How to use it well
Enter each option's price and quantity in the same unit: grams with grams, millilitres with millilitres, washes with washes. The calculator returns each option's per-unit cost, which is better, and the percentage saving.
The unit itself is arbitrary as long as it matches. Compare coffee per gram, detergent per wash, streaming per month of actual use. The technique generalises far beyond the supermarket.
When the bigger pack loses
Bulk discounts often invert during promotions: a small pack on offer frequently beats the large pack's everyday price. Shrinkflation also erodes the comparison, when pack sizes quietly fall while prices hold, so the per-unit habit protects you even on products you buy weekly.
The other bulk trap is waste. A per-unit bargain on something perishable you will not finish is a per-used-unit loss. Price the quantity you will genuinely consume.
Frequently asked questions
The packs use different units. How do I compare?
Convert to a common unit first: 1 kg is 1,000 g, 1 litre is 1,000 ml. Then enter both quantities in that unit.
Is the bigger pack always cheaper per unit?
Usually but not reliably. Promotions on small sizes frequently undercut large-pack everyday prices, which is exactly when the mental shortcut fails.
What is shrinkflation?
Reducing pack size while keeping the price, which raises the unit price invisibly. Comparing per-unit over time is the only way to spot it.
Do supermarket shelf labels already show unit prices?
Many do, in small print, though sometimes in inconsistent units across similar products, which defeats the comparison. Doing your own division settles it.
Does this work for subscriptions?
Yes. Compare cost per month of actual use, or per unit of the thing you consume: per delivery, per screen, per gigabyte.
Should I always buy the better unit price?
Only if you will use it all before it expires and can afford the larger outlay now. Waste and cash flow are real costs the unit price does not capture.