Top 10 Things to Know About 400 GSM Fleece and What GSM Means
Ever bought a fleece that felt cosy at first but bobbled, or one that overheated on a brisk walk? Knowing GSM, and why 400 GSM is considered a midweight, helps you understand how fabric weight relates to warmth, breathability, and durability.
This guide explains what GSM measures, how pile structure, stitch density, and fabric construction affect insulation and pilling, and how to test weight and drape for freedom of movement. You will also find practical care tips, sustainability checkpoints, and simple layering ideas to choose, maintain, and style a 400 GSM fleece that performs for everyday wear.
1. Understand GSM and how it is measured
GSM, or grams per square metre, describes the mass of a textile per unit area and is measured by cutting a known-area sample, weighing it on a precision scale, and scaling the mass to one square metre so results can be reproduced or verified. In practice, a higher GSM usually indicates more mass per area, which often correlates with greater insulation and durability, but pile height, fibre type, knit density, and finishing can change how heavy or warm a fleece actually feels. Be aware of common labelling differences and measurement pitfalls: suppliers may quote fabric GSM or finished-garment GSM, measure samples before or after brushing or backing, and add coatings or linings that alter the reported figure.
When comparing or sourcing, always ask for the GSM of the finished fabric, check accompanying specifications such as pile height, fibre content, and request a tactile sample or a laboratory certificate to confirm claims. For simple quality control, cut and weigh an in-house sample to verify supplier numbers, and track changes in mass after washing or finishing to reveal shrinkage, compaction, or material loss. Using these checks helps verify supplier claims, and lets you compare like for like when selecting fleece for insulation or durability.
Choose a denser 400 GSM crew for extra warmth.
2. Choose midweight fleece for cold weather protection
At around 400 GSM, fleece sits in the midweight category between lightweight fleeces near 200 to 300 GSM and heavyweight options at 600 GSM or more, and higher GSM increases fibre mass and trapped air which raises insulation without necessarily adding bulk. Pair a fitted, moisture-wicking base layer with a 400 GSM fleece as the insulating mid layer and a lightweight windproof shell on top to balance warmth, breathability, and protection from gusts. That combination preserves core heat while allowing sweat to move away from the body.
Choose 400 GSM for lower to moderate intensity outdoor activity, commuting, or stationary work in cold conditions, and select more breathable cuts or zipped vents if you expect sustained exertion to avoid overheating and sweat retention. Look for a fit with enough room for a base layer without excess bulk, articulated sleeves, and a draft-collaring zip to reduce heat loss, and prefer tightly knit face fabrics or brushed interiors to improve wind resistance and loft retention. Maintain insulation by washing in cool water, avoiding fabric softener, and drying on low heat or by air to preserve loft and wicking. Remove pilling with a fabric shaver as needed, and repair small seam or zip damage early to extend usable life.
Choose a roomy 400 GSM fleece for everyday warmth.
3. Inspect pile structure for effective insulation
Pile anatomy determines insulation: pile height, fibre crimp, and fibre type all influence how much stationary air the fabric traps, with dense, upright pile reducing heat loss and long, loose pile able to trap air but prone to matting. Inspect an exposed fabric edge or a small tear under a bright light or loupe to see the pile base and backing, and judge construction density from what you observe. For a quick, repeatable check, pinch a small patch, compress it, then release and time how quickly the pile springs back, recording recovery rather than relying on assumptions.
Hold the fleece up to a light to check backing and stitch density, since visible gaps or open stitches provide pathways for convective heat loss and indicate looser construction. Look for visual and tactile clues to fibre structure, such as split ends, uniform fluffiness, or a silkier sheen, which suggest hollow, crimped, or multi-filament fibres that trap more air per gram. Run your hand across the pile and gently tug individual fibres to reveal early pilling, loose tufts, or a flattened nap; shedding or matting predicts reduced long-term loft. To preserve insulating performance, avoid prolonged compression in storage and revive flattened pile with a low-heat tumble dry when necessary, and keep notes of what you find so future comparisons are evidence based.
Choose a mid-weight fleece for dependable everyday warmth.
