The cheapest protein powder in the UK is rarely the one with the lowest tub price. Bag size, scoop size, and the protein percentage of the powder all change the real cost of a serving. A £25 tub at 60g of protein per 100g of powder works out worse value than a £30 tub at 80g of protein per 100g.
Two metrics resolve this: cost per serving (useful for comparing same size scoops) and cost per 25g of protein (useful for comparing across brands with different scoop sizes and protein densities). This guide gives you the formulas, a worked example with three real UK products, and the common mistakes that lead UK buyers to overpay.
Why tub price is misleading
UK protein powder is sold in confusingly different formats. Bag sizes range from 250g sample tubs to 5kg bulk bags. Scoop sizes range from 25g to 40g. Protein content ranges from 55g to 92g per 100g of powder. A direct comparison of two tub prices is meaningless because none of those variables are held constant.
The marketing makes it worse. A 1kg tub priced at £29.99 next to a 2.5kg bag priced at £52 looks like the smaller tub is cheaper. The 2.5kg bag is roughly 30 percent cheaper per gram of powder. Add a difference in protein density and the gap widens further.
The maths is simple. The point of doing it is that nobody else does, which is exactly why so many people pay too much.
MyProtein Impact Whey Protein Powder
810g bag
UK's bestselling whey concentrate with 82g protein per 100g. Regularly discounted 40-60% — never pay full price.
See cheapest price →Buy direct from MyProtein →MyProtein Impact Whey Protein Powder
810g bag
UK's bestselling whey concentrate with 82g protein per 100g. Regularly discounted 40-60% — never pay full price.
See cheapest price →Buy direct from MyProtein →Quick verdict
Pros
- + 82g protein per 100g high purity
- + Cheapest per serving during sales
- + Huge 60+ flavour range
- + 5kg bags for long-term savings
Cons
- – Full price is poor value
- – Needs discount code to compete
- – Reformulations change taste occasionally
Quick verdict
Pros
- + 82g protein per 100g high purity
- + Cheapest per serving during sales
- + Huge 60+ flavour range
- + 5kg bags for long-term savings
Cons
- – Full price is poor value
- – Needs discount code to compete
- – Reformulations change taste occasionally
The two formulas you actually need
Cost per serving:
(Price ÷ bag size in grams) × scoop size in grams = cost per serving
Use this when you are comparing two products with the same protein content per 100g and the same scoop size. For example, two whey concentrates at 75g of protein per 100g with a 30g scoop.
Cost per 25g of protein:
(Price ÷ (bag size in grams × protein per 100g ÷ 100)) × 25 = cost per 25g of protein
This is the metric that handles every variable: bag size, scoop size, and protein density. It is the only fair way to compare a 1kg tub of whey isolate at 88g per 100g against a 2.5kg bag of whey concentrate at 72g per 100g. Use this formula across every product type and every UK retailer.
Worked example, three real UK products
Three real UK whey products at typical 2026 sale prices, run through both formulas:
Product A: MyProtein Impact Whey, 2.5kg, £30 on Impact Week. Protein per 100g: 82g. Scoop: 25g.
Cost per serving: (30 ÷ 2500) × 25 = £0.30 per scoop.
Total protein in bag: 2500 × 82 ÷ 100 = 2050g.
Cost per 25g protein: (30 ÷ 2050) × 25 = £0.37.
Product B: Bulk Pure Whey, 2.5kg, £42 standard sale. Protein per 100g: 80g. Scoop: 30g.
Cost per serving: (42 ÷ 2500) × 30 = £0.50 per scoop.
Total protein in bag: 2500 × 80 ÷ 100 = 2000g.
Cost per 25g protein: (42 ÷ 2000) × 25 = £0.53.
Product C: Optimum Nutrition Gold Standard, 2.27kg, £55 on sale. Protein per 100g: 79g. Scoop: 30g.
Cost per serving: (55 ÷ 2270) × 30 = £0.73 per scoop.
Total protein in bag: 2270 × 79 ÷ 100 = 1793g.
Cost per 25g protein: (55 ÷ 1793) × 25 = £0.77.
The ranking on cost per 25g of protein: MyProtein £0.37, Bulk £0.53, Gold Standard £0.77. The premium protein costs more than twice as much per 25g of actual protein as the cheap one. The protein content gap is a few grams per 100g. The price gap is much larger than the quality gap.
Bulk Pure Whey Protein
2.5kg bag
Budget-friendly Informed Sport tested whey from Bulk with 50+ flavours. Consistently among the cheapest per serving in the UK.
See cheapest price →Buy direct from Bulk →Bulk Pure Whey Protein
2.5kg bag
Budget-friendly Informed Sport tested whey from Bulk with 50+ flavours. Consistently among the cheapest per serving in the UK.
