Supermarket prices are now so baffling customers can end up paying TWICE as much for a product by choosing the wrong-sized packet, tub or bottle.
And buying in bulk no longer guarantees a better deal, an investigation into pricing at the big stores found.
Martin Isark, founder of consumer website Can I Eat It?, which did the research, said: “Shoppers automatically grab the bigger pack.
Supermarkets are banking on that. They know most shoppers don’t compare the prices of the same product in different sizes.”
Getting the best deal is made harder still because deals vary wildly from supermarket to supermarket.
Morrisons shoppers wanting Cheddar cheese save £6 a kilo by buying a big block of Cathedral City rather than a small one.
But at Tesco it is £4 a kilo to buy a small block of rival Pilgrims Choice.
Martin advised shoppers to ignore the price tag for the product and instead check shelf labels which show a price per weight or size.
Most products now have a price per kilo or price per litre listed under the main cost on the ticket.
This makes it simpler to compare a small packet or bottle with a big one to see which is better value.
Mr Isark, a professional food and drink taster, found there were massive differences in price depending on size.
At Waitrose, a 105g tin of John West salmon is £2.49 while a 213g tin is £3, a difference of 51p.
The price per kilogram shows the small tin is almost £10 a kilo more expensive, at £23.72 per kg, compared to the big tin, which works out at £14.09 per kg.
But teabags at the same store cost £5 for a box of 240 – £7.18 per kg – while a box of 160 bags is £2.24, which is just £4.83 per kilogram.
Mr Isark added: “Don’t get caught out. Always check the price per volume if you want the best price on the shelf.”
How prices compare
Where big is best
AsdaLurpak Spreadable 1kg: £3.87 (£3.87 per kg) 250kg: £1.75 (£7 per kg) - Big pack is £3.13/kg cheaper
Morrisons Cathedral City Mature Cheddar 550g: £3 (£5.45 per kg) 350g: £4 (£11.43 per kg) - Big block is £5.98/kg cheaper
WaitroseJohn West Wild Red Salmon 105g tin: £2.49 (£23.72/kg) 213g tin: £3 (£14.09/kg) - Big tin £9.63/kg cheaper
Sainsbury's Napolina Extra Virgin Olive Oil 1 litre: £4 (£4 per litre) 250ml: £2.30 (£9.20 per litre)- Big bottle is £5.20 per litre cheaper
Tesco Hellmann’s Real Mayonnaise 800g: £2.99 (£3.70 per kg) 200g: £1.39 (£7 per kg) - Big jar is £3.30 a kg cheaper
Where small is best
Morrisons Nestle Shreddies 500g: £1.24 (£2.48 per kg) 750g: £3.38 (£4.51 per kg) * Small pack is £2.14 cheaper
Asda Akash Basmati Rice 5kg: £5 (£1 per kg) 10kg: £16 (£1.60 per kg) - Small pack is 60p/kg cheaper
Waitrose PG Tips Pyramid Teabags 160bags/464g: £2.24 (£4.83/kg) 240bags/696g: £5 (£7.18/kg ) - Small pack is £2.35/kg cheaper
Sainsbury's Nescafe Original Instant Coffee 330g: £4 (£13.30 per kg) 500g: £11 (£22/kg) - Small jar is £8.70/kg cheaper
Tesco Pilgrims Choice Mature Cheddar 350g: £2 (£5.72 per kg) 550g: £5.50 (£10 per kg) - Small pack is £4.28/kg cheaper