Serious Mass Gainer
by Optimum Nutrition
Calories that count: a high-protein, high-carb formula built for serious muscle and weight gain.
- Up to 50g protein per serving
- Low fat - under 3g per 100g
- High calorie for bulking phases
- For shakes, smoothies, baking and more
FlavourVanilla
Best price today
In stock£63.10
£1.16 / 100g
You save £15.81vs highest tracked price today
Secure retailer checkout
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Transparent protein profile
What's inside
Values per 100g, with the amount in one 335g serving
15g protein
50.3g per 335g serving
75g carbohydrates
251.3g per 335g serving
2g fat
6.4g per 335g serving - very lean
376 kcal
1260 kcal per 335g serving
View full nutritional information
| Nutrient | Per 100g | Per serving (335g) |
|---|---|---|
| Protein | 15g | 50.3g |
| Calories | 376 kcal | 1260 kcal |
| Carbohydrates | 75g | 251.3g |
| Fat | 1.9g | 6.4g |
Know when to buy
Price history
Lowest price on record, any flavour or size
£0.63/100g (£34.40 for 5.45kg on 16 Aug 2026)
5.45kg · all flavours · price per pack
- Amazon UKlatest £63.10, lowest £34.40, highest £79.99 in the last 90 days
Quick answers
Is this right for you?
Who is it for?
Optimum Nutrition Serious Mass Gainer is designed for hard-gainers and anyone in a dedicated bulking phase who struggles to eat enough calories. Mass gainers combine protein with carbs and calories to support weight gain and muscle growth during intense training blocks.
What can you expect?
A calorie-dense shake designed to help you gain weight. Mass gainers pack protein alongside a significant amount of carbohydrates and calories per serving. Servings tend to be larger than standard protein powders, so expect thicker shakes and bigger scoops. With 15g of protein per 100g (around 50g per 335g serving), Optimum Nutrition Serious Mass Gainer sits in a competitive position for a mass gainer product.
What else do you need to know?
Optimum Nutrition Serious Mass Gainer is a mass gainer protein powder made by Optimum Nutrition. It comes in 2.66kg, 5.45kg sizes. Every price on WheyWise is read from the retailer’s own page and re-checked daily and sorted by cost per 100g, so you can see both the cheapest option and how current it is. All links to Amazon and retailer sites are affiliate links - this is how WheyWise stays free.
Frequently asked questions
Where is the cheapest place to buy Optimum Nutrition Serious Mass Gainer in the UK?
The best price we have verified today is £1.16 per 100g for the 5.45kg bag, checked 30 Sept 2026. Use the buy button above to go straight to it.
How much protein does Serious Mass Gainer contain per 100g?
Serious Mass Gainer contains 15g of protein per 100g, with 50.3g per 335g serving. It is a mass gainer product.
Is Optimum Nutrition Serious Mass Gainer currently in stock?
At the last check, 5 out of 5 tracked variants were in stock. Stock status carries the same fetch time as the price shown above, so check the retailer listing before ordering.
Pick your pack
2 sizes · any flavour · cost per 100g for each
The 5.45kg works out at £1.16 per 100g. The 2.66kg costs £1.47. Same powder, 1.3x the price per gram.
Where to buy in the UK
5.45kg · 3 offers for this selection · sorted by cost per 100g
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