Quick answer
Our pick: Under Armour ColdGear Fitted Mock
- Best for the cold: UA ColdGear Fitted Mock (~£39)
- Warm, compression fit: UA ColdGear Comp Mock (~£45)
- Cold-weather legs: UA ColdGear Leggings (~£25)
- Best for hot weather: UA HeatGear Compression LS (~£23)
- Hot-weather legs: UA HeatGear Leggings (~£24)
The full reasoning, and everything we ruled out, is below.
If you take one thing from this page, take this: HeatGear is the hot-weather line and ColdGear is the cold-weather line. That sounds obvious written down, but the number of people who buy a HeatGear compression top expecting it to keep them warm, then stand around freezing in it, tells you the names do not land the way they should. So before you spend anything, get the two lines straight.
HeatGear is thin, tight and built to keep you cool. It is Under Armour's original compression base layer, it wicks sweat hard, and it is designed for hot days, the gym and high-output efforts where the enemy is overheating. It is UA's biggest seller by a mile. It is not a warmth layer.
ColdGear is the opposite. It is a dual-layer fabric with a warm, brushed inner face against your skin and a smooth outer, and it is built to hold heat in the cold. This is the one I actually own and have worn for a couple of winters, and it is the one you want if the question is "which Under Armour base layer for the cold".
That is the whole answer. The rest of this page is why, the fit question that catches people out, and honestly where UA sits against proper merino. This is a brand deep-dive that sits under our complete base layers guide and the wider base layers hub, so if you have not settled on Under Armour at all, start there.
HeatGear vs ColdGear at a glance
The two lines side by side. Tap a name to jump to the detail, or a buy button to check today's price.
| Product | Price | Buy |
|---|---|---|
| Hot weatherUA HeatGear Compression LS | £18.74 | Amazon |
| Thin single-layer, 79% polyester / 21% elastane · ultra-tight compression fit · UA says 'keeps you cool', ventilated, fast-dryingBest for Summer, gym and high-output warm days, or a light wicking layer worn under other layers. Not a warmth layer | ||
| UA HeatGear Leggings | £23.95 | Amazon |
| Thin compression legging · same cool, wicking hot-weather HeatGear fabricBest for Warm-weather training and as a thin wicking base under shorts or trousers in the heat | ||
| Best for coldUA ColdGear Fitted Mock | £34.50 | Amazon |
| Dual-layer: warm brushed inner face, smooth fast-drying outer · snug 'fitted' cut, not full compression · UA says 'keeps you warm'Best for The one to buy for the cold. Winter runs, cold gym sessions and as a warm synthetic base in a layering system | ||
| UA ColdGear Comp Mock | £29.69 | Amazon |
| Warm dual-layer ColdGear fabric in a tighter compression fitBest for Cold-weather warmth if you specifically want the muscle-hugging compression feel and close moisture management | ||
| UA ColdGear Leggings | £35.99 | Amazon |
| Warm brushed-inner ColdGear leggingBest for The bottom half of a cold-weather UA base layer, under trousers on cold days | ||
Amazon prices refresh automatically with the site’s nightly build, so they can lag a day and can change at any time. The price at checkout is the one that applies.
The one mistake almost everyone makes
The mistake is reading the names as a promise about warmth. People see "HeatGear" and their brain files it under "heat, warm, keeps me warm", so they buy the thin compression top for winter and wonder why they are cold. HeatGear does not mean "generates heat". It means "gear for the heat", for hot weather, and Under Armour's own copy on it says, in as many words, "keeps you cool". It is a cooling top. Wearing it as your only layer in the cold is like wearing a string vest and expecting a jumper.
ColdGear is "gear for the cold", and that one does keep you warm. Read the names as the weather they are for, not the sensation they give you, and it clicks: HeatGear for when you are too hot, ColdGear for when you are too cold.
There is a grain of sense in owning both, mind. A thin HeatGear top is a perfectly good light wicking layer to wear under other things, and plenty of people use one that way year round. But if the reason you are here is the cold, HeatGear is not the answer and ColdGear is.
HeatGear: the hot-weather line
HeatGear is the fabric Under Armour built its name on, and the compression long sleeve is its single biggest seller, with review counts running into the tens of thousands. That popularity is exactly why it gets bought for the wrong job. It is a very good top. It is just a very good top for being hot, not for being cold.
The fabric is a thin, single-layer stretch knit, around 79% polyester and 21% elastane on the compression top (the exact blend varies a little across the HeatGear range, but it is always polyester-heavy with a big slug of elastane for the stretch). That elastane is what gives it the second-skin squeeze. Under Armour's own words for it are "keeps you cool", "ventilated" and "fast-drying", with mesh panels under the arms and across the back to dump heat. Everything about it is engineered to move sweat off you fast and let warmth escape, so you stay cool when you are working hard in the heat.
Where it is genuinely great. Summer running and hiking, the gym, football and any high-output warm-weather effort where overheating is the problem. It is also a fine thin wicking layer to wear under something else, because it is light and dries quickly. As a hot-weather compression base it earns its sales.
Where it is wrong. As a cold-weather warmth layer, full stop. It traps almost no still air, it is built to vent heat, and on a cold day that is precisely the opposite of what you want next to your skin. If you are cold, do not reach for HeatGear.
