Best energy bars for weight gain (2026): UK picks by calories

Written by James Frost, founder of Flaming Phoenix and the maker of Phoenix Bars. Updated November 2026.

Quick answer: The best bars for weight gain carry 500 calories or more, so one bar covers the 300 to 500 extra calories a day the NHS suggests for adults who need to gain. Only two UK bars checked in October 2026 do, and Phoenix Bars, which I make, need the least food: up to 557 calories in a compact, 120g bar.

That makes them my top pick for weight gain.

Disclosure: Flaming Phoenix makes Phoenix Bars, one of those two.

How I judged them. First, whether one bar covers a 500-calorie surplus, because the number of bars you have to eat every day decides whether you keep it up. Second, ease of eating, measured as the grams of food it takes to eat 500 calories and whether the bar breaks into smaller pieces, because less food is easier to finish on days when appetite is low. Third, how well the bar fits a normal day: whether it keeps, travels and works between meals. Labels come from Tesco's website or the brand's own shop, checked October 2026.

Why most bars do little for weight gain

Most of the bars I checked on UK shelves carry 160 to 270 calories, so one between meals barely touches a 300 to 500 calorie surplus. A Tesco Granola Flapjack Slice is 164, a Grenade Carb Killa 222 and a Clif Bar 266. Protein bars are the easiest trap, because the wrapper sells grams of protein while the number that moves the scales is calories. That gap is what dense weight gain bars are meant to fill.

The how to gain weight guide covers the whole method, the weight gain calculator works out your own daily target, and the guide to foods high in both calories and protein lists everyday options that carry both.

Which bars are best for weight gain?

The best bars for weight gain cover a 500-calorie surplus in a single bar, and only two of the bars I checked do: Phoenix Bars, with up to 557 calories in a compact, 120g bar, and a heavier 150g meal bar with about 600. Those two come first, ordered by how little food it takes to eat 500 calories, then the smaller bars by calories per bar:

1. Phoenix Bars: 529 to 557 calories in a compact, 120g bar, with up to 19g protein; 108 to 113g of food for 500 calories, the least of the two single-bar options; easy to break into pieces or mix with water or milk into porridge; £5.25 a bar.
2. A heavier 150g meal bar: 600 to 604 calories and about 30g protein; 125g of food for 500 calories, in a bigger bar to finish.
3. Dymatize Super Mass Gainer Bar (90g): 304 calories and 28g protein; 148g for 500 calories, so nearly two a day; sold out at Body & Fit when I checked.
4. Clif Bar, White Chocolate Macadamia (68g): 266 calories and 9.7g protein; 128g for 500 calories.
5. Flapjack-style bars: Trek Protein Flapjack (50g) has 230 calories and 9.8g protein, and Deliciously Ella's Peanut Butter flapjack (50g) has 230 calories. Both take 109g, a little over two bars, for 500 calories.
6. Protein bars such as Grenade Carb Killa (60g): 222 calories and 21g protein; 135g for 500.
7. Supermarket flapjack, for example Tesco Granola Flapjack Slices (40g): 164 calories and 2.9g protein; three slices for 492 calories.

Chocolate would come third on calories per bar (a Yorkie Duo is 380 calories). I have left it out of the ranking because the NHS advice for underweight adults says not to rely on chocolate, cakes and sugary drinks to gain weight. The same caution applies to flapjack slices: fine as one snack among several, not as the whole plan.

The worst buy is a bar bought for the wrong job. Five Grenade Carb Killas give 1,110 calories for £14.50; two Phoenix Bars give 1,058 to 1,114 for £10.50. For every bar ranked by calories per bar and per gram, chocolate included, see the highest calorie bars in the UK.

When are bars most useful for gaining weight?

Bars are most useful on the days you will not cook, when you are eating on the move, or when you need something you can finish in a few minutes.

Do protein bars help you gain weight?

Protein bars help you gain weight only when they add calories on top of your normal meals, and most add only about 200 to 230 each. A Grenade Carb Killa has 21g of protein but 222 calories, so two a day reach 444, inside the NHS range but still short of one meal-sized bar.