4. Assess fabric construction and stitch density
To assess stitch density and gauge, count loops or stitches across a 1 centimetre span on the knit side using a magnifier to estimate tightness, because higher stitch density usually yields a firmer hand, lower wind permeability, and greater resistance to snagging, while lower density gives softer drape and increased breathability. Inspect the back and cut edges to identify loopback, single-brushed, or double-faced constructions, as the visible structure predicts performance: loopback knits offer stretch and moisture management, while double-faced fleeces balance loft and durability. Run your fingers over the pile to judge brushing depth, uniformity, and springback, since a deep, even pile traps more air for insulation but may show fibre migration and pilling if the fibre finish is poor. Check seams and stitch types for overlock or coverstitch, consistent stitch length, and extra rows at hems and pocket openings, because tight, even stitching and reinforced stress points reduce seam failure and edge curling over time.
Perform simple hands on quality checks by holding the fabric up to light to assess openness, stretching it to observe recovery, and rubbing a small area to gauge initial pilling tendency. Also examine dye penetration and pile uniformity, and use seams or cut edges to confirm construction and stitch density findings. Consistent results across these tests indicate controlled knitting and finishing processes that influence warmth retention, durability, and long term performance.
Choose a dense 400 GSM fleece for lasting warmth.
5. Balance breathability and moisture control
Pile height, yarn density, and overall GSM combine to control how much air a fleece traps and how quickly moisture can evaporate from the fabric. High loft increases insulation and reduces evaporative cooling, so for higher activity choose lower-pile constructions, loopback or microfleece faces, or ventilated knits that preserve airflow. Because hydrophobic fibres such as polyester shed liquid water and dry faster than cotton, polyester or polyester-blend fleeces tend to move sweat away from the skin and recover breathability more quickly, so choose synthetics for active use and natural fibres for low-intensity comfort.
Design and fit features such as vents, full or partial zips, pit zips, mesh-lined pockets, raglan sleeves, and looser cuts increase airflow and let moisture escape during exertion. Treat a 400 GSM piece as a midlayer: wear a moisture-wicking base layer next to the skin, add the fleece for insulation, and use a breathable, waterproof outer shell only when needed, shedding or unzipping layers to restore evaporative cooling rather than relying on the fleece alone. Simple tests help evaluate performance: wet a small patch to compare drying times, and consult published moisture vapour transmission rate (MVTR) or breathability figures when available to compare fabrics objectively. Care matters: avoid fabric softener, de-pill compressed areas, and gently restore loft after washing to keep air channels open and maintain moisture handling.
Wear a midweight cotton-faced crew for relaxed, breathable comfort.
6. Test weight and drape for freedom of movement
Carry out a hang and fold test on a palm-sized swatch: hold it by one corner and note whether the fabric forms smooth, rounded folds and springs back, or hangs boxy with sharp, persistent creases, which indicate bulk and stiffness. Stretch a small patch, release it, and assess permanent distortion and pile flattening to judge recovery; rapid rebound and retained pile depth predict preserved drape and comfort under repeated movement. Compare side-by-side swatches because identical GSMs can behave differently depending on brushing and pile length, or loopback and microfleece constructions.
Run a movement trial with a finished garment or mock-up pattern, performing squats, overhead reaches, and torso twists while noting tugging at seams, binding across shoulders, or restriction at hips so you can mark problem motions and seam areas. Use those marks to alter pattern lines or add gussets, and prefer raglan or articulated sleeves with tapered bulk around joints to maintain mobility without excessive insulation loss. Also test garments with typical layers or a rucksack to reveal realistic drape and movement constraints that a swatch cannot show. Together, these practical checks let you weigh knit structure, pile characteristics, and design interactions rather than relying on GSM alone.
Try a mid-weight crew for structured, mobile layering.
7. Reduce pilling to maximise lifespan
Pilling appears when short or loose fibres abrade, knot and form small balls, and while a dense 400 GSM pile resists overall bulk loss it still pills at high-friction points such as cuffs, seams, and under backpack straps, which diminishes appearance and reduces insulation where the nap flattens. To limit abrasion in the wash, turn garments inside out, fasten zips and hook and loop fasteners, wash with similar fabrics on a gentle cycle using a liquid detergent, place items in a mesh laundry bag for extra protection, and use a low spin to reduce fibre agitation. Avoid high tumble dryer temperatures that can melt or weaken synthetic fibres and accelerate pilling; air dry flat when possible, or tumble dry on low and remove items promptly to prevent excess rubbing while warm.
Remove pills by testing any tool on an inconspicuous area, then using a fabric shaver, sweater stone, or a careful disposable razor to lift pills, and brush the nap with a soft garment brush to restore loft and even the surface. Taking pills off early prevents larger fibre tangles, which become harder to remove and trap more debris. Reduce wear-related pilling by alternating layers, avoiding rough surfaces and abrasive accessories, and storing garments clean and folded in a breathable space to limit dust and loose fibres. Mend loose seams or pulls promptly to stop new pills forming and to preserve the fleece structure and performance.
