# Unit price calculator: how to compare pack sizes and find the real best buy per gram or per litre

> Two packs, two prices and two sizes make it hard to see the cheaper one. Dividing price by quantity gives a unit price you can compare directly. This guide covers the method, the unit mix-ups to avoid, why the bigger pack is not always cheaper and how waste changes the answer.

Published 2026-10-09 by Toolcloak. 3 minute read. Original: https://toolcloak.pages.dev/articles/unit-price-calculator-how-to-compare-pack-sizes-and-find-the-real-best-buy-per-gram-or-per-litre
Related free tool: [Unit Price Comparison Calculator](https://toolcloak.pages.dev/tools/unit-price-calculator)

A **unit price** is the price of one unit of a product, such as one gram, one millilitre or one piece. It is the only fair way to compare two packs of different sizes. The calculation is one division, but shop labels, multi-buy offers and different units make it easy to pick the wrong pack.

> **In short**
> - **Unit price = price ÷ quantity.** Compare the unit prices, not the pack prices.
> - Pack A costs ₹120 for 500 g, which is **₹0.24 per g**. Pack B costs ₹210 for 1,000 g, which is **₹0.21 per g**.
> - Pack B is cheaper by **12.5%** per gram, even though it costs more on the shelf.
> - Both packs must use the **same unit**. Convert kilograms to grams or litres to millilitres first.

## The method

**Example: two packs of the same product**

1. **Pick one unit.** Use grams for both packs.
2. **Divide each price by its quantity.** A: ₹120 ÷ 500 = **₹0.24**. B: ₹210 ÷ 1,000 = **₹0.21**.
3. **Compare.** ₹0.21 is lower than ₹0.24, so **Pack B** is the better deal.
4. **Find the saving.** (0.24 − 0.21) ÷ 0.24 = **12.5%** cheaper per gram.

*The tool takes two prices, two quantities and a unit name, and shows the better deal, both unit prices and the percentage saved.*

Try it: [Unit Price Comparison Calculator](https://toolcloak.pages.dev/tools/unit-price-calculator). Enter the price and quantity of both packs, plus the unit, to see which one is cheaper per unit.

## Make the unit easier to read

A unit price of ₹0.24 per gram is correct, but it is hard to feel. Scale it to a size you recognise, such as 100 g or 1 kg, or use the same scale on every product you compare.

| Pack | Per gram | Per 100 g | Per kg |
| --- | --- | --- | --- |
| A: ₹120 for 500 g | ₹0.24 per g | ₹24 per 100 g | ₹240 per kg |
| B: ₹210 for 1,000 g | ₹0.21 per g | ₹21 per 100 g | ₹210 per kg |
| C: ₹250 for 1,000 g | ₹0.25 per g | ₹25 per 100 g | ₹250 per kg |

*Pack C is the same size as B but costs more, so it is the worst buy per gram.*

**Price per kg of the three packs (₹)**

|  | Per kg |
| --- | --- |
| A (500 g) | ₹240 |
| B (1 kg) | ₹210 |
| C (1 kg) | ₹250 |

*A bigger pack is not always cheaper. C is double the size of A and costs more per kilogram.*

## Mistakes that give the wrong answer

- **Mixed units.** Typing 500 for one pack and 1 (for 1 kg) for the other makes the 1 kg pack look hundreds of times dearer. Use grams for both, or kilograms for both.
- **Pack count against weight.** Six small tubs of 90 g are not the same as one 500 g tub. Compare weight, not the number on the front.
- **Servings instead of weight.** Serving sizes differ between brands, so compare the total weight or volume.
- **Offers that change the quantity.** 'Buy 2 get 1 free' means you pay for 2 and get 3 packs, so divide the price by 3 packs.
- **Shrinking packs.** The same price with a smaller pack is a hidden price rise, and a unit price makes it visible.

**A multi-buy offer**

| Single pack | Buy 2, get 1 free |
| --- | --- |
| ₹0.24 per g | ₹0.16 per g |
| ₹120 for 500 g | Pay ₹240 for 1,500 g |
| No offer | ₹240 ÷ 1,500 g |
| Pay the full unit price | A third off the unit price |

*Always divide by everything you actually receive. An offer is a good one only if you will use all of it.*

## When the bigger pack costs more in the end

Unit price assumes you use every gram. If food spoils or you never finish it, the effective price is higher. If a quarter of the 1 kg pack B is wasted, you pay ₹210 for 750 g that you use, which is ₹280 per kg. That is more than the ₹240 per kg of pack A, which you would use up completely.

**Effective cost per kg if some of it is wasted**

|  | Value | Note |
| --- | --- | --- |
| Pack A, all used | ₹240 | 500 g pack |
| Pack B, all used | ₹210 | 1 kg pack |
| Pack B, 25% wasted | ₹280 | Only 750 g used |
| Saving if all of B is used | 12.5% | Against pack A |

*Bulk buying pays off for things you use steadily and that keep well, such as rice, oil or detergent, and less for fresh or short-life items.*

> **The break-even waste:** Pack B stays cheaper than pack A as long as you waste less than about 12.5% of it. Beyond that, the smaller pack is the better buy.

## Use it for more than groceries

- **Household items** such as detergent, tissue paper and cooking oil.
- **Fuel and liquids,** where you compare price per litre.
- **Subscriptions,** where you can compare price per month across plans.
- **Printing and stationery,** comparing price per sheet or per piece.

## Frequently asked questions

### How do I calculate the unit price?

Divide the price by the quantity. For a pack of 500 grams at 120 rupees, the unit price is 120 ÷ 500 = 0.24 rupees per gram.

### How do I compare two products of different sizes?

Work out the unit price of each, in the same unit, and compare. The one with the lower unit price is cheaper per gram, litre or piece.

### Is the bigger pack always cheaper?

No. Some brands price larger packs higher per unit than the smaller ones. Always check the unit price. Also consider whether you will use the whole quantity.

### What unit should I use?

Any unit works as long as both products use the same one. Convert kilograms to grams, or litres to millilitres, before you enter the quantities.

### How do I work out a buy-two-get-one-free offer?

Pay for two, receive three. Divide the total price you pay by the total quantity you receive, which is three packs, to get the unit price.

### Can I compare more than two products?

The [Unit Price Calculator](https://toolcloak.pages.dev/tools/unit-price-calculator) compares two at a time. For three or more, work out the unit price of each pair and keep the cheapest.