Unit Price Comparison Calculator

Price per 100 g and per kg for common pack sizes

A raw price per unit on a food pack is an awkward number to read, so shelf labels scale it up to a familiar quantity. Both columns below are the same division the calculator does, multiplied back up: price divided by grams, times 100 for the per 100 g figure and times 1000 for the per kg figure. The prices are worked examples of one product sold in eight sizes, not any particular shop's prices, and they make the point that matters. The per kg column does not fall steadily as the pack grows. The 1.5 kg pack here is dearer per kilogram than the 750 g one.

Per 100 g is shown to three decimal places because the third one decides some of these rows: 0.427 against 0.450 is a real gap that rounds away to 0.43 and 0.45. Litres and millilitres work identically, with per 100 ml and per litre in place of per 100 g and per kg.

Price per ml and per litre for common bottle sizes

Anything sold by volume works the same way, with millilitres in place of grams. A price per ml is too small a number to read on a shelf label, which is why drinks are almost always labelled per litre: the same division scaled up by 1000 instead of by 100. The rows below are the eight sizes a supermarket drinks aisle actually stocks, priced as one worked example. Read the per litre column, not the pack price, and the ranking stops matching the size order.

Two rows here go the wrong way. The 750 ml bottle is dearer per litre than the 500 ml one, and the 2 litre bottle is dearer per litre than both the 1 litre carton and the 5 litre box, so the largest container on the shelf is not the winner and neither is the second largest. That is normal rather than a trick: pack prices are set by what sells at that size, not by scaling one price up.

Multibuy offers converted to a price per item

An offer only becomes comparable once you turn it back into the price of one pack: divide what you actually hand over by the number of items you actually leave with. Every row below starts from the same £2.00 pack holding 400 g, so the effective price column ranks the offers directly against each other. Percentages off are worked out against that full price. If you want to check a percentage on your own figures, the discount calculator and the percentage calculator do that part.

Three of these rows land on exactly the same £1.50 an item despite reading very differently on the shelf: second item half price, a straight 25 percent off, and 1.00 off a pair. Buy one get one free is the strongest at 50 percent, and 3 for the price of 2 is worth 33.3 percent, which beats 25 percent off but only if you want all three. An offer you take on a quantity you would not otherwise buy is not a saving.

Making the units match before you compare

The calculator assumes both sizes are in the same unit, and the most common mistake is feeding it one pack in grams and one in kilograms, which makes the metric pack look a thousand times dearer. If one label is imperial and the other metric, convert first. Multiply a price per the unit in the first column by the figure in the last column to get a price per kilogram or per litre, which is the form UK shelf labels use. Recipe volumes convert the same way, and the cooking measurement converter handles those directly.

To go the other way, divide instead of multiplying: a price per kilogram divided by 2.205 is the price per pound. Note that a UK pint is 568.261 ml against the US pint's 473.176 ml, so a price per pint means two different things depending on which side of the Atlantic the label was printed, and the gap is a fifth.

Comparing two packs without dividing

Dividing a price by a pack size in your head is unpleasant, and it is not necessary. To rank two packs you only need to know which unit price is lower, not what either one is, and multiplication answers that on its own. Multiply the first pack's price by the second pack's size, then the second pack's price by the first pack's size. The smaller of the two answers names the cheaper pack, because it is the one carrying that pack's price. Equal answers mean the two packs are exactly the same value.

It works because both unit prices share the same denominator once you multiply through by both sizes: comparing price A over size A against price B over size B is the same comparison as price A times size B against price B times size A. The verdict and saving columns below are the calculator's own comparison, so if the cross products ever disagreed with them the table would show it.

The cross products carry no units and mean nothing on their own, so there is no point reading 1,875 as pounds or as grams. Only the comparison between the two matters. Note the third and fifth rows, where the smaller pack wins: 500 ml at £1.40 beats 750 ml at £2.25, and 1.5 litres at £3.30 beats the 5 litre box at £11.50. The last row is the exact tie, where 1.25 times 1000 and 5.00 times 250 both come to 1,250.

How much a bigger pack can cost and still be worth it

The most useful form of this calculation in a shop is backwards. You already know what the small pack costs, and the question is what the big one would have to be priced at to beat it. Take the small pack's price per unit and multiply it by the bigger pack's size. That is the break-even price: at exactly that figure the two packs are identical value, and above it the small pack wins no matter how much bigger the other one looks. The rows below start from a 500 g pack at £2.50, which is £5.00 per kg.

The third column is the same £5.00 on every row, which is the point rather than a rounding accident: break-even means the per-unit price has not moved. The last two columns exist because matching value is not a reason to buy the bigger pack. A 1 kg pack at £5.00 is worth no more than two 500 g packs, and it only starts paying for the extra money up front and the storage somewhere nearer the £4.50 or £3.75marks. Reverse the whole thing to check a small pack against a big one you have already seen: divide the big pack's price by its size, multiply by the small size, and compare that with the small pack's ticket.

Price per sheet, per wash and per tablet

Plenty of household products are not usefully measured by weight at all. Toilet roll is bought by the roll but used by the sheet, and roll sizes vary enough that price per roll tells you almost nothing. Laundry liquid is sold in millilitres but used in washes, and a concentrated bottle gets more washes out of fewer millilitres. Pick the unit you actually consume, put that count into the size field, and the comparison works exactly as it does for grams. The rows below are worked examples.

The per 100 units column exists because a price per sheet of 0.0025 is hard to hold in your head next to 0.0031, while 0.25 against 0.31 per 100 sheets is obvious at a glance. Scaling the unit price up does not change which product wins, only how readable the gap is.

Where unit price stops being the right answer