size across UK, US, and European sizing systems.
| UK | US Men's | US Women's | EU | Foot Length (cm) |
|---|---|---|---|---|
| 3 | — | 5 | 36 | 22.5 |
| 4 | — | 6 | 37 | 23.5 |
| 5 | — | 7 | 38 | 24.0 |
| 6 | 7 | 8 | 39 | 24.5 |
| 7 | 8 | 9 | 41 | 25.5 |
| 8 | 9 | 10 | 42 | 26.5 |
| 9 | 10 | 11 | 43 | 27.0 |
| 10 | 11 | 12 | 44 | 28.0 |
| 11 | 12 | 13 | 45 | 28.5 |
| 12 | 13 | 14 | 46 | 29.5 |
Product-Specific Guidance
Women's Dresses & Separates
Theory dresses are engineered for a sleek, column silhouette emphasising clean lines. Popular styles include structured crepe shift dresses and column midi dresses. Sizing guidance: Most dresses fit true to size, though crepe and silk styles offer minimal stretch. If you're between sizes or prefer a looser fit, size up. For petite builds, consider sizing down. The brand's US sizing (00–14) translates to UK sizes 4–20.
Trousers & Tailored Bottoms
Theory's trouser expertise is legendary. Styles like the Treeca trousers feature cropped ankles with slim fit and subtle stretch, making them ideal for office wear and travel. Sizing guidance: Measure your waist carefully, as Theory trousers are tailored with precise waist shaping. If you have curvier hips, size up one size from your waist measurement. The brand offers mid-rise, high-rise, and cropped lengths; always check inseam options before purchasing.
Blazers & Jackets
Theory blazers are prized for precision tailoring with fitted sleeves and clean shoulders. Styles like the Tenia Blazer exemplify the brand's commitment to modern suiting. Sizing guidance: Measure your bust carefully. If your shoulders are broad, size up to avoid restriction across the back. Remember that these pieces are designed for layering — consider whether you'll wear knits underneath and size accordingly. Sleeve length may require tailoring, especially for those under 5'4" or over 5'8".
Silk Tops & Blouses
Theory's silk and satin pieces include camis, blouses, and shirt dresses featuring elegant drape and minimal cling. Popular styles like the Karenia Silk Shirt offer relaxed, button-down styling. Sizing guidance: These pieces typically run true to size, though silk fabrics have minimal stretch. If you're broad in the shoulders, size up. For a fitted silhouette, order your usual size; for a more relaxed fit, go up one size. Remember that silk wrinkles easily during transit — steam gently rather than iron.
Knitwear & Jersey
Theory's knitwear emphasises refined simplicity with fine-gauge knits and structured stitching. Jersey and rib styles offer some natural give whilst maintaining shape. Sizing guidance: Most knitwear runs true to size. If between sizes, size up for comfort and longevity. Jersey pieces are slightly more forgiving than tailored items; silk knits require gentle care and may shrink slightly if not hand-washed. Always check fibre content, as Theory uses various blends including cashmere, merino, and cotton-blend fabrics.
Sizing Tips
- Theory US-to-UK Conversion: Theory primarily uses US sizing. Women's sizes convert as: US 00=UK 4, US 2=UK 8, US 4=UK 10, US 6=UK 12, US 8=UK 14, US 10=UK 16, US 12=UK 18, US 14=UK 20. Always check the product label, as some UK retailers list both.
- Measure Both Your Waist & Hips: Theory's tailored aesthetic means fit varies significantly between styles. A dress might fit true to size, whilst trousers in the same size could be too snug at the hip. Measure both points and compare against the specific product's dimensions before purchasing.
- Consider Fabric Type & Stretch: Theory uses varying fabrics with different stretch properties. Precision Ponte and stretch cottons have mild elasticity; crepe and Good Wool are structured with minimal give. Check the product description carefully — a size that fits in stretch cotton may be tight in crepe.
