TL;DR
For comparing two grocery products in seconds with nothing to sign up for, use the Calc Garden unit price calculator: type each product's price and pack size and it shows both prices per unit, which is cheaper and the exact percent you save, entirely in your browser. This is not an "only free one" claim. Omni Calculator is free and no-signup too and converts between weight and volume units, PricePerPiece compares up to ten items with a shop and brand on each, and Bang Your Buck auto-converts units for bulk buys. The real choice is a fast two-way answer with the saving spelled out versus a fuller tool for many items or mixed units.
What makes a unit price calculator "best"
Every unit price calculator runs the same core sum: divide each product's price by its size, then compare the results. Since that arithmetic is shared, the tools separate on what they wrap around it, and that is where the right pick is decided. Three things matter. The first is how many products you can line up at once: a two-way tool is quick for this pack versus that pack, but useless for a whole trolley. The second is whether it converts units, because a price per ounce and a price per 100 grams cannot be compared until they are in the same unit. The third is how the answer is presented: a bare price per unit is fine, but a named winner and a percent saved tell you what to do without a second glance.
Those pull against each other, so there is no single winner. A lean two-way calculator is best when you are standing at the shelf deciding between two sizes of the same thing and want a verdict in five seconds. A multi-item tool is better when you are keeping a price-book across shops or comparing five brands at once, and a converting tool is essential the moment the packs are measured differently. The table below lines up four free options against exactly these points.
The best free unit price calculators compared
In the table a Yes means the tool does it cleanly, "Partial" means it does it with caveats, and a No means it does not. All four are free with no account required; the pricing row reflects each product as of 2026.
| Capability | Calc Garden | Omni Calculator | PricePerPiece | Bang Your Buck |
|---|---|---|---|---|
| Price (as of 2026) | Free, no signup | Free, no signup | Free, no signup | Free, no signup |
| Price per unit for each product | Yes | Yes | Yes | Yes |
| Compare two products side by side | Yes | Yes | Yes | Yes |
| Compare three or more at once | No | No | Yes | Yes |
| Converts between units (oz, lb, kg, litre) | No | Yes | No | Yes |
| Names the cheaper product | Yes | Yes | Yes | Yes |
| Shows the exact percent you save | Yes | Partial | No | Partial |
| Label each item with shop and brand | No | No | Yes | Yes |
| Updates live as you type | Yes | Partial | No | No |
| Shareable link to your figures | Yes | No | Partial | No |
Read it honestly. Omni Calculator is the strongest free tool when the packs are measured differently, because it converts across weight and volume and offers separate modes for items, items-with-weight and items-with-volume, so a six-pack of bottles lines up against a single large one. PricePerPiece is the one to use for a full trolley: it takes up to ten items on one screen, lets you tag each with a shop and brand, and highlights the cheapest, which makes it a decent running price-book. Bang Your Buck sits between the two, auto-converting units for bulk-buying analysis across several items. Where Calc Garden wins is the first five seconds of a two-way decision: two prices, two sizes, and a verdict that already spells out the percent saved and updates the instant you type, plus a link you can copy to send the figures to whoever is doing the shopping.
How unit price is worked out
The formula is short enough to check by hand, which is exactly why it is worth seeing. Take each product's price, divide by its size to get a price per unit, then compare, and express the gap as a percentage of the dearer one:
unit price = price / size; saving = (dearer unit price - cheaper unit price) / dearer unit price
Work through the example the Calc Garden unit price calculator lands on by default. Product A costs 2.50 for 500 grams, so its unit price is 2.50 divided by 500, or 0.005 per gram. Product B costs 3.20 for 750 grams, so its unit price is 3.20 divided by 750, about 0.00427 per gram. Product B is cheaper per gram, and the saving is 0.005 minus 0.00427, or 0.00073, divided by 0.005, which is about 14.7 percent. The bigger pack wins here, but only because you compared per gram rather than per pack: the 3.20 ticket is the larger cash outlay even though each gram is cheaper.
That last point is why unit price beats eyeballing the shelf price. The cheaper total is not always the cheaper product, and the cheaper product is not always the one you should buy, but you cannot see either until both packs are on the same per-unit footing.
Same units in, or the answer is nonsense
The one rule that catches people is that both sizes must be in the same unit. Calc Garden does no conversion: it divides price by size exactly as entered, so a price per gram compared against a price per millilitre or a price per ounce is a meaningless number, not a smaller one. Put both packs in grams, or both in millilitres, or both as a count, and the comparison holds. If one product is 500 grams and the other is 1.2 kilograms, convert the kilograms to 1200 grams first.
This is the gap where a converting tool earns its place. If your two packs are genuinely in different units, for instance one priced per ounce and one per 100 grams, reach for Omni Calculator or Bang Your Buck, both of which convert weight and volume for you. For UK groceries the units are usually already aligned, because shelf-edge labels quote a price per 100 grams or per litre, so a same-unit tool handles most real comparisons without any conversion at all.
When a lower unit price is not the better buy
Unit price answers one question well: which option gives you more product per pound. It does not answer whether you should buy it. A larger pack with the lower unit price is a false economy if it spoils before you finish it, which is the usual trap with fresh produce, dairy and bread. It is also the wrong call if you have nowhere to store it, or if the bigger ticket stretches a tight weekly budget for a saving of a few pence per gram you will not notice.
So use the number as the start of the decision, not the end. Check the use-by date against how fast you actually get through the item, glance at the cash difference rather than only the per-unit one, and remember that a multi-buy offer only beats the base unit price if you would have bought that quantity anyway. The discount calculator helps when the deal is dressed up as a percentage off rather than a bigger pack.
When to pick each one
Reach for Omni Calculator when the two packs are measured in different units and you want the conversion done for you, and for PricePerPiece when you are comparing more than two items or keeping a price-book of shops and brands across visits. Bang Your Buck is the pick for bulk-buying analysis across several items with automatic unit conversion.
Reach for the Calc Garden unit price calculator when it is this pack versus that pack, the sizes are already in the same unit, and you want a verdict with the percent saved in five seconds and no signup. From there, two related tools cover the rest of the shop: the pizza size value calculator applies the same value logic to price per square inch, and the discount calculator turns a percentage off into the actual saving. For the wider set of explainers, the guides index collects the rest, including why bigger pizzas are often better value, which is the same per-unit maths applied to area.