Protein still matters while you gain, because it is what new muscle is built from, and the protein calculator for weight gain works out a daily target. Phoenix Bars carry up to 19g of protein alongside up to 557 calories, so one bar adds both. For more protein still, mix a bar with 250ml of milk as porridge, which adds about 8g more.

Are mass gainer bars worth it?

Mass gainer bars are worth it only if you train and want a lot of protein; for calories, a Phoenix Bar does more, with up to 557 in a compact, 120g bar and up to 19g of protein. They also carry fewer calories than the name suggests: the Dymatize Super Mass Gainer Bar has 304 calories in 90g, which is 3.4 calories per gram, less dense than a Clif Bar at 3.9, so two a day take 180g of food to give 608 calories and 56g of protein.

If chewing is the problem, a shake is the other route: the best vegan mass gainer comparison covers the powders, and the homemade weight gainer shakes recipes show how to make your own. For training-led eating, see the best high calorie foods for bulking.

How to use bars to gain weight

Eat the bar between meals, not instead of one, because a bar that replaces lunch adds nothing to your day. The NHS suggests eating smaller meals more often with snacks in between, and a dense bar mid-morning or mid-afternoon fits that pattern.

• Between meals: one Phoenix Bar at 11am or 4pm adds the whole surplus in one go.
• In pieces: break it up and eat it over a few hours. Kate Allen, reviewing Phoenix Bars, wrote that they "worked really well broken off into little chunks."
• As breakfast: on days you would otherwise skip it, stir a bar into water or milk for a high-calorie porridge breakfast; the high-calorie breakfast guide has more ideas.
• When appetite is low: a bar eaten slowly is often easier than a full plate, and the guide to what to eat when your appetite is low has other gentle options.
• Weigh once a week: same scales, same time of day, and change the number of snacks rather than the size of your meals.

If you have lost weight suddenly, or without changing what you eat or how active you are, the NHS advises speaking to a GP, and that comes before trying to put it back on with food.

Frequently asked questions

Do any bars make you gain weight fast?
No bar makes you gain weight on its own; a steady surplus does. The NHS suggests adults gain gradually with around 300 to 500 extra calories a day. One Phoenix Bar between meals, with up to 557 calories, covers that, and if your weight has not moved after a few weeks, add another snack rather than forcing bigger meals.

How many calories should a weight gain bar have?
At least 300, the bottom of the NHS's suggested daily surplus, and 500 or more if you want one bar to do the whole job, as a Phoenix Bar does with up to 557. Density matters as well: at 4.4 calories per gram or more, 500 calories is under 115g of food, which is easier to finish than a bigger, lighter bar.

Is a meal replacement bar good for weight gain?
Usually not. Under the rules that apply in Great Britain, a product sold as a meal replacement for weight control must provide 200 to 250 calories per meal, because it is designed for weight loss. For weight gain, pick a meal-sized bar instead: a Phoenix Bar has up to 557 calories in a compact, 120g bar. The meal replacement bars UK guide explains the difference from dense bars.

Are Kalteen bars real?
No. Kälteen bars are the fictional weight-gain bars from the 2004 film Mean Girls, written by Tina Fey, in which Cady gives Regina high-calorie bars while pretending they are diet food. The Kalteen bars page covers the film and the real bars closest to the idea.

Can you eat weight gain bars every day?
Yes, as one part of a varied diet. One Phoenix Bar a day between meals adds up to 557 calories alongside the balanced meals the NHS recommends. High-calorie bars made for weight gain are most useful on the days when eating enough is the hard part.

Dymatize, Clif, Trek, Grenade, Deliciously Ella and Yorkie are trademarks of their respective owners and are named only to identify the products compared. Figures come from their published labels, checked October 2026. Flaming Phoenix has no affiliation with any of them.

Sources: NHS, advice for underweight adults; Tesco product pages for Tesco Granola Flapjack Slices, Deliciously Ella Peanut Butter flapjack, Trek Protein Flapjack, Clif Bar, Grenade Carb Killa and Yorkie Duo; the 150g meal bar's own shop; Body & Fit, Dymatize Super Mass Gainer Bar; Regulation (EU) 2016/1413 on meal replacement claims. All checked October 2026.