Why tub price is the wrong number to look at
A 1kg bag of whey at £20 and a 2.5kg bag at £44 look like a clear win for the smaller bag until you divide by weight. The 2.5kg bag is actually cheaper per 100g. Now factor in that the smaller bag is 80g protein per 100g and the larger one is 74g, and the ranking shifts again.
This is the core problem with protein powder pricing in the UK. Pack sizes range from 500g to 5kg. Protein percentages range from 60g per 100g for some mass gainers to 92g per 100g for premium isolates. Promotional discounts move daily. None of that is visible in the headline price.
Cost per gram of protein is the number ProteinDeals calculates for every product in its database, and it removes all of that noise. It is pack price divided by total weight divided by protein percentage. One number, any product, any size.
How to calculate it yourself
Step one: find the total pack price including any applied discount. Step two: find the pack weight in grams. Step three: find the protein per 100g on the nutrition label. Step four: divide price by weight in grams to get price per gram of powder. Step five: divide that by protein percentage as a decimal.
Example: a 2.5kg bag costs £42. That is 2,500g. Protein is 79g per 100g, so 0.79 as a decimal. £42 divided by 2,500 is 1.68p per gram of powder. Divided by 0.79 gives 2.13p per gram of protein.
A 1kg bag at £21 with 82g protein per 100g: £21 divided by 1,000 is 2.1p per gram of powder. Divided by 0.82 gives 2.56p per gram of protein. The 2.5kg bag wins on value even though it has slightly lower protein density, because pack size drives the cost down enough.
This is why bulk buying almost always wins. The only exception is when a smaller pack carries an unusually aggressive discount that overrides the size advantage.
Which formats win on cost per gram
Large whey concentrate bags (2.5kg–5kg) from Bulk or Myprotein consistently produce the lowest cost per gram of protein in the UK market when a normal promotion is running. Expect 2.2p–3.2p per gram depending on the current deal.
Whey isolate sits roughly 15–25% higher per gram of protein than concentrate. The higher protein density (85–92g per 100g vs 75–82g for concentrate) partially offsets the higher list price, but isolate rarely beats concentrate on pure value.
Vegan protein (pea, rice, or blends) typically sits 10–20% above whey concentrate. The gap has narrowed significantly since 2022 as competition in the category increased.
Mass gainers are almost always the worst value on a per-gram-of-protein basis because you are paying for carbohydrate filler. A 3kg mass gainer at £35 might look cheap until you notice the protein percentage is 22g per 100g. Most of the weight is maltodextrin.
What drives the daily ranking changes
UK protein pricing is not stable. Bulk and Myprotein both run rolling promotions including discount codes, bundle offers, clearance lines and loyalty discounts that can move the cost per gram of a product by 20–40% from one week to the next.
This is why a static best-value list goes stale within days. The live comparison tables on ProteinDeals pull current prices and recalculate cost per gram of protein automatically. The product at number one today may not be number one tomorrow.
What stays consistent is that large concentrate bags from major UK brands almost always occupy the top of the value ranking. The exact product and brand at the top shifts, but bulk whey concentrate almost never loses its structural advantage.
Red flags to watch for
Protein spiking is one thing to watch. Some cheaper products artificially boost their protein reading on lab tests by adding nitrogen-rich fillers like creatine, glycine or taurine that register as protein in the standard Kjeldahl test but are not complete proteins. Reputable UK brands are generally fine, but very cheap unknown-brand products occasionally have inflated protein claims.
Serving size manipulation is another. A product showing 20g protein per serving sounds similar to one showing 25g, but if the first uses a 25g scoop and the second uses a 35g scoop, the per-100g numbers are 80g and 71g respectively. Always read per-100g, not per-serving.
Small packs can also create an illusion of cheapness. Some products appear cheap per gram because they are sold in small trial sizes where the per-gram economics do not apply. A 500g bag priced to look reasonable rarely beats a 2.5kg bag from the same brand.
The simplest protection against all of this is to use cost per gram of protein as your number, read the per-100g protein figure from the nutrition label, and use the live comparison table rather than one-off Google searches.