Pick a cotton-faced midweight crew to reduce pilling.
8. Follow proper care to preserve fleece performance
Turn fleece inside out, close zips and fastenings, and wash with like colours on a gentle cycle using a mild, non-biological detergent to reduce surface abrasion and trapped residue. Use cool to warm water to limit agitation that flattens the pile and reduces insulation, and skip fabric softener and bleach because softeners coat fibres and bleach weakens them, both accelerating loss of loft and increasing pilling. Washing this way preserves moisture wicking and breathability, helping the garment retain its thermal performance.
After washing, reshape and lay fleece flat to air dry away from direct sunlight and radiators to preserve loft, and if you must tumble dry use the lowest heat and remove items while slightly damp so the pile can recover. Reduce friction by washing fleece with soft items or inside a laundry bag, and avoid abrasive surfaces to minimise pilling. When pills do form, remove them carefully with a targeted fabric shaver or lint comb to recover a smoother surface and maintain thermal efficiency. Store folded in a breathable container, away from damp and strong odours, and occasionally rinse with an extra cool water cycle or a splash of white vinegar to strip detergent residue and restore natural wicking.
Keep heavyweight fleece looking new—follow these care tips
9. Choose sustainable fibres and confirm responsible sourcing
Choose fibres with their trade-offs in mind: recycled polyester offers warmth and durability, organic cotton or wool improve biodegradability, and lyocell or hemp reduce overall environmental impact. Ask suppliers for exact fibre percentages, the origin of the feedstock, and knitted-at-GSM lab data that shows how the chosen fibre behaves in a heavy, 400 GSM construction. Request current certificates such as the Global Recycled Standard, Global Organic Textile Standard, OEKO-TEX Standard 100, the Responsible Wool Standard, or bluesign, and verify certificate numbers online. Also ask suppliers to state which supply chain stage each certificate covers so you can match claims to specific processes and facilities.
Require clear chain of custody by naming the spinner, mill, and dyehouse and by explaining whether recycled content is handled as segregated, mass balance, or blended, then use batch-level traceability to test the strength of sustainability claims. To minimise microplastic release and other use-phase impacts, specify fibre length, yarn construction, and finishing that reduce pilling and fibre loss, and request third-party wash tests or microfibre release data while including care guidance on labels and recommending washing bags or filters. Design for end-of-life by preferring mono-fibre constructions or easily separable trims, asking about recyclability rates and take-back programmes, and documenting material composition on product tags to improve the odds of responsible disposal or recycling.
Choose a certified cotton-faced fleece for durable, lower-impact layering.
10. Layer and style midweight fleece for everyday wear
A 400 GSM midweight fleece works well as an outer layer on mild days and as a midlayer beneath a waterproof or insulated shell in colder conditions, because its loft traps warm air while remaining breathable. Pairing a close-fitting base layer, the fleece, and a breathable shell lets you manage moisture and trap heat efficiently, and full or half zips provide quick ventilation while a high collar or hood blocks wind. Zipped pockets, elasticated cuffs, and hems reduce heat loss and keep valuables secure, while mobile seams and slightly trimmed cuts preserve freedom of movement when you layer under a coat.
On the high street, elevate a relaxed look by pairing midweight fleece with tailored trousers and boots, or slide it under a classic overcoat for a contrast of texture and silhouette. Use a colourful fleece as an accent under neutral outer layers to add character without overwhelming an outfit, and select slightly trimmed silhouettes if you plan to layer frequently. To keep loft and appearance, wash on a gentle programme, skip fabric softener, dry flat or loosely on a line, and remove bobbles with a fabric comb to extend the garment's life.
GSM measures a fabric's mass per square metre, and a 400 GSM fleece typically delivers midweight insulation that balances trapped air, breathability, and durability. To predict how a specific garment will perform, check pile height, stitch density, fibre content, and finished-garment GSM, then test drape, recovery, and pilling on a swatch or trial fit.
Follow the practical checks and care steps described in the headings, inspect pile structure and construction, manage moisture with layers and vents, and remove pills early to preserve loft. When you confirm materials, construction, and responsible sourcing, you will choose and maintain a 400 GSM fleece that suits your activity, extends usable life, and reduces environmental impact.
