The reliable way to compare an FMCG product after its price or pack changes is to calculate the price for the same unit of quantity—such as 100 grams, 100 millilitres, a litre, or one item. Record the old and new shelf prices, quantities, retailer, dates and promotion conditions, then compare the resulting unit prices. This reveals the combined cost per declared unit; it does not prove why a manufacturer changed a price or pack.
A larger pack is not automatically better value. A smaller pack can cost less at the checkout yet more per gram or millilitre, while a promotion can reverse the normal ranking. A dated, like-for-like unit-price record prevents those traps and gives you a defensible way to manage household or business purchasing.
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Why shelf price alone gives the wrong answer
The number printed on a shelf label is the price for one package, not necessarily the price for the amount you need. Two bottles can have different volumes, two cereal boxes can contain different weights, and multipacks can contain different item counts. Looking only at the total ticket price therefore mixes quantity and price into one figure.
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This is especially important when a familiar package changes quietly. A current label can show today’s cost per 100 g, but it cannot establish what the product used to cost or contain. A historical claim requires dated records, photographs of the old package, archived listings or another contemporaneous source.
Choose a meaningful common unit
Use the physical measure that normally defines the category:
- Weight: grams or kilograms for products such as flour, coffee, confectionery and pet food.
- Volume: millilitres or litres for drinks, detergents, oils and other liquids.
- Count: one item for eggs, nappies, batteries or other individually countable goods.
Pick one basis and keep it unchanged within the comparison. Price per 100 g is often easy to read for solid goods; price per 100 ml works for smaller liquid packs; price per litre suits large containers. NIST’s Unit Pricing Guide (SP 1181, 2025) advises using a consistent legal measurement within a product category and cautions against measures such as “loads,” “uses” or “servings.” Those descriptions can vary with behaviour and do not provide a stable physical quantity.
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Do not compare weight with volume merely because both figures look similar. A 500 g powder and a 500 ml liquid are different physical measures. If products use different forms or concentrations, note that limitation rather than pretending the units are interchangeable.
The calculation
The basic equation is:
unit price = shelf price ÷ pack quantity
Convert the result to the common reader-friendly basis. If a 750 g pack costs £3.60, the price per 100 g is £3.60 ÷ 750 × 100 = 48p. For a 1.5 litre bottle at £2.70, the price per litre is £2.70 ÷ 1.5 = £1.80.
To measure change between two observations, use:
percentage change in unit price = (new unit price ÷ old unit price − 1) × 100%
A positive result is an increase; a negative result is a decrease. This is a derived calculation, not a statistic published by the retailer or manufacturer, so retain the underlying prices and quantities beside your result.
A repeatable comparison workflow
1. Capture the offer exactly as sold
For every observation, write down the product name, brand or variant, declared quantity, shelf price, retailer, date and whether the price is promotional. Record a multibuy condition or minimum purchase requirement too. A “two for £5” offer is not equivalent to a single-pack price unless you can actually buy and use two packs under that condition.
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2. Check that the products are genuinely comparable
Keep flavour, formulation, strength and preparation method alike. A concentrated detergent, premium recipe or different coffee roast may have a different usable output even when the package weight matches. If you switch brands or variants, label the comparison as a value choice rather than a pure price change.
3. Convert quantities to one basis
Change kilograms to grams, litres to millilitres or multipacks to item counts before dividing. For example, 1.2 kg becomes 1,200 g; a six-pack of 330 ml cans contains 1,980 ml. Use the same denominator for every row in your notes.
4. Calculate each unit price
Divide the displayed price by the declared quantity and multiply by the chosen standard unit. Keep at least one decimal place during calculation, then round only the displayed result. Early rounding can make two close offers appear tied when they are not.
5. Compare old and new unit prices
Apply the percentage-change equation to the unrounded figures. Report the dates with the result, for example, “unit price rose 8.4% between the 12 March and 18 June observations.” Do not call this an inflation rate for the whole category; it describes the recorded offer.
6. Separate price and quantity changes
State the shelf-price movement and pack-quantity movement independently before discussing the unit-price outcome. A higher unit price can result from a higher shelf price, a smaller pack, or both. Keeping the two movements visible prevents a reader from mistaking a quantity change for a simple price increase.
7. Preserve the evidence
Save dated receipts, shelf-label photographs and package photographs where possible. For online prices, retain the product page, displayed quantity and promotion terms on the date checked. A current page cannot, by itself, establish a historical change.
Published example: three orange-juice packs
UK government consumer guidance published in 2015 demonstrates why pack size and total price do not determine value ranking. The figures below are that guidance’s worked example, not a current market survey.
| Pack | Shelf price | Calculated price per 100 ml |
|---|---|---|
| 600 ml bottle | 95p | 15.8p |
| 1 litre carton | £2.00 | 20p |
| 1.5 litre family pack | £3.50 | 23.3p |
On total price, the 600 ml bottle is cheapest and the family pack is most expensive. On the common 100 ml basis, the order is the same but the gap is clearer: the largest pack has the highest unit price. The example shows why “family size” is a description, not a guarantee of a discount.
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Reading a price increase and a pack change together
Suppose an old package cost £2.00 and contained 500 g, while a later package costs £2.10 and contains 450 g. The shelf price rose 5%, but the quantity fell 10%. The old unit price is 40p per 100 g; the new unit price is approximately 46.7p per 100 g, an increase of about 16.7%. Reporting only the 10p shelf-price movement understates the change faced by someone buying the same amount of product.