See cheapest price →Buy direct from Bulk →Quick verdict
Pros
- + Among cheapest UK whey options
- + Huge 50+ flavour range
- + 22g protein per 30g scoop
- + Informed Sport batch tested
Cons
- – 70-75% protein purity only
- – Some flavours overly artificial
- – Can cause bloating if lactose-sensitive
Quick verdict
Pros
- + Among cheapest UK whey options
- + Huge 50+ flavour range
- + 22g protein per 30g scoop
- + Informed Sport batch tested
Cons
- – 70-75% protein purity only
- – Some flavours overly artificial
- – Can cause bloating if lactose-sensitive
Common mistakes UK buyers make
Five recurring errors when comparing UK protein powder value.
Comparing tub prices instead of cost per 25g of protein. A 1kg tub at £30 looks cheaper than a 2.5kg tub at £55 until you do the maths and realise the 2.5kg tub is 27 percent cheaper per gram of powder.
Ignoring protein percentage. A budget protein at 60g protein per 100g costs more per 25g of protein than a slightly pricier one at 80g per 100g, even when the cheap tub looks cheaper. Always read the back label.
Comparing pre-sale RRP, not actual sale price. MyProtein's RRP is fictional. Use the price you actually pay. Bulk and the Protein Works are the same.
Forgetting shipping. A £25 bag with £6 shipping is a £31 bag. Direct to consumer brands often have free shipping above £40 to £55, so combining purchases drops the per kilogram price.
Using cost per scoop alone. A 25g scoop and a 40g scoop are not comparable. Cost per 25g of protein normalises this.
The shortcut: how WheyWise does this for you
The maths above takes about 60 seconds per product to run by hand. WheyWise was built specifically so you do not have to. Every product in the database (1,958 UK protein powders across 85+ retailers as of April 2026) is normalised to cost per 25g of protein automatically and updated weekly.
The main comparison page sorts by cost per 25g of protein by default. The cheapest protein per 25g page surfaces only the lowest cost options across all categories. The whey concentrate, whey isolate, vegan and casein category pages each rank within their type.
You still benefit from understanding the formulas. Knowing why one product is cheaper per 25g of protein than another helps you spot fake bargains (a £20 tub at 50g protein per 100g is not a bargain) and helps you weigh up whether a sale is genuinely a deal or just a cleverly named price drop.
Factoring in discount codes and bag size
Two adjustments worth making after the basic formula.
Apply the actual discount code. MyProtein and the Protein Works both run frequent multi tier codes (40 percent off, 50 percent off, 60 percent off on Impact Week or seasonal promos). Your cost per 25g of protein is the post code price, not the sticker price. Sign up to brand emails to see codes early.
Step up bag size where you can. A 2.5kg bag is typically 15 to 25 percent cheaper per kilogram than a 1kg bag of the same product. A 5kg bag is usually another 5 to 10 percent cheaper per kilogram than 2.5kg. If you have committed to a brand and flavour, scale up.
Check whether the same product is cheaper at Amazon UK. For brands like Applied Nutrition, Optimum Nutrition, Mutant Whey and Warrior, Amazon UK is sometimes meaningfully cheaper than the brand's own site. For Bulk and MyProtein, direct is almost always cheapest. WheyWise tracks both and shows the cheapest UK retailer per product. See the full comparison page for current cheapest seller per product.
Frequently asked questions
What is cost per serving for protein powder? Cost per serving is the price of one scoop of powder, calculated as (price ÷ bag size) × scoop size. It is useful when comparing two products with similar protein densities and scoop sizes. For different brands and formats, cost per 25g of protein is more accurate.
How do I compare protein powder prices fairly? Use cost per 25g of protein, which normalises bag size, scoop size and protein percentage in one metric. Apply actual sale prices and discount codes, not RRP. WheyWise calculates this automatically for every UK product on the comparison page.
What is a good cost per serving for whey protein in the UK? In 2026, anything under £0.40 per 25g of protein is excellent value. £0.40 to £0.55 is reasonable. Above £0.70 you are paying a premium for brand or certification. The cheapest UK options live on the cheapest protein per 25g list.
Why does scoop size matter when comparing protein powders? Scoop size determines how much protein you actually consume per serving. A 25g scoop at 80g protein per 100g delivers 20g of protein, while a 30g scoop at 70g protein per 100g delivers 21g. The cost per 25g of protein metric controls for this and gives you a directly comparable number.
Is bigger bag size always cheaper per serving? Almost always, yes. UK direct to consumer brands consistently price 2.5kg and 5kg bags 15 to 25 percent cheaper per 100g of powder than 1kg tubs of the same product. The exception is short term sales where a smaller bag may be discounted more aggressively. Check both formats before buying.
