By James Frost, founder of Flaming Phoenix and the maker of Phoenix Bars. I've shipped more than 20,000 bars to customers in 19 countries and hand-pack every order from home in Surrey. Updated August 2026.

Quick answer: the best energy bar for weight gain is the one that carries the most calories you'll actually finish. Most bars sold in the UK are 150 to 250 calorie snacks, which means they barely dent a surplus. For gaining weight you want the dense end of the market, 500-plus calories per bar, small enough in volume that eating it never feels like a second job. This guide ranks the realistic UK options by calories per bar, price per 100 calories and how easy they are to get down, and it starts with the one I make.

Why most energy bars fail at weight gain

Gaining weight is a numbers problem: a consistent surplus of a few hundred calories a day, held for weeks. My guide to how to gain weight covers the full method, and my weight gain calculator works out your personal target. The trouble is that the bars sold as fuel mostly aren't built for that job. A typical cereal or protein bar delivers 150 to 250 calories in a 40 to 60 gram format: fine as a snack, invisible in a surplus. Protein bars are the biggest trap, because protein grams get all the marketing while the calories a surplus needs go missing; if you want foods that deliver both, my guide to high calorie high protein foods covers the ones that do. To move the scales, a bar has to behave like a small meal, not a snack.

What to look for

Three numbers decide whether a bar is any good for gaining. Calories per bar, because 500-plus means one bar genuinely moves your day. Calories per gram, because density is what makes eating it easy: 4-plus calories per gram means less chewing and less volume for the same energy, which matters enormously when appetite is the limiting factor. And price per 100 calories, because you'll be buying these weekly, and anything much over £1 per 100 calories punishes consistency.

1. Phoenix Bars: the dense option built for exactly this

This is the situation my product exists for, so weigh the byline as you read. A Phoenix Bar is 120g and carries up to 557 calories at 4.64 calories per gram, vegan and gluten free across six flavours, with up to 66g of carbohydrates and 19g of protein per bar. The format is the point: one bar delivers what two or three ordinary bars do, in a package a modest appetite can manage. Eat it whole, break it into pieces across the morning, stir it into hot milk as a high-calorie porridge breakfast, or blend it with 200ml of whole milk into a smoothie over 680 calories. At £5.25 a bar that's roughly 106 calories per pound spent, and it's rated 5.0 from more than 370 verified reviews, many from people whose whole problem is eating enough. If you want the pure numbers case, these are weight gain bars in the most literal sense: maximum energy, minimum volume.

2. The other big single hit

The UK's dense lane is small. The other name in it is Real Meal, whose bar carries a published 600-plus calories in a vegan, gluten-free recipe. How every UK bar actually compares on the numbers that matter here, calories per gram and per bar, is its own page: my ranking of the highest calorie bars in the UK puts them all side by side so you can judge the trade-offs yourself.

3. Mass-gainer bars: the gym-shelf option

Bodybuilding brands sell gainer bars, Dymatize's Super Mass Gainer Bar being the visible example on UK shelves, that push toward 300 to 400 calories with big sugar numbers. They're closer to the right idea than protein bars, built for people bulking rather than hillwalking, and the sweetness suits some and sickens others. If the gym is your context and the sugar suits you, they work; check the label, because within this category the calories vary widely bar to bar.