The reverse can also occur. A retailer may raise the shelf price while increasing the quantity enough to lower the cost per gram. Unit pricing records the consumer outcome without assuming that either change is beneficial or harmful in isolation.
When the pack contains countable goods, use the same logic per item. If a multipack changes from 12 to 10 units, divide the price by the item count and identify any changes in item size or specification that make a simple count comparison incomplete.
How to compare several packs on today’s shelf
For a live shopping decision, calculate every available pack on the same basis rather than assuming a volume discount. Compare regular and promotional prices separately, because a temporary offer can make a normally expensive pack the cheapest option for one visit.
- Unit price: the cost per common weight, volume or item.
- Offer conditions: promotion dates, loyalty requirements, multibuy rules and purchase limits.
- Like-for-like scope: the same variant and formulation where the goal is a pure size comparison.
- Practical quantity: storage space, spoilage risk and whether you will use the entire pack before it deteriorates.
The last factor is a personal decision that arithmetic cannot settle. A lower price per kilogram may not reduce your real spending if half the larger pack is wasted or if buying it triggers a promotion you would not otherwise use.
Promotions, retailers and dates can change the result
Write “regular” or “promotional” beside every price. Do not compare a current sale price with an old regular price and label the difference a permanent increase. Likewise, a loyalty-card price may not be available to every shopper. Record the retailer because the same pack can have different prices, and record the date because shelf prices move.
For a historical series, use consistent observation points—such as the same retailer and a regular price each month—or clearly label changes in retailer and promotion status. If you must combine sources, explain the break rather than presenting a continuous trend that the records cannot support.
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How to establish that a pack really changed
A high current unit price does not prove shrinkflation. First establish that the quantity changed. Compare dated packaging, receipts or archived product descriptions showing the old declaration with the new one. The US Federal Trade Commission’s 2024 consumer advice notes that old packaging may be needed to identify a package change.
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Only after confirming the quantity difference should you describe downsizing. NIST defines the effect succinctly: “The effect of shrinkflation is to increase the product’s unit price.” The term is most useful when the quantity is reduced without a commensurate price reduction; it should not be applied automatically to every small pack or every reformulation.
What broader statistics do—and do not—tell you
NIST’s program material summarizes a 2023 Bureau of Labor Statistics estimate that approximately 30% of consumer goods had been downsized and likely affected by shrinkflation over the preceding four years. That is a historical estimate presented through a secondary source, covers consumer goods broadly rather than FMCG alone, and should not be treated as a current prevalence rate for a particular shelf or country.
The US Government Accountability Office’s 2025 report says downsizing has a small overall effect on inflation because much household spending is on goods that cannot be shrunk, while products such as coffee and cereal can be more affected. That macroeconomic observation does not predict what happened to an individual product or household basket.
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A simple worksheet or spreadsheet layout
Create one row for each dated observation with these fields:
- Product and exact variant.
- Declared quantity and unit.
- Price paid or displayed price.
- Retailer and store or website.
- Date checked.
- Promotion, loyalty or multibuy condition.
- Quantity converted to the common basis.
- Calculated unit price.
If quantity is in cell B2 and price in C2, a spreadsheet formula for price per base unit is =C2/B2. If B2 is grams and you want price per 100 g, use =C2/B2*100. Add a second formula for the change between rows: =new_unit/old_unit-1, then format it as a percentage. Keep the source values visible so another person can reproduce the calculation.
Common mistakes and how to correct them
Comparing total prices
Problem: A £4 pack appears worse than a £3 pack even though it contains twice as much. Fix: Convert both to price per the same physical unit.
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Assuming the biggest pack is the bargain
Problem: A family pack is treated as discounted by definition. Fix: Calculate each size; the UK juice example shows the largest pack can have the highest unit price.
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Mixing promotions with regular prices
Problem: A temporary multibuy is used to claim a lasting price reduction. Fix: Label the offer and state any minimum purchase.
Using servings or “uses” as the denominator
Problem: Different people define a serving differently, and concentrated products may deliver different outputs. Fix: Use the declared legal weight, volume or count, as advised by NIST, and discuss performance separately.
Calling a current unit price historical evidence
Problem: Today’s label is treated as proof of what the product contained last year. Fix: Find dated records or old packaging before claiming a size reduction or past price.
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Problem: A reformulated or concentrated product is compared as though it were identical. Fix: Record the variant and qualify any comparison that is not like-for-like.
Rounding too early
Problem: Rounded unit prices hide small but real differences. Fix: calculate with full precision and round only the displayed answer.
What unit pricing cannot tell you
Unit price measures value per declared quantity, not quality, taste, nutritional profile, durability, concentration or waste. It also does not reveal a company’s motive. A manufacturer may change materials, formulation, logistics or positioning; the arithmetic alone cannot distinguish those reasons.
Nor does one product prove a category-wide trend. To describe a broader movement, collect comparable observations across products and dates, identify retailer and promotion differences, and avoid generalising a single pack’s result.
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A decision rule for managing FMCG costs
- Need the same product? Match variant and specification, then compare dated unit prices.
- Choosing among today’s sizes? Rank all packs by unit price after checking promotion conditions.
- Suspect shrinkflation? Verify the old and new declared quantities before making the claim.
- Changing products? Show the unit-price difference and separately flag formulation, quality or performance differences.
- Planning future purchases? Keep a dated log so a later comparison uses evidence rather than memory.
This process turns a confusing shelf-price change into three visible facts: what the package costs, how much it contains, and what that means per consistent unit. It is precise enough for a spreadsheet, transparent enough for a budget discussion and cautious enough to avoid attributing motives that the numbers cannot establish.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