ColdGear: the cold-weather line (the one I own)
This is the one I can talk about from my own back, not a spec sheet. I own the ColdGear Fitted Mock and I have worn it through a couple of winters of gym sessions and cold morning runs. It does the job it is sold for. On a cold start it takes the edge off straight away, and once I am moving it moves sweat off me without going clammy. It is the top I reach for when it is properly cold and I am going to be working, and it has held up to a lot of washes without pilling or going baggy.
The reason it works and HeatGear does not is the fabric. ColdGear is a dual-layer knit: a warm, brushed inner face that sits against your skin and traps a layer of still air, and a smooth, fast-drying outer. Still air is what actually insulates you, so that brushed inner is doing real warmth work, while the smooth outer keeps it from feeling bulky and lets you layer over it cleanly. Under Armour's line on it is simply "keeps you warm", and for once the marketing matches the garment. It is still a wicking synthetic, so it moves sweat like HeatGear does, it just holds heat while it is at it.
ColdGear Fitted Mock. The one I own and the one I would point most people at. The mock neck is genuinely useful in the cold, it closes off the draught at your collar that a crew leaves open. The "fitted" cut is snug but, and this matters, Under Armour describe it themselves as "without the squeeze of compression", so it is close without strangling you. For winter running, cold-weather training and as the warm base in a layering system, this is the pick.
ColdGear Comp Mock. The same warm ColdGear fabric in a tighter, true compression fit. If you actively like the muscle-hugging compression feel and the very close moisture management it gives, this is the warm version of that. If you do not care about compression, save the money and take the Fitted.
ColdGear Leggings. The bottom half. Same warm brushed-inner fabric in a legging, to go under trousers on cold days. Cheap, warm enough for most UK winter use, and an easy way to complete a head-to-toe UA base layer.
Compression or relaxed: the fit question
Here is the thing to be honest about before you buy, because it is the main variable and the reason a lot of returns happen. Under Armour runs tight. The whole brand grew out of compression, and even the pieces that are not badged "compression" are cut close. So the choice is not really warm-versus-cool once you are in the ColdGear line, it is how much squeeze you want.
At the tight end sits everything HeatGear and the ColdGear Comp Mock, all true compression, an ultra-tight second-skin fit that hugs the muscles. Some people love it: it feels supportive, it never shifts, and the fabric stays in close contact with your skin so moisture management is very direct. Runners and gym users often want exactly this.
The ColdGear Fitted Mock sits a notch looser. Snug and form-fitting, but as UA put it, without the compression squeeze. For most hillwalking and everyday cold-weather use, this is the sweet spot, close enough to work as a base layer, not so tight it feels like shapewear.
What Under Armour does not really do is a properly relaxed base layer. If what you actually want is loose, cosy warmth that you forget you are wearing, a boxy merino or synthetic crew from another brand will suit you better than any UA piece, and that is worth knowing before you order. If you like a close, athletic fit, UA is right up your street. If you hate anything tight, it is not. Size up if you are between sizes or you want a bit more room; UA sizing is on the trim side.
Which should I buy?
By use case, so you can find yourself:
- You want Under Armour for the cold. Buy the ColdGear Fitted Mock. This is the answer to the question in the title. Warm brushed fabric, sensible snug fit, mock neck for the draught. Add the ColdGear Leggings for the legs.
- You want the cold-weather warmth but love a compression fit. The ColdGear Comp Mock. Same warmth, tighter squeeze.
- You run hot, or you are shopping for summer, the gym or high-output warm days. The HeatGear Compression Long Sleeve. Cool, thin, fast-wicking, and its huge review count is deserved for that job.
- You want a thin wicking layer to wear under other layers, any season. HeatGear again, used as a liner rather than a warmth piece.
- You want loose, cosy warmth and dislike anything tight. Neither, honestly. Look at a relaxed merino or synthetic crew in the complete base layers guide.
Where Under Armour fits, and where it doesn't
I like my ColdGear and I keep wearing it, so this is not a hatchet job. But it is worth being straight about where it sits, because Under Armour is not a specialist outdoor base layer brand and it is not pretending to be one.
Neither ColdGear nor anything else in the UA base layer range is a heavyweight merino built for an Arctic week. When I put my own kit together for an active week at minus twenty-five in Finland, the ColdGear I own has a place in it, but as a mid-layer over a warmer top, not as the single warm layer against my skin. For that trip the warmth-for-weight, the staying-warm-when-damp and the odour resistance of a proper 260gsm merino, or a dedicated brushed synthetic like the Odlo, are a step above what ColdGear does. If deep, static cold is your actual use case, read that piece and the complete base layers guide, because there are warmer tools for it.
What ColdGear is, and this is not a small thing, is a capable, genuinely cheap, widely available winter base layer that you can pick up almost anywhere. For UK winter running, cold gym work, general cold-weather layering and anyone who already trusts the brand and likes the fit, it does the job and it does not cost much. Layered up, it is a sensible warm base. Just do not ask it to be the one heroic layer that beats an Arctic night on its own, and whatever you do, do not buy its HeatGear cousin for the cold.