- Tailoring Budget: Theory pieces are designed for precision fit but may require minor alterations for your body. If purchasing trousers or dresses online, budget for potential tailoring — especially for inseam or hem adjustments. Theory's sophisticated construction holds tailoring well.
- Shoulder Width Matters: Many Theory customers with broader shoulders size up in blazers and tops. If your shoulder width exceeds 17.5", consider trying a size L or XL in women's, or sizing up in blazer styles to ensure comfort and movement.
- Try Before Committing: When ordering from UK retailers like Net-a-Porter or Selfridges, take advantage of free returns. Theory's silhouette is quite specific — some styles will suit your body shape better than others. Ordering two sizes is a smart strategy for learning your preference.
- Care for Longevity: Theory pieces are investment pieces. Most require gentle care — hand-wash silk items, lay flat to dry, and avoid high-heat washing. Proper care ensures your Theory wardrobe remains pristine and maintains its shape season after season.
- Petite & Tall Considerations: Theory doesn't specifically offer petite or tall ranges. Petite customers (under 5'3") should size down and budget for hemming; taller customers (over 5'9") should check inseam lengths carefully and consider that blazers may be slightly cropped.
- Width Fittings Not Available: Unlike some British brands, Theory doesn't offer width fittings (D, E, EE). Standard width is assumed. If you have narrow feet or wide feet, footwear may require specialist insoles or a different brand.
Fabric Guide & Care
Precision Ponte & Stretch Cotton: These signature Theory fabrics feature light stretch and soft hold, ideal for travel and office wear. They maintain shape well and are generally machine-washable on a gentle cycle. True to size; expect mild give in the fabric.
Good Wool & Crepe: Structured luxury suiting fabrics with excellent drape and no cling. These offer minimal stretch and require dry-cleaning for blazers; crepe dresses can often be hand-washed. True to size; expect a tailored silhouette.
Silk & Satin: Found in camis, blouses, and shirt dresses, these fabrics are elegant with minimal stretch. Hand-wash in cool water and lay flat to dry. Silk wrinkles easily and adds a luxe feel; these pieces require careful handling but reward with timeless style.
Jersey & Knitwear: Theory's jersey and knit pieces include fine-gauge options in cashmere, merino, and cotton blends. These offer some natural give and are often hand-wash recommended, though some cotton blends tolerate gentle machine washing. Lay flat to dry to prevent stretching.
International Shipping & Returns
Theory ships globally through its website and selected UK retailers including Selfridges and Net-a-Porter. When ordering internationally, note that you're ordering in US sizes — use the UK conversion chart above. Most retailers offer free returns within 28 days, making it wise to order two sizes if uncertain. Always check the retailer's returns policy before purchasing, as policies vary by location.
FAQs
Q: What size should I order in Theory if I'm a UK size 12?
A: UK 12 typically converts to US 8 in Theory. However, always measure your bust, waist, and hips and compare against the specific product's size chart, as fit varies by style and fabric.
Q: Do Theory trousers run small?
A: No, Theory trousers run true to size in the waist, but are tailored with a fitted hip. If you have curvier hips, consider sizing up one size from your waist measurement.
Q: Is Theory good for petite women?
A: Theory offers excellent tailoring but doesn't have a dedicated petite range. Petite women (under 5'3") should size down and budget for hemming on dresses and trousers. Blazers may be slightly cropped.
Q: Can I return Theory clothes purchased from UK retailers?
A: Yes, most UK retailers (Selfridges, Net-a-Porter) offer free returns within 28 days. Check the specific retailer's policy before purchasing. Direct purchases from Theory's website may have different terms depending on your location.
Q: How does Theory men's sizing compare to British brands?
A: Theory uses US sizing for men's, so sizes run approximately one size smaller than traditional British brands. A Theory medium is comparable to a British large. Always check measurements rather than relying on size alone.
Q: What's the difference between Theory's US 00 and US 0?
A: US 00 is the smallest size (UK 4, bust approximately 31–32 inches) and is designed for petite builds. US 0 is UK 6, suitable for those with approximately 32–33 inch bust. Both are significantly smaller than standard UK